cruise flight level

LearnToFly.ca ✈ Learn To Fly ✈ Canada

VFR and IFR Cruising Altitudes

August 28, 2010 by Geoff McKay 2 Comments

What Flight Level should I cruise at?

Is Fifteen Feet O.K. for my Cruising Altitude? Viper North owner Jeff “Biscuit” Lewis flys his Czechoslovakian designed Aero Vodochody L-29 Delfin jet at 15 feet above the deck. How’s that for a cruising altitude?

Pilots must be aware of VFR and IFR Cruising Altitudes. VFR (Visual Flight Rules) and IFR (Instrument Flight Rules) Cruising Altitudes apply for safe flight operations.

Automobile Analogy Here’s a simple analogy. Car drivers know they have their own side of the road to travel on. There is even a yellow line separating the two sides of the road. Cars on each side of the road must stay on their own side to avoid a collision with oncoming traffic. Similarly, pilots use different cruising altitudes for vertical separation. These “roads” or “airways” in the sky are similar to traffic lanes to separate converging traffic.

Vertical Separation In the air, pilots use vertical separation to help avoid collisions with other oncoming traffic. Pilots fly at different altitudes for different directions of flight. This altitude separation works like traffic lanes to keep aircraft flying in different directions from colliding into each other.

When operating below 18,000 feet MSL

• On a magnetic course of zero degrees through 179 degrees, any odd thousand foot MSL altitude +500 feet (such as 3,500, 5,500, or 7,500)
• On a magnetic course of 180 degrees through 359 degrees, any even thousand foot MSL altitude +500 feet (such as 4,500, 6,500, or 8,500)

When operating above 18,000 feet MSL, maintain the altitude or flight level assigned by ATC.

Hemispherical Cruising Altitudes – VFR and IFR Cruising Altitudes

VFR Cruising Altitudes

VFR Cruising Altitudes VFR Pilots flying on a magnetic course (track) of 0 degrees through 179 degrees should fly any odd thousand foot MSL (Mean Sea Level) altitude plus 500 feet. Example VFR Cruising altitudes would be 3,500 feet, 5,500 feet, 7,500 feet etc.

VFR Pilots flying on a magnetic course (track) of 180 degrees through 359 degrees should fly any even thousand foot MSL altitude plus 500 feet. Example VFR Cruising altitudes would be 4,500 feet, 6,500 feet, 8,500 feet etc.

Vertical Separation These VFR Cruising Altitudes provides a minimum of 1,000 feet clearance or vertical separation from other VFR airplanes heading in opposing directions.

These altitudes are based on your course or ground track, and not necessarily your heading being flown because of variance caused by cross-wind effects.

East is Odd As a memory aid, I always think of people from out east speaking with an ‘Odd’ accent. East directions, from 0 degrees through 179 degrees represented on the right (or east) side of the diagram therefore use ODD 1,000 foot altitudes plus 500 feet. Conversely, the West directions, from 180 degrees through 359 degrees on the left (or west) side of the diagram use EVEN 1,000 foot altitudes plus 500 feet.

Who’s flying at the 1000’s? If the VFR Pilots are cruising at the 1,000’s PLUS 500 feet, who is flying at each of the 1,000 foot levels? The IFR Pilots.

IFR Cruising Altitudes

IFR Cruising Altitudes IFR Pilots flying on a magnetic course (track) of 0 degrees through 179 degrees should fly on an odd thousand foot MSL altitude. Example IFR Cruising altitudes would be 5,000 feet, 7,000 feet, 9,000 feet etc.

IFR Pilots flying on a magnetic course (track) of 180 degrees through 359 degrees should fly on an even thousand foot MSL altitude. Example IFR Cruising altitudes would be 4,000 feet, 6,000 feet, 8,000 feet etc.

Remember, Cruising Altitudes are based on your course or ground track, and the pilot must consider cross-wind variances to their heading being flown.

Vertical Separation These IFR Cruising Altitudes provides a minimum of 1,000 feet clearance or vertical separation from other IFR airplanes heading in opposing directions. In addition, we can see the IFR traffic is separated from the VFR traffic by minimum 500 feet.

See and Avoid Pilots should always be actively scanning for other airplane traffic. The VFR pilot should see and avoid other air traffic. And, it is comforting to know there is a built-in vertical separation for safety based on these established cruising altitudes!

Flight Level (FL) Sometimes altitudes in feet are abbreviated as Flight Levels. A Flight Level is a standard nominal altitude in hundreds of feet. The Flight Level altitudes are calculated from the International standard pressure datum of 1013.25 hPa (29.92 inHg), or the average sea-level pressure.  This may not be the same as the aircraft’s true altitude either above mean sea level or above ground level due to variances in atmospheric conditions (from standard pressure) where the airplane is being flown.

Flight levels are described by a number, which is this nominal altitude (“pressure altitude”) in feet, divided by 100. Therefore an apparent altitude of, for example, 35,000 feet is referred to as “flight level 350”.

Flight levels are usually designated in writing as FLxxx, where xxx is a one-to-three digit number indicating the pressure altitude in units of 100 feet.  For instance, FL200 indicates the pressure altitude of 20,000 feet. The phrase “flight level” makes it clear that this refers to the standardized pressure altitude.

At or Below 3,000 Remember, these VFR and IFR cruising altitudes only apply to pilots operating aircraft at more than 3,000 feet AGL (Above Ground Level). Pilots operating at or below 3,000 feet AGL may fly at other altitudes. Also, these cruising altitudes do not apply when the airplane is turning or manoeuvring while practicing flight manoeuvres such as stalls, steep turns, and other activities.

Subscribe and never miss a LearnToFly.ca article!

Subscribe to the free newsletter now. (No spam, we promise.) Published by LearnToFly Inc.

You may cancel anytime. Refer to our Privacy Policy or Contact Us for more details.

@LearnToFly.ca

About Geoff McKay

LearnToFly.ca Editor. Jesus Follower. Information Technology Executive. PMP. Publisher and developer of incredible online brands. Follow me: Twitter / LinkedIn

November 5, 2011 at 11:54 pm

Where are these cruising altitudes based on flight tracks for VFR and IFR mentioned in the AIM or CARSs. In these documents they that these rules exist above 3000 AGL. Even the From The Ground Up mentions these cruising altitudes.

November 9, 2011 at 10:12 am

Canadian Aviation Regulations (CARS) Part VI: General Operating and Flight Rules: Subpart 2: Division 1: Section 602.34

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

LearnToFly.ca Website

• Terms of Use
• Privacy Policy

We'd love to hear from you. Toll-Free: 1.866.96.PILOT

Contact LearnToFly Inc.

• Submit News
• Add Your School

Connect with us

Simple Flying

How do pilots decide their cruise altitude.

The higher an airplane goes the lower the fuel burn, and it also allows the airplane to achieve greater cruise speeds more efficiently.

Airplanes fly at high altitudes for many reasons. The higher an airplane goes the lower the fuel burn, and it also allows the airplane to achieve greater cruise speeds more efficiently. Furthermore, high altitudes ensure that the aircraft is well away from the most severe weather which can lead to turbulence and icing.

So, how is the cruise altitude for a particular flight determined? This article focuses on the main factors that determine the altitude of an airplane in a flight.

The service ceiling and the absolute ceiling

When an aircraft is certified, a ceiling called service ceiling is calculated by the manufacturer. This is typically defined as the altitude at which a jet aircraft can maintain a climb rate of at least 300 ft/minute. This ceiling varies with weight changes. For pilots operating an aircraft, the altitude of the main concern is the maximum certified altitude.

This altitude is fixed and is a part of the aircraft limitations mentioned in the flight manual. In most passenger aircraft, the maximum certified altitude is a structural limitation enforced due to the pressurization system. The higher an aircraft flies, the greater the pressure difference between the aircraft cabin and the atmosphere. This can, in the long term, damage the fuselage of the aircraft.

The absolute ceiling, also known as the aerodynamic ceiling, is the altitude at which the low-speed stall speed and high-speed stall speed converge. This altitude is also known as the coffin corner. You can read more about this phenomenon in this article: Aerodynamic Phenomenon: A Detailed Look At the Coffin Corner.

The optimum altitude

In normal line operations, airplanes are operated at an altitude called the optimum altitude. This altitude is the most efficient altitude to operate an aircraft as it leads to increased range and less fuel burn.

The optimum altitude is determined in several ways. In its most basic derivation, it is all about increasing the range or the fuel mileage of the aircraft. Large jetliners cruise using Mach number as a speed reference. As an aircraft climbs, its speed or True Air Speed (TAS) increases, and the local speed of sound decreases. The combined result of this is an ever-increasing Mach number.

Initially, the increase in TAS is quite beneficial as it allows the aircraft to cover more ground with less fuel burn. However, as mentioned before, this increases the Mach number. As the Mach number increases, there is a corresponding increase in compressibility drag (drag due to the aircraft approaching the speed of sound).

At some point, this drag increase can overcome the benefits of the increasing TAS and start to reduce the range of the aircraft. So, the altitude at which the effects of compressibility drag do not negatively affect the range of the aircraft is known as the optimum altitude.

One of the biggest factors that affect the optimum altitude is the weight of the aircraft. The heavier the aircraft the more lift-induced drag the aircraft generates (due to the increased operating angle of attack). This means that the speed for the best range increases, which ultimately increases the Mach number.

As the weight of the aircraft decreases, there is a decrease in drag and the speed for best range falls off allowing a decrease in Mach number which allows the aircraft to climb higher as it is no longer limited by the compressibility drag associated with large Mach numbers.

The most optimum or efficient altitude is not only affected by aerodynamics. The environment plays a major role as well, particularly the prevailing winds and temperature. In modern aircraft, the Flight Management System (FMS) calculates the optimum altitude by considering these factors. For this, the pilots are required to input accurate data into the FMS.

This includes entering cruise winds and updating the temperature for various altitudes. During the dispatch phase of the flight, the pilots are provided data on forecast winds and temperature for normal cruise levels of the aircraft. The pilots can then input these data into the FMS and then once in the air the FMS calculates the most optimum altitude based on the input data.

One might now wonder how winds can affect the range or the optimum altitude. The reason is simple. In tailwind conditions, the aircraft gets a push from the winds, which increases the ground range of the aircraft. In a headwind, it is the opposite. The range is reduced as the headwind reduces the ground speed of the aircraft. Thus, when accurate wind data is available, the FMS may give a lower optimum altitude because a favorable wind (a strong tailwind) results in a longer range.

Want answers to more key questions in aviation? Check out the rest of our guides here .

The entered Cost Index (CI) also plays a major role. CI is a time-to-fuel ratio. A higher CI indicates that a particular airline has lower fuel costs compared to time-related costs. A lower CI, on the other hand, indicates the airline spends more on fuel when compared to time-related spending. Each airline has a CI calculated based on their operations and this is mentioned in the flight plan. The pilots can then enter this value in the FMS and the FMS uses this information to further optimize the optimum cruise altitude.

Step climbs

Step climbs or cruise climbs is a climbing technique whereby pilots initially remain at a higher or a lower altitude than the optimum altitude. As discussed earlier, as an aircraft's weight decreases the drag reduces which increases the optimum altitude. In a long-range flight, the fuel burn results in large changes in weight which can keep modifying the optimum altitude throughout the flight.

In the initial parts of the flight, the pilots may cruise at a lower altitude than the optimum altitude. This happens most of the time due to the fact a heavier aircraft has lower climb rates, and a slow climbing aircraft can be a nuisance to both the pilots of other aircraft and air traffic control. Once the fuel is burnt enough, and the weight reduced the pilots can initiate a climb to the optimum altitude.

If the aircraft performance permits, a higher altitude than the optimum can be chosen. This way, as the weight of the aircraft, reduces the aircraft can settle to the optimum altitude later in the flight. This is the best option given all the other conditions, such as weather and ATC complies as this prevents the aircraft from being stuck at lower altitudes for the majority of the flight.

In real life, pilots may not be able to get the optimum altitude for a cruise on every flight. This mainly occurs due to ATC restrictions (other aircraft occupying the altitude, airspace restrictions, etc.). Weather and turbulence are other factors that may prevent pilots from achieving the desired optimum altitude. In such situations, the pilots should try to remain as close as possible to the optimum altitude. Generally, remaining within 2000 ft on either side of the optimum altitude does not affect cruise performance that significantly.

Demystifying Flight Levels: An Inside Look at FL600 and Extreme Altitudes

Flight levels. Just the mention of those two words can evoke images of airliners streaking across the sky at mind-boggling heights. But unless you‘re a pilot or air traffic controller, understanding what a flight level actually represents can be confusing.

As a planespotting enthusiast and aviation geek, I want to demystify flight levels for you. In this comprehensive guide, we‘ll dig into the nitty-gritty of flight levels, with a special focus on the rare air of FL600 and above. Get ready for an inside look into the exciting world of high altitude aviation!

Altitudes vs Flight Levels

The first key concept to understand is the difference between altitudes in feet and flight levels.

Below 18,000 feet above mean sea level (MSL), aviation refers to height in feet. So when you‘re on a commercial flight cruising at 35,000 feet, that‘s the actual altitude – no conversion needed.

But once above 18,000 feet, we enter the realm of flight levels. Essentially, a flight level represents altitude calculated based on air pressure, not height above the ground.

The pressure used is a standard 29.92 inches of mercury, equivalent to 1013.2 hectopascals. This pressure exists naturally at approximately 18,000 feet MSL.

To express flight levels succinctly, altitude is stated in hundreds of feet. So flight level 510 represents an altitude of 51,000 feet. Easy, right?

Now, flight levels are important because unlike height above the ground, pressure varies smoothly with increasing altitude. So flight levels provide a consistent frame of reference for tracking aircraft position, regardless of weather influence on barometric pressure.

Below is a conversion chart to help keep altitudes and flight levels straight:

Commercial Airliners and Flight Levels

For most commercial flights, cruise altitudes rarely exceed FL400, or 40,000 feet. In fact, only a handful of airliners can even reach FL410 and above. Let‘s look at some examples:

• Boeing 737 series – Typical cruise FL350
• Airbus A320 family – Typical cruise FL380
• Boeing 777 / Airbus A350 – Can reach FL430+
• Concorde (retired) – Cruise up to FL600

The service ceilings of modern airliners reflect vast improvements from early passenger aircraft. In the 1920s and 30s, airliners like the Ford Trimotor typically cruised between 5,000 and 12,000 feet MSL.

Advances in aerodynamics, wing design, and engine power have enabled service ceilings for commercial aircraft to steadily increase decade after decade:

• 1940s – 30,000 feet becomes standard
• 1950s – Early jets like Boeing 707 reach FL390
• 1960s – Boeing 747 enters service, crosses FL410
• 1970s – Concorde breaks mach 2 near FL600

Today, even typical single-aisle jets cruise in the mid to upper 30s, altitudes only dreamed of just a generation or two ago. The slow elevation of flight levels reflects the remarkable progress in commercial aviation technology.

When Do Flight Levels Begin?

You might be wondering, at what point do flight levels kick in? What‘s the transition altitude between stating height in feet and standardized flight levels?

The transition altitude varies around the world, but in the United States it is 18,000 feet MSL. Some key facts:

• Below 18,000 feet MSL, altitude is stated in feet
• At 18,000 feet, pilots adjust altimeters to standard setting of 29.92 inHg
• All altitudes above 18,000 feet are referred to by flight level

Therefore, in the US, FL180 represents the transition point between feet and flight levels. It‘s essential for pilots to switch altimeter settings at this point to ensure accurate readings above 18,000 feet.

Fun fact – In much of Europe, the transition altitude is lower, typically around 10,000 to 13,000 feet AMSL. So European controllers will begin using flight levels earlier during climbs and descents compared to the USA or Canada.

Demystifying the Rare Air of FL600 and Above

Now that we understand the essentials of flight levels, let‘s zoom in on the extreme altitudes of FL600 and above. This is aviation‘s rare air, reachable only by specialized aircraft types.

For civilian aviation, FL600 represents a functional ceiling. While modern long-haul jets can cross into the lower 50s in altitude, very few have service ceilings exceeding FL600.

In fact, the now retired Concorde SST is one of the only civilian planes to have regularly cruised at FL600. Most airliners simply aren‘t built to operate for prolonged periods at such altitudes.

So what allows high-performance military aircraft to seemingly defy the limits of commercial aviation? Key factors include:

Specialized materials – Fuselages and wings designed with titanium, composites to withstand stress and heat Engine power – Immense thrust generated by afterburners and highly optimized intakes Aerodynamics – Shapes optimized for supersonic flight and maneuvers Life support – Pressurized cockpits and G-suits to protect pilots

These technologies have allowed modern fighters like the F-22 Raptor to reach dizzying altitudes exceeding FL650 during intercepts and special missions. Likewise, spy planes such as the now retired SR-71 Blackbird could sustain speeds over Mach 3 above FL800!

In the end, reaching such heights depends on a delicate balance of lift, thrust, and minimizing drag. It‘s a tremendous feat of aeronautical engineering that few production aircraft have yet achieved.

Highest Altitudes Reached by Aircraft Type

While exact capabilities remain classified, sources indicate modern spy planes like the rumored SR-72 may be able to sustain cruising in the 80,000 to 85,000 feet range. That would place them beyond the reach of virtually any anti-aircraft system or intercepting fighter.

Key Takeaways on Flight Levels

We‘ve covered a lot of ground exploring the esoteric world of flight levels. Here are some key points to remember:

• Flight levels use standardized pressure to express altitudes above 18,000 feet
• Only a handful of specialized aircraft can reach FL600 and above
• Commercial airliners generally fly between FL250 and FL410
• The transition between feet and flight levels occurs at 18,000 feet in the USA
• Flight levels enable consistent altitude reporting regardless of weather

Understanding these concepts provides valuable insight into the invisible architecture of the skies. Next time you track your flight‘s progress via inflight WiFi, you can picture exactly what flight level corresponds to your aircraft‘s cruising altitude.

So while you may never fly on an SR-71 streaking past FL800, I hope this guide has shed light on flight levels and brought you into the rarefied air of high altitude aviation. Let me know if you have any other topics you want explored – planespotting is my passion and I love sharing the thrill of aircraft with new audiences!

How useful was this post?

Click on a star to rate it!

Average rating 0 / 5. Vote count: 0

No votes so far! Be the first to rate this post.

Share this:

You may like to read,.

• What is the expression snookered?
• Is "Hey" or "Hi" More Flirty? A Data-Driven Analysis
• What are 99 to 1 Odds? A Thorough Betting Guide for Long Shots
• Are XXL Pokémon Rare in Pokémon Go?
• What do Italians Call Juventus? A Deep Dive into the Nicknames and History of Italy‘s Most Successful Club
• The Rise and Fall of 2CP: An Overwatch Saga
• Hey friend! Let‘s break down what GTG and TTYL mean in texting
• Why is Saudi Gold So Yellow? An In-Depth Look

3. Initial Climb

• Descent Planning and Descent
• Approach and ILS Landing
• After Landing and Taxi to Gate
• Powering Down
• Advanced Guides
• A320neo Pilot Briefing
• Standard Operating Procedures
• Airliner Flying Guide
• Terms and Abbreviations
• Development Corner
• Release Notes

Takeoff, Climb and Cruise

This guide will explain the correct procedures to accomplish takeoff, climb and establish cruise altitude.

The level of detail in this guide is meant to get a FlyByWire A320neo beginner safely up in the air and to cruise level under normal conditions, while simplifying details which are not (yet) important for a beginner.

A beginner is defined as someone familiar with flying a GA aircraft or different types of airliners. Aviation terminology and know-how is a requirement to fly any airliner, even in Microsoft Flight Simulator.

Further reading: A320 Autoflight Also, you will find many great videos on YouTube on how to fly the FlyByWire A32NX. Check out the FlyByWire YouTube Channel as well: FlyByWire on YouTube

MSFS Start from Gate or Runways

Microsoft Flight Simulator allows you to start your flight from cold & dark at a gate or directly from the runway with the aircraft ready for takeoff.

For this guide, we assume you started cold & dark at the gate and taxied to the runway holding point as per the previous chapters of this beginner guide.

If you did start on the runway, you can skip the first part (Lineup) and directly continue reading Takeoff .

Prerequisites

Aircraft is in TAXI state as per previous chapters

Download FlyByWire Checklist

Chapters / Phases

This guide will cover these phases:

• Initial climb

Base Knowledge About the Airbus A320 for Flight

This list is focussed on differences to other non-Airbus airliners, a user might be used to.

Fly-by-wire system Traditional mechanical and hydro-mechanical flight control systems use a series of levers, rods, cables, pulleys and more, which pilots move to adjust control surfaces to aerodynamic conditions. Their "hands on" design gives pilots a direct, tactile feel for how the aircraft is handling aerodynamic forces as they fly. On the other hand, mechanical systems are also complicated to operate, need constant monitoring, are heavy and bulky, and require frequent maintenance.

In fly-by-wire systems, when the pilot moves flight controls, those movements are converted into electronic signals, which are then interpreted by the aircraft's Electrical Flight Control System (EFCS) to adjust actuators that move flight control surfaces. Computers also monitor sensors throughout the aircraft to make automatic adjustments that enhance the flight.

Because fly-by-wire is electronic, it is much lighter and less bulky than mechanical controls, allowing increases in fuel efficiency and aircraft design flexibility, even in legacy aircraft. And to prevent flight critical failure, most fly-by-wire systems also have triple or quadruple redundancy back-ups built into them. source: BAE Systems

See also: Fly-by-wire Wikipedia

Autotrim The A320 has a feature called "Autotrim", which makes it unnecessary to hold the sidestick or use the trim wheel for holding the current pitch. This system is always active, even when the Autopilot is off (in Normal Law, which means under normal circumstances with a fully functional aircraft).

Autothrust The A320 has Autothrust which is similar to Autothrottle (e.g., in a Boeing), but it does not move the thrust levers. Basically, the thrust levers are only moved by the pilot and never move on their own. The thrust levers act as a maximum allowed power setting for the Autothrust system. During normal flight (after takeoff) the levers stay in the CL climb detent, and the Autothrust system will set engine power accordingly.

Autopilot The A320's Autopilot system works a bit differently from other manufacturer's systems. The A320 FCU controls allow setting certain values and then push or pull the knobs. Pushing usually means automatic control (Managed Mode) and pulling will use the manually selected value (Selected Mode).

Microsoft Flight Simulator knobs

Flight phases The A320 uses flight phases to manage different parts of a flight. These are preflight, takeoff, climb, cruise, descent, approach, go around, done. They match the PERF pages in the MCDU (see Preparing the MCDU ).

Protections The A320 includes many protections for the pilot, which make it nearly impossible to stall or overspeed the aircraft. It's beyond this beginner-guide to go into details (Normal law, Alternate Law, ...)

• ATC (Ground or Tower) has instructed us to hold at a runway holding point and wait until we are cleared to "line up" or "take off".
• Aircraft is still in TAXI state (see previous chapters) and parking brakes are set.

Typically, it is here at the latest that we are asked to switch to Tower ATC frequency for takeoff clearance.

While approaching the runway holding point or at the latest at the runway holding point the "Before takeoff checklist" needs to be completed.

Before takeoff checklist

The "Before Takeoff" checklist is divided into two parts:

• "Down to the line" (or "Above the line") means before "ATC Takeoff Clearance".
• "Below the line" means after T.O. clearance (when lined up) but before starting the roll.

Preparation and "Down to the line" Checklist pre-T.O.-clearance

The following steps from TAXI setup need to be done and checked:

Check OVHD panel: APU off, no lights visible under normal circumstances (exception: Pack 1+2 might be OFF if part of procedure)

Check Flight Controls

Check Flight Instruments

Advise Cabin Crew

• The cabin crew will notify the pilots with a "Cabin Ready" button once they are ready for takeoff.
• In the FBW A320, this is simulated by pressing the CALLS ALL button on the left of the overhead panel.
• This will set the "Cabin Ready" status as shown in the ECAM .

Check correct FLAPS setting (must be in line with PERF TAKE OFF page)

Check V 1 , V R , V 2 speeds and also, if required, FLX temperature setting (PERF TAKE OFF page)

• check squawk ID number
• Set to AUTO or On
• Set ALT RPTG to ON

Check COM frequency

• Tip: set the standby frequency of COM 1 to the Departure frequency to be able to quickly change after takeoff

Check ECAM - no blue writing should be visible for these:

AUTO BRK MAX

• CABIN READY
• TO CONFIG NORMAL

Press T.O. Config button below the ECAM to check takeoff configuration

Check radar panel:

• Set Weather Radar to Sys 1 to show weather on ND
• Check if Predictive Windshear Alerts (PWS) is set to AUTO (should have been set to AUTO during TAXI)

Entering Runway

Before we start rolling, we visually check that no other aircraft is on final approach. We can also use TCAS on the ND to check for aircraft in the vicinity.

If everything is clear, we release the parking brake and slowly roll onto the runway in the direction of takeoff and come to a stop on the runway's center line.

There is also a rolling start where we would not stop but directly apply thrust for takeoff once we are straight on the runway. But, as a beginner, a full stop is recommended, so we can double-check everything.

When we reached our starting point, we stop and set the parking brakes.

If we were only cleared to "line up" we wait here until we get clearance to take off .

This concludes Lineup .

• Aircraft is on runway and fully setup for takeoff as per previous chapters.

Preparation and "Below the line" Checklist post-T.O.-clearance

After ATC (Tower) gives clearance to "line up" or "take off" we are allowed to enter the runway.

To "line up" means that we roll onto the runway and stop at our starting point. We MUST wait for ATC to give us "takeoff clearance" before we can continue.

"Cleared for takeoff" means we are allowed to actually start the takeoff when aligned with the runway.

Shortly before we start our takeoff roll, we do the following steps:

Check PACKS as required (some airlines take off with Packs OFF to allow more power to thrust and save fuel - not necessarily required)

Turn on landing lights ( LAND ) and check if STROBE light is in AUTO or ON

The correct switch settings are:

• RWY TURN OFF lt is ON - NOSE light is at T.O. (T.O. = takeoff)
• LAND lights are both ON
• STROBE is on ON or AUTO
• BEACON , NAV & LOGO should have been on during taxi already
• WING is OFF . It is usually only on for wing inspection and to detect ice accretion on the wing

Lights at Takeoff

Setting the RWY TURN OFF light to ON , the LAND lights to ON and the NOSE light to T.O. minimizes bird strike hazard during takeoff.

Check ENG MODE SEL as required (should be on MODE NORM )

Set TCAS to TA or TA/RA and traffic to ALL or ABV

A typical standard takeoff follows these steps:

Airline SOPs

Some airline's SOPs (standard operating procedures) might have a different order for these steps.

Release parking brake and hold down manual brakes.

Apply thrust slowly to about 50 % N1 until both engines are stabilized (N1 stays constant at around 50 %) while still holding the brakes.

Push sidestick forward half the way to put pressure on the front gear

Release brakes and apply FLX/MCT or TO GA power. (depending on if you have configured a FLEX temperature, and the runway is long enough for a FLEX start)

The PFD Flight Mode Annunciator ( FMA ) now shows several things which we should check when the aircraft starts rolling:

From the left:

• Thrust: set to MAN FLX + 60
• Active (green): SRS (pitch guidance to maintain V 2  + 10)
• Armed (blue): CLB mode (is next after SRS is done)
• Active: RWY (automatic runway axis follow up through ILS use)
• Armed: NAV (navigation guidance according to HDG knob)
• Autopilots are off
• Flight Director 1 and 2 are ON
• A/THR (Autothrust) is armed (not active yet)

Vertical and lateral guidance are only shown via Flight Director, as we have not turned on the Autopilot yet and need to be followed manually by the pilot.

Keep the aircraft on the center line while accelerating down the runway.

There are three important speeds for takeoff, which we have configured earlier when programming the MCDU 's PERF page for takeoff. These are shown on the PFD 's speed tape.

V 1 : The speed beyond which takeoff should no longer be aborted. V 1 is depicted as a cyan "1" next to the speedband in the PFD .

V R : Rotation speed. The speed at which the pilot begins to apply control inputs to cause the aircraft nose to pitch up, after which it will leave the ground. V R is depicted as a cyan circle next to the speedband in the PFD .

V 2 : Takeoff safety speed. The speed at which the aircraft may safely climb with one engine inoperative. V 2 is depicted by a magenta triangle next to the speedband in the PFD .

On a long enough runway, V 1 (depicted by "1") and V R (depicted by "o") are often very close together and can't be clearly distinguished on the PFD speed tape.

At about 80 knots, slowly release the forward pressure on the sidestick until about 100 knots, when the sidestick should be in the neutral position.

The throttle hand remains on the thrust levers until reaching V 1 to be able to quickly abort the start. Remove the hand from the thrust levers at V 1 to avoid accidentally aborting after V 1 .

At V R apply smooth positive backward stick movement on the sidestick and aim for a rotation rate (pitch rate) of 3 deg/sec for about 5 seconds (15° - 18° pitch attitude). Once airborne, follow the flight director's guidance for pitch attitude. Tip: Count one-one thousand, two-one-thousand, etc. and hit 15 degrees at five-one-thousand - practice this.

Once we have confirmed "positive climb" we retract the landing gear.

We confirm that the landing gear is up by looking at the landing gear annunciators, and the lower ECAM Wheels page.

This concludes Takeoff .

• Aircraft has left the ground and is climbing at about 15°.
• Gear is up.
• Thrust levers are in FLX MCT or TO GA detent.
• Flaps are still in T.O. position.

After takeoff, the aircraft will use FLX/MCT or TO GA thrust until thrust reduction altitude is reached (typically ~ 1500 ft above runway, this is part of the MCDU setup)

After reaching thrust reduction altitude, the PFD FMA now shows a flashing LVR CLB message to instruct the pilot to move thrust levers to the CL detent.

Pull the throttle back into the CL detent.

This activates the Autothrust system ( FMA shows A/THR in white now). In the A320 (and most Airbus models) we will not touch the thrust levers again before final approach and landing (under normal flight conditions).

The aircraft will now climb to the altitude selected in the FCU (in our case, 5.000ft).

Activate the Autopilot at this point by pressing the AP1 button on the FCU .

The FMA now shows AP1 in white in the upper-right corner.

FCU Autopilot Controls

The FCU (Flight Control Unit) shows three important values:

• SPD "---" : means the Autopilot is in Managed Speed mode (e.g., 250 kt < 10 000 ft, 290 kt above). If we pull the SPD knob we can select a speed which the Autopilot will then apply.
• HDG "---" : means the lateral navigation is in Managed HDG Mode and the Autopilot follows the planned route. Dialing the HDG knob will let us select a heading and by pulling the knob we tell the Autopilot to fly this heading (Selected Heading Mode).
• ALT "5000" : means the selected altitude is 5000 ft

When reaching S-speed retract flaps. S-speed is signified with an S next to the speed band in the PFD .

Flaps during takeoff and climb

Depending on the start configuration, there will be different markers next to the speedband in the PFD to show when to retract flaps:

• CONF-2 (Flaps position 2): At "F" and positive speed trend
• CONF-1+F (Flaps position 1): At "S" and positive speed trend

We always retract flaps by only one step at a time. So, when we took off with FLAPS 2 ( CONF-2 ) we retract FLAPS at "F" to FLAPS 1 . Then at "S" we retract them to FLAPS 0 .

The TAXI and RWY TURN OFF lights are automatically switched off when the landing gear is retracted. The flight crew should still move the switches to the OFF position as part of after take off flows.

We do this in case the auto-turn-off has failed. This would mean the lights sitting on the front gear, which are now within the gear housing, will start increasing in temperature.

Lastly, we disarm the SPEED BRAKE and turn on the PACKS if we turned them off for takeoff.

Now complete the "After takeoff checklist"

• Landing gear up
• Flaps retracted
• Check Baro setting: above transition altitude (defined in the ECAM PERF page) set it to STD by pulling the baro knob. A flashing baro value in the PFD will remind us in case we forgot.

This is usually a good time to contact ATC Departure to check in with your current altitude. In most cases, ATC will now give us a higher climb altitude. If we did not receive a higher altitude, we have to level off at the previously cleared altitude (cleared by ATC or navigational charts). If we have the Autopilot activated, it will level off automatically at the Selected Altitude.

This concludes the Initial Climb .

• Aircraft is climbing to or is at our initially cleared climb altitude.
• After takeoff checklist is completed.
• ATC has given us clearance for further climb.

Dial the newly cleared altitude into the FCU . (e.g., 15 000 ft) and push for managed climb (CLB) or pull for open climb (OP CLB)

The aircraft will now continue climbing while managing thrust and pitch level. The Autopilot ensures that the aircraft stays at the Selected or Managed Speed setting and climbs to the new altitude while managing thrust automatically.

Thrust level is THR CLB , vertical mode is CLB (ALT mode armed), lateral mode is NAV.

Typically, the climb to the flight plan's cruise level (e.g., FL210) happens in several steps (step climbs). Each to be instructed and cleared by ATC.

It is not recommended to use V/S for climbing or descending in the A320 (at least not for beginners) as the V/S guidance has priority over the speed guidance, and speed needs to be watched very closely when using V/S.

Passing 10,000ft Turn off and retract the landing lights and when the aircraft is stable (weather, no turn, etc.) you can turn off the seatbelt signs. The aircraft will now accelerate to CLB speed (defined in MCDU PERF CLB page).

Repeat the climb process above until cruise level (e.g., FL240) is reached.

MCDU and PFD at cruise level:

This concludes the Climb .

• Aircraft has leveled off at planned cruise level.
• Speed is cruise speed as per ECAM PERF CRZ page.
• Autopilot is ON.
• Speed is in Managed Mode.

This is usually the quietest time of the flight. It allows time to double-check the systems by going through all ECAM pages, etc.

Regular ATC frequency changes with altitude and position check-ins are common.

Here are some typical activities which might happen during cruise mostly on request from ATC or other circumstances like weather, traffic, etc.

Altitude change (also called flight level change) Like before, during climb, set your new altitude in the FCU and push the ALT knob. The aircraft will descent or climb to the new altitude automatically.

Course change with Selected Heading (given or cleared by ATC) Dial heading knob to the desired heading and pull knob for Selected Heading Mode. The aircraft will automatically change course to the new heading. If you want the aircraft to follow the planned route again, you can push the knob for Managed Heading Mode.

Direct course to a waypoint (DIR TO) ATC regularly instructs us to go "direct to (waypoint) XYZ". Use the ECAM DIR page to select the waypoint from the flight plan's list of waypoints. In rare cases it is a waypoint not in the current flight plan, then you can enter a new waypoint in the MCDU and put it into the upper left entry field. Select DIRECT* on the right-bottom to execute the change.

ATC requests specific speed Sometimes ATC requests a specific speed to keep separation between aircraft. Pull the speed knob to switch to Selected Speed Mode. The current speed will be preselected. Dial to the desired speed. The aircraft will immediately begin to target the new speed by either increasing or decreasing thrust.

At some point (200 - 300 NM from destination) we would start with descent-planning and setting up the aircraft for descent and approach.

Descent, Approach, and Landing will be covered in later chapters of this beginner guide.

This concludes the Cruise .

Continue with Descent Planning and Descent

• school Campus Bookshelves
• menu_book Bookshelves
• perm_media Learning Objects
• login Login
• how_to_reg Request Instructor Account
• hub Instructor Commons
• Download Page (PDF)
• Download Full Book (PDF)
• Periodic Table
• Physics Constants
• Scientific Calculator
• Reference & Cite
• Tools expand_more
• Readability

selected template will load here

This action is not available.

4: Performance in Straight and Level Flight

• Last updated
• Save as PDF
• Page ID 75448

• James F. Marchman
• Virginia Tech via Virginia Tech Libraries' Open Education Initiative

Introduction

Now that we have examined the origins of the forces which act on an aircraft in the atmosphere, we need to begin to examine the way these forces interact to determine the performance of the vehicle. We know that the forces are dependent on things like atmospheric pressure, density, temperature and viscosity in combinations that become “similarity parameters” such as Reynolds number and Mach number. We also know that these parameters will vary as functions of altitude within the atmosphere and we have a model of a standard atmosphere to describe those variations. It is also obvious that the forces on an aircraft will be functions of speed and that this is part of both Reynolds number and Mach number.

Many of the questions we will have about aircraft performance are related to speed. How fast can the plane fly or how slow can it go? How quickly can the aircraft climb? What speed is necessary for lift‑off from the runway?

In the previous section on dimensional analysis and flow similarity we found that the forces on an aircraft are not functions of speed alone but of a combination of velocity and density which acts as a pressure that we called dynamic pressure. This combination appears as one of the three terms in Bernoulli’s equation

\[P+\frac{1}{2} \rho V^{2}=P_{0}\]

which can be rearranged to solve for velocity

\[V=\sqrt{2\left(P_{0}-P\right) / \rho}\]

In chapter two we learned how a Pitot‑static tube can be used to measure the difference between the static and total pressure to find the airspeed if the density is either known or assumed. We discussed both the sea level equivalent airspeed which assumes sea level standard density in finding velocity and the true airspeed which uses the actual atmospheric density. In dealing with aircraft it is customary to refer to the sea level equivalent airspeed as the indicated airspeed if any instrument calibration or placement error can be neglected. In this text we will assume that such errors can indeed be neglected and the term indicated airspeed will be used interchangeably with sea level equivalent airspeed.

\[V_{I N D}=V_{e}=V_{S L}=\sqrt{\frac{2\left(P_{0}-P\right)}{\rho_{S L}}}\]

It should be noted that the equations above assume incompressible flow and are not accurate at speeds where compressibility effects are significant. In theory, compressibility effects must be considered at Mach numbers above 0.3; however, in reality, the above equations can be used without significant error to Mach numbers of 0.6 to 0.7.

The airspeed indication system of high speed aircraft must be calibrated on a more complicated basis which includes the speed of sound:

\[V_{\mathrm{IND}}=\sqrt{\frac{2 a_{S L}^{2}}{\gamma-1}\left[\left(\frac{P_{0}-P}{\rho_{S L}}+1\right)^{\frac{\gamma-1}{\gamma}}-1\right]}\]

where \(a_{sl}\) = speed of sound at sea level and ρ SL = pressure at sea level . Gamma is the ratio of specific heats (Cp/Cv) for air.

Very high speed aircraft will also be equipped with a Mach indicator since Mach number is a more relevant measure of aircraft speed at and above the speed of sound.

In the rest of this text it will be assumed that compressibility effects are negligible and the incompressible form of the equations can be used for all speed related calculations. Indicated airspeed (the speed which would be read by the aircraft pilot from the airspeed indicator) will be assumed equal to the sea level equivalent airspeed. Thus the true airspeed can be found by correcting for the difference in sea level and actual density. The correction is based on the knowledge that the relevant dynamic pressure at altitude will be equal to the dynamic pressure at sea level as found from the sea level equivalent airspeed:

An important result of this equivalency is that, since the forces on the aircraft depend on dynamic pressure rather than airspeed, if we know the sea level equivalent conditions of flight and calculate the forces from those conditions, those forces (and hence the performance of the airplane) will be correctly predicted based on indicated airspeed and sea level conditions. This also means that the airplane pilot need not continually convert the indicated airspeed readings to true airspeeds in order to gauge the performance of the aircraft. The aircraft will always behave in the same manner at the same indicated airspeed regardless of altitude (within the assumption of incompressible flow). This is especially nice to know in take‑off and landing situations!

4.1 Static Balance of Forces

Many of the important performance parameters of an aircraft can be determined using only statics; ie., assuming flight in an equilibrium condition such that there are no accelerations. This means that the flight is at constant altitude with no acceleration or deceleration. This gives the general arrangement of forces shown below.

In this text we will consider the very simplest case where the thrust is aligned with the aircraft’s velocity vector. We will also normally assume that the velocity vector is aligned with the direction of flight or flight path. For this most basic case the equations of motion become:

T – D = 0

L – W = 0

Note that this is consistent with the definition of lift and drag as being perpendicular and parallel to the velocity vector or relative wind.

Now we make a simple but very basic assumption that in straight and level flight lift is equal to weight,

We will use this so often that it will be easy to forget that it does assume that flight is indeed straight and level. Later we will cheat a little and use this in shallow climbs and glides, covering ourselves by assuming “quasi‑straight and level” flight. In the final part of this text we will finally go beyond this assumption when we consider turning flight.

Using the definition of the lift coefficient

\[C_{L}=\frac{L}{\frac{1}{2} \rho V_{\infty}^{2} S}\]

and the assumption that lift equals weight, the speed in straight and level flight becomes:

\[V=\sqrt{\frac{2 W}{\rho S C_{L}}}\]

The thrust needed to maintain this speed in straight and level flight is also a function of the aircraft weight. Since T = D and L = W we can write

Therefore, for straight and level flight we find this relation between thrust and weight:

The above equations for thrust and velocity become our first very basic relations which can be used to ascertain the performance of an aircraft.

4.2 Aerodynamic Stall

Earlier we discussed aerodynamic stall. For an airfoil (2‑D) or wing (3‑D), as the angle of attack is increased a point is reached where the increase in lift coefficient, which accompanies the increase in angle of attack, diminishes. When this occurs the lift coefficient versus angle of attack curve becomes non‑linear as the flow over the upper surface of the wing begins to break away from the surface. This separation of flow may be gradual, usually progressing from the aft edge of the airfoil or wing and moving forward; sudden, as flow breaks away from large portions of the wing at the same time; or some combination of the two. The actual nature of stall will depend on the shape of the airfoil section, the wing planform and the Reynolds number of the flow.

We define the stall angle of attack as the angle where the lift coefficient reaches a maximum, CLmax, and use this value of lift coefficient to calculate a stall speed for straight and level flight.

Note that the stall speed will depend on a number of factors including altitude. If we look at a sea level equivalent stall speed we have

It should be emphasized that stall speed as defined above is based on lift equal to weight or straight and level flight. This is the stall speed quoted in all aircraft operating manuals and used as a reference by pilots. It must be remembered that stall is only a function of angle of attack and can occur at any speed. The definition of stall speed used above results from limiting the flight to straight and level conditions where lift equals weight. This stall speed is not applicable for other flight conditions. For example, in a turn lift will normally exceed weight and stall will occur at a higher flight speed. The same is true in accelerated flight conditions such as climb. For this reason pilots are taught to handle stall in climbing and turning flight as well as in straight and level flight.

For most of this text we will deal with flight which is assumed straight and level and therefore will assume that the straight and level stall speed shown above is relevant. This speed usually represents the lowest practical straight and level flight speed for an aircraft and is thus an important aircraft performance parameter.

We will normally define the stall speed for an aircraft in terms of the maximum gross takeoff weight but it should be noted that the weight of any aircraft will change in flight as fuel is used. For a given altitude, as weight changes the stall speed variation with weight can be found as follows:

It is obvious that as a flight progresses and the aircraft weight decreases, the stall speed also decreases. Since stall speed represents a lower limit of straight and level flight speed it is an indication that an aircraft can usually land at a lower speed than the minimum takeoff speed.

For many large transport aircraft the stall speed of the fully loaded aircraft is too high to allow a safe landing within the same distance as needed for takeoff. In cases where an aircraft must return to its takeoff field for landing due to some emergency situation (such as failure of the landing gear to retract), it must dump or burn off fuel before landing in order to reduce its weight, stall speed and landing speed. Takeoff and landing will be discussed in a later chapter in much more detail.

4.3 Perspectives on Stall

While discussing stall it is worthwhile to consider some of the physical aspects of stall and the many misconceptions that both pilots and the public have concerning stall.

To the aerospace engineer, stall is C Lmax , the highest possible lifting capability of the aircraft; but, to most pilots and the public, stall is where the airplane looses all lift! How can it be both? And, if one of these views is wrong, why?

The key to understanding both perspectives of stall is understanding the difference between lift and lift coefficient. Lift is the product of the lift coefficient, the dynamic pressure and the wing planform area. For a given altitude and airplane (wing area) lift then depends on lift coefficient and velocity. It is possible to have a very high lift coefficient C L and a very low lift if velocity is low.

When an airplane is at an angle of attack such that C Lmax is reached, the high angle of attack also results in high drag coefficient. The resulting high drag normally leads to a reduction in airspeed which then results in a loss of lift. In a conventionally designed airplane this will be followed by a drop of the nose of the aircraft into a nose down attitude and a loss of altitude as speed is recovered and lift regained. If the pilot tries to hold the nose of the plane up, the airplane will merely drop in a nose up attitude. Pilots are taught to let the nose drop as soon as they sense stall so lift and altitude recovery can begin as rapidly as possible. A good flight instructor will teach a pilot to sense stall at its onset such that recovery can begin before altitude and lift is lost.

It should be noted that if an aircraft has sufficient power or thrust and the high drag present at C Lmax can be matched by thrust, flight can be continued into the stall and post‑stall region. This is possible on many fighter aircraft and the post‑stall flight realm offers many interesting possibilities for maneuver in a “dog-fight”.

The general public tends to think of stall as when the airplane drops out of the sky. This can be seen in almost any newspaper report of an airplane accident where the story line will read “the airplane stalled and fell from the sky, nosediving into the ground after the engine failed”. This kind of report has several errors. Stall has nothing to do with engines and an engine loss does not cause stall. Sailplanes can stall without having an engine and every pilot is taught how to fly an airplane to a safe landing when an engine is lost. Stall also doesn’t cause a plane to go into a dive. It is, however, possible for a pilot to panic at the loss of an engine, inadvertently enter a stall, fail to take proper stall recovery actions and perhaps “nosedive” into the ground.

4.4 Drag and Thrust Required

As seen above, for straight and level flight, thrust must be equal to drag. Drag is a function of the drag coefficient C D which is, in turn, a function of a base drag and an induced drag.

C D = C D0 + C Di

We assume that this relationship has a parabolic form and that the induced drag coefficient has the form

C Di = KC L 2

We therefore write

C D = C D0 + KC L 2

K is found from inviscid aerodynamic theory to be a function of the aspect ratio and planform shape of the wing

where e is unity for an ideal elliptical form of the lift distribution along the wing’s span and less than one for non‑ideal spanwise lift distributions.

The drag coefficient relationship shown above is termed a parabolic drag “polar” because of its mathematical form. It is actually only valid for inviscid wing theory not the whole airplane. In this text we will use this equation as a first approximation to the drag behavior of an entire airplane. While this is only an approximation, it is a fairly good one for an introductory level performance course. It can, however, result in some unrealistic performance estimates when used with some real aircraft data.

The drag of the aircraft is found from the drag coefficient, the dynamic pressure and the wing planform area:

Realizing that for straight and level flight , lift is equal to weight and lift is a function of the wing’s lift coefficient, we can write:

The above equation is only valid for straight and level flight for an aircraft in incompressible flow with a parabolic drag polar.

Let’s look at the form of this equation and examine its physical meaning. For a given aircraft at a given altitude most of the terms in the equation are constants and we can write

The first term in the equation shows that part of the drag increases with the square of the velocity. This is the base drag term and it is logical that for the basic airplane shape the drag will increase as the dynamic pressure increases. To most observers this is somewhat intuitive.

The second term represents a drag which decreases as the square of the velocity increases. It gives an infinite drag at zero speed, however, this is an unreachable limit for normally defined, fixed wing (as opposed to vertical lift) aircraft. It should be noted that this term includes the influence of lift or lift coefficient on drag. The faster an aircraft flies, the lower the value of lift coefficient needed to give a lift equal to weight. Lift coefficient, it is recalled, is a linear function of angle of attack (until stall). If an aircraft is flying straight and level and the pilot maintains level flight while decreasing the speed of the plane, the wing angle of attack must increase in order to provide the lift coefficient and lift needed to equal the weight. As angle of attack increases it is somewhat intuitive that the drag of the wing will increase. As speed is decreased in straight and level flight, this part of the drag will continue to increase exponentially until the stall speed is reached.

Adding the two drag terms together gives the following figure which shows the complete drag variation with velocity for an aircraft with a parabolic drag polar in straight and level flight.

4.5 Minimum Drag

One obvious point of interest on the previous drag plot is the velocity for minimum drag. This can, of course, be found graphically from the plot. We can also take a simple look at the equations to find some other information about conditions for minimum drag.

The requirements for minimum drag are intuitively of interest because it seems that they ought to relate to economy of flight in some way. Later we will find that there are certain performance optima which do depend directly on flight at minimum drag conditions.

At this point we are talking about finding the velocity at which the airplane is flying at minimum drag conditions in straight and level flight. It is important to keep this assumption in mind. We will later find that certain climb and glide optima occur at these same conditions and we will stretch our straight and level assumption to one of “quasi” ‑level flight.

We can begin with a very simple look at what our lift, drag, thrust and weight balances for straight and level flight tells us about minimum drag conditions and then we will move on to a more sophisticated look at how the wing shape dependent terms in the drag polar equation (CD0 and K) are related at the minimum drag condition. Ultimately, the most important thing to determine is the speed for flight at minimum drag because the pilot can then use this to fly at minimum drag conditions.

Let’s look at our simple static force relationships:

L = W, T = D

D = W x D/L

which says that minimum drag occurs when the drag divided by lift is a minimum or, inversely, when lift divided by drag is a maximum .

This combination of parameters, L/D , occurs often in looking at aircraft performance. In general, it is usually intuitive that the higher the lift and the lower the drag, the better an airplane. It is not as intuitive that the maximum lift‑to drag ratio occurs at the same flight conditions as minimum drag. This simple analysis, however, shows that

MINIMUM DRAG OCCURS WHEN L/D IS MAXIMUM .

Note that since C L / C D = L/D we can also say that minimum drag occurs when C L /C D is maximum. It is very important to note that minimum drag does not connote minimum drag coefficient .

Minimum drag occurs at a single value of angle of attack where the lift coefficient divided by the drag coefficient is a maximum:

D min occurs when (C L / C D ) max

As noted above, this is not at the same angle of attack at which C D is at a minimum . It is also not the same angle of attack where lift coefficient is maximum . This should be rather obvious since C Lmax occurs at stall and drag is very high at stall.

Since minimum drag is a function only of the ratio of the lift and drag coefficients and not of altitude (density), the actual value of the minimum drag for a given aircraft at a given weight will be invariant with altitude. The actual velocity at which minimum drag occurs is a function of altitude and will generally increase as altitude increases.

If we assume a parabolic drag polar and plot the drag equation

for drag versus velocity at different altitudes the resulting curves will look somewhat like the following:

Note that the minimum drag will be the same at every altitude as mentioned earlier and the velocity for minimum drag will increase with altitude.

We discussed in an earlier section the fact that because of the relationship between dynamic pressure at sea level with that at altitude, the aircraft would always perform the same at the same indicated or sea level equivalent airspeed. Indeed, if one writes the drag equation as a function of sea level density and sea level equivalent velocity a single curve will result.

To find the drag versus velocity behavior of an aircraft it is then only necessary to do calculations or plots at sea level conditions and then convert to the true airspeeds for flight at any altitude by using the velocity relationship below.

4.6 Minimum Drag Summary

We know that minimum drag occurs when the lift to drag ratio is at a maximum, but when does that occur; at what value of C L or C D or at what speed?

One way to find C L and C D at minimum drag is to plot one versus the other as shown below. The maximum value of the ratio of lift coefficient to drag coefficient will be where a line from the origin just tangent to the curve touches the curve. At this point are the values of C L and C D for minimum drag. This graphical method of finding the minimum drag parameters works for any aircraft even if it does not have a parabolic drag polar.

Once C Lmd and C Dmd are found, the velocity for minimum drag is found from the equation below, provided the aircraft is in straight and level flight

As we already know, the velocity for minimum drag can be found for sea level conditions (the sea level equivalent velocity) and from that it is easy to find the minimum drag speed at altitude.

It should also be noted that when the lift and drag coefficients for minimum drag are known and the weight of the aircraft is known the minimum drag itself can be found from

It is common to assume that the relationship between drag and lift is the one we found earlier, the so called parabolic drag polar . For the parabolic drag polar

it is easy to take the derivative with respect to the lift coefficient and set it equal to zero to determine the conditions for the minimum ratio of drag coefficient to lift coefficient, which was a condition for minimum drag.

The above is the condition required for minimum drag with a parabolic drag polar.

Now, we return to the drag polar

and for minimum drag we can write

which, with the above, gives

From this we can find the value of the maximum lift‑to‑drag ratio in terms of basic drag parameters

And the speed at which this occurs in straight and level flight is

So we can write the minimum drag velocity as

or the sea level equivalent minimum drag speed as

4.7 Review: Minimum Drag Conditions for a Parabolic Drag Polar

At this point we know a lot about minimum drag conditions for an aircraft with a parabolic drag polar in straight and level flight. The following equations may be useful in the solution of many different performance problems to be considered later in this text. There will be several flight conditions which will be found to be optimized when flown at minimum drag conditions. It is therefore suggested that the student write the following equations on a separate page in her or his class notes for easy reference.

EXAMPLE 4.1

An aircraft which weighs 3000 pounds has a wing area of 175 square feet and an aspect ratio of seven with a wing aerodynamic efficiency factor (e) of 0.95. If the base drag coefficient, C DO , is 0.028, find the minimum drag at sea level and at 10,000 feet altitude, the maximum lift‑to-drag ratio and the values of lift and drag coefficient for minimum drag. Also find the velocities for minimum drag in straight and level flight at both sea level and 10,000 feet. We need to first find the term K in the drag equation.

K = 1 / ( π ARe ) = 0.048

Now we can find

We can check this with

The velocity for minimum drag is the first of these that depends on altitude.

At sea level

To find the velocity for minimum drag at 10,000 feet we an recalculate using the density at that altitude or we can use

It is suggested that at this point the student use the drag equation

and make graphs of drag versus velocity for both sea level and 10,000 foot altitude conditions, plotting drag values at 20 fps increments. The plots would confirm the above values of minimum drag velocity and minimum drag.

4.8 Flying at Minimum Drag

One question which should be asked at this point but is usually not answered in a text on aircraft performance is “Just how the heck does the pilot make that airplane fly at minimum drag conditions anyway?”

The answer, quite simply, is to fly at the sea level equivalent speed for minimum drag conditions. The pilot sets up or “trims” the aircraft to fly at constant altitude ( straight and level ) at the indicated airspeed (sea level equivalent speed) for minimum drag as given in the aircraft operations manual. All the pilot need do is hold the speed and altitude constant.

4.9 Drag in Compressible Flow

For the purposes of an introductory course in aircraft performance we have limited ourselves to the discussion of lower speed aircraft; ie, airplanes operating in incompressible flow. As discussed earlier, analytically, this would restrict us to consideration of flight speeds of Mach 0.3 or less (less than 300 fps at sea level), however, physical realities of the onset of drag rise due to compressibility effects allow us to extend our use of the incompressible theory to Mach numbers of around 0.6 to 0.7. This is the range of Mach number where supersonic flow over places such as the upper surface of the wing has reached the magnitude that shock waves may occur during flow deceleration resulting in energy losses through the shock and in drag rises due to shock‑induced flow separation over the wing surface. This drag rise was discussed in Chapter 3.

As speeds rise to the region where compressiblility effects must be considered we must take into account the speed of sound a and the ratio of specific heats of air, gamma .

Gamma for air at normal lower atmospheric temperatures has a value of 1.4.

Starting again with the relation for a parabolic drag polar, we can multiply and divide by the speed of sound to rewrite the relation in terms of Mach number.

The resulting equation above is very similar in form to the original drag polar relation and can be used in a similar fashion. For example, to find the Mach number for minimum drag in straight and level flight we would take the derivative with respect to Mach number and set the result equal to zero. The complication is that some terms which we considered constant under incompressible conditions such as K and CDO may now be functions of Mach number and must be so evaluated.

Often the equation above must be solved itteratively.

4.10 Review

To this point we have examined the drag of an aircraft based primarily on a simple model using a parabolic drag representation in incompressible flow . We have further restricted our analysis to straight and level flight where lift is equal to weight and thrust equals drag.

The aircraft can fly straight and level at a wide range of speeds, provided there is sufficient power or thrust to equal or overcome the drag at those speeds. The student needs to understand the physical aspects of this flight.

We looked at the speed for straight and level flight at minimum drag conditions. One could, of course, always cruise at that speed and it might, in fact, be a very economical way to fly (we will examine this later in a discussion of range and endurance). However, since “time is money” there may be reason to cruise at higher speeds. It also might just be more fun to fly faster. Flight at higher than minimum-drag speeds will require less angle of attack to produce the needed lift (to equal weight) and the upper speed limit will be determined by the maximum thrust or power available from the engine.

Cruise at lower than minimum drag speeds may be desired when flying approaches to landing or when flying in holding patterns or when flying other special purpose missions. This will require a higher than minimum-drag angle of attack and the use of more thrust or power to overcome the resulting increase in drag. The lower limit in speed could then be the result of the drag reaching the magnitude of the power or the thrust available from the engine; however, it will normally result from the angle of attack reaching the stall angle. Hence, stall speed normally represents the lower limit on straight and level cruise speed.

It must be remembered that all of the preceding is based on an assumption of straight and level flight . If an aircraft is flying straight and level at a given speed and power or thrust is added, the plane will initially both accelerate and climb until a new straight and level equilibrium is reached at a higher altitude. The pilot can control this addition of energy by changing the plane’s attitude (angle of attack) to direct the added energy into the desired combination of speed increase and/or altitude increase. If the engine output is decreased, one would normally expect a decrease in altitude and/or speed, depending on pilot control input.

We must now add the factor of engine output, either thrust or power, to our consideration of performance. It is normal to refer to the output of a jet engine as thrust and of a propeller engine as power. We will first consider the simpler of the two cases, thrust.

4.11 Thrust

We have said that for an aircraft in straight and level flight, thrust must equal drag. If the thrust of the aircraft’s engine exceeds the drag for straight and level flight at a given speed, the airplane will either climb or accelerate or do both. It could also be used to make turns or other maneuvers. The drag encountered in straight and level flight could therefore be called the thrust required (for straight and level flight). The thrust actually produced by the engine will be referred to as the thrust available.

Although we can speak of the output of any aircraft engine in terms of thrust, it is conventional to refer to the thrust of jet engines and the power of prop engines . A propeller, of course, produces thrust just as does the flow from a jet engine; however, for an engine powering a propeller (either piston or turbine), the output of the engine itself is power to a shaft. Thus when speaking of such a propulsion system most references are to its power. When speaking of the propeller itself, thrust terminology may be used.

The units employed for discussions of thrust are Newtons in the SI system and pounds in the English system. Since the English units of pounds are still almost universally used when speaking of thrust, they will normally be used here.

Thrust is a function of many variables including efficiencies in various parts of the engine, throttle setting, altitude, Mach number and velocity. A complete study of engine thrust will be left to a later propulsion course. For our purposes very simple models of thrust will suffice with assumptions that thrust varies with density (altitude) and throttle setting and possibly, velocity. We already found one such relationship in Chapter two with the momentum equation. Often we will simplify things even further and assume that thrust is invariant with velocity for a simple jet engine.

If we know the thrust variation with velocity and altitude for a given aircraft we can add the engine thrust curves to the drag curves for straight and level flight for that aircraft as shown below. We will normally assume that since we are interested in the limits of performance for the aircraft we are only interested in the case of 100% throttle setting. It is obvious that other throttle settings will give thrusts at any point below the 100% curves for thrust.

In the figure above it should be noted that, although the terminology used is thrust and drag , it may be more meaningful to call these curves thrust available and thrust required when referring to the engine output and the aircraft drag, respectively.

4.12 Minimum and Maximum Speeds

The intersections of the thrust and drag curves in the figure above obviously represent the minimum and maximum flight speeds in straight and level flight. Above the maximum speed there is insufficient thrust available from the engine to overcome the drag (thrust required) of the aircraft at those speeds. The same is true below the lower speed intersection of the two curves.

The true lower speed limitation for the aircraft is usually imposed by stall rather than the intersection of the thrust and drag curves. Stall speed may be added to the graph as shown below:

The area between the thrust available and the drag or thrust required curves can be called the flight envelope. The aircraft can fly straight and level at any speed between these upper and lower speed intersection points. Between these speed limits there is excess thrust available which can be used for flight other than straight and level flight. This excess thrust can be used to climb or turn or maneuver in other ways. We will look at some of these maneuvers in a later chapter. For now we will limit our investigation to the realm of straight and level flight.

Note that at the higher altitude, the decrease in thrust available has reduced the “flight envelope”, bringing the upper and lower speed limits closer together and reducing the excess thrust between the curves. As thrust is continually reduced with increasing altitude, the flight envelope will continue to shrink until the upper and lower speeds become equal and the two curves just touch. This can be seen more clearly in the figure below where all data is plotted in terms of sea level equivalent velocity. In the example shown, the thrust available at h 6 falls entirely below the drag or thrust required curve. This means that the aircraft can not fly straight and level at that altitude. That altitude is said to be above the “ceiling” for the aircraft. At some altitude between h 5 and h 6 feet there will be a thrust available curve which will just touch the drag curve. That altitude will be the ceiling altitude of the airplane, the altitude at which the plane can only fly at a single speed. We will have more to say about ceiling definitions in a later section.

Another way to look at these same speed and altitude limits is to plot the intersections of the thrust and drag curves on the above figure against altitude as shown below. This shows another version of a flight envelope in terms of altitude and velocity. This type of plot is more meaningful to the pilot and to the flight test engineer since speed and altitude are two parameters shown on the standard aircraft instruments and thrust is not.

It may also be meaningful to add to the figure above a plot of the same data using actual airspeed rather than the indicated or sea level equivalent airspeeds. This can be done rather simply by using the square root of the density ratio (sea level to altitude) as discussed earlier to convert the equivalent speeds to actual speeds. This is shown on the graph below. Note that at sea level V = Ve and also there will be some altitude where there is a maximum true airspeed.

4.13 Special Case of Constant Thrust

A very simple model is often employed for thrust from a jet engine. The assumption is made that thrust is constant at a given altitude. We will use this assumption as our standard model for all jet aircraft unless otherwise noted in examples or problems. Later we will discuss models for variation of thrust with altitude.

The above model (constant thrust at altitude) obviously makes it possible to find a rather simple analytical solution for the intersections of the thrust available and drag (thrust required) curves. We will let thrust equal a constant

therefore, in straight and level flight where thrust equals drag, we can write

where q is a commonly used abbreviation for the dynamic pressure.

and rearranging as a quadratic equation

Solving the above equation gives

In terms of the sea level equivalent speed

These solutions are, of course, double valued. The higher velocity is the maximum straight and level flight speed at the altitude under consideration and the lower solution is the nominal minimum straight and level flight speed (the stall speed will probably be a higher speed, representing the true minimum flight speed).

There are, of course, other ways to solve for the intersection of the thrust and drag curves. Sometimes it is convenient to solve the equations for the lift coefficients at the minimum and maximum speeds. To set up such a solution we first return to the basic straight and level flight equations T = T 0 = D and L = W .

solving for CL

This solution will give two values of the lift coefficient. The larger of the two values represents the minimum flight speed for straight and level flight while the smaller C L is for the maximum flight speed. The matching speed is found from the relation

4.14 Review for Constant Thrust

The figure below shows graphically the case discussed above. From the solution of the thrust equals drag relation we obtain two values of either lift coefficient or speed, one for the maximum straight and level flight speed at the chosen altitude and the other for the minimum flight speed. The stall speed will probably exceed the minimum straight and level flight speed found from the thrust equals drag solution, making it the true minimum flight speed.

As altitude increases T 0 will normally decrease and V MIN and V MAX will move together until at a ceiling altitude they merge to become a single point.

It is normally assumed that the thrust of a jet engine will vary with altitude in direct proportion to the variation in density. This assumption is supported by the thrust equations for a jet engine as they are derived from the momentum equations introduced in chapter two of this text. We can therefore write:

EXAMPLE 4.2

Earlier in this chapter we looked at a 3000 pound aircraft with a 175 square foot wing area, aspect ratio of seven and C DO of 0.028 with e = 0.95. Let us say that the aircraft is fitted with a small jet engine which has a constant thrust at sea level of 400 pounds. Find the maximum and minimum straight and level flight speeds for this aircraft at sea level and at 10,000 feet assuming that thrust available varies proportionally to density.

If, as earlier suggested, the student, plotted the drag curves for this aircraft, a graphical solution is simple. One need only add a straight line representing 400 pounds to the sea level plot and the intersections of this line with the sea level drag curve give the answer. The same can be done with the 10,000 foot altitude data, using a constant thrust reduced in proportion to the density.

Given a standard atmosphere density of 0.001756 sl/ft 3 , the thrust at 10,000 feet will be 0.739 times the sea level thrust or 296 pounds. Using the two values of thrust available we can solve for the velocity limits at sea level and at l0,000 ft.

= 63053 or 5661

V SL = 251 ft /sec (max)

or = 75 ft /sec (min)

Thus the equation gives maximum and minimum straight and level flight speeds as 251 and 75 feet per second respectively.

It is suggested that the student do similar calculations for the 10,000 foot altitude case. Note that one cannot simply take the sea level velocity solutions above and convert them to velocities at altitude by using the square root of the density ratio. The equations must be solved again using the new thrust at altitude. The student should also compare the analytical solution results with the graphical results.

As mentioned earlier, the stall speed is usually the actual minimum flight speed. If the maximum lift coefficient has a value of 1.2, find the stall speeds at sea level and add them to your graphs.

4.15 Performance in Terms of Power

The engine output of all propeller powered aircraft is expressed in terms of power. Power is really energy per unit time. While the propeller output itself may be expressed as thrust if desired, it is common to also express it in terms of power.

While at first glance it may seem that power and thrust are very different parameters, they are related in a very simple manner through velocity. Power is thrust multiplied by velocity. The units for power are Newton‑meters per second or watts in the SI system and horsepower in the English system. As before, we will use primarily the English system. The reason is rather obvious. The author challenges anyone to find any pilot, mechanic or even any automobile driver anywhere in the world who can state the power rating for their engine in watts! Watts are for light bulbs: horsepower is for engines!

Actually, our equations will result in English system power units of foot‑pounds per second. The conversion is

one HP = 550 foot-pounds/second.

We will speak of two types of power; power available and power required. Power required is the power needed to overcome the drag of the aircraft

P req = D x V

Power available is equal to the thrust multiplied by the velocity.

P av = T x V

It should be noted that we can start with power and find thrust by dividing by velocity, or we can multiply thrust by velocity to find power. There is no reason for not talking about the thrust of a propeller propulsion system or about the power of a jet engine. The use of power for propeller systems and thrust for jets merely follows convention and also recognizes that for a jet, thrust is relatively constant with speed and for a prop, power is relatively invariant with speed.

Power available is the power which can be obtained from the propeller. Recognizing that there are losses between the engine and propeller we will distinguish between power available and shaft horsepower . Shaft horsepower is the power transmitted through the crank or drive shaft to the propeller from the engine. The engine may be piston or turbine or even electric or steam. The propeller turns this shaft power (Ps) into propulsive power with a certain propulsive efficiency, η p .

The propulsive efficiency is a function of propeller speed, flight speed, propeller design and other factors.

It is obvious that both power available and power required are functions of speed, both because of the velocity term in the relation and from the variation of both drag and thrust with speed. For the ideal jet engine which we assume to have a constant thrust, the variation in power available is simply a linear increase with speed.

It is interesting that if we are working with a jet where thrust is constant with respect to speed, the equations above give zero power at zero speed. This is not intuitive but is nonetheless true and will have interesting consequences when we later examine rates of climb.

Another consequence of this relationship between thrust and power is that if power is assumed constant with respect to speed (as we will do for prop aircraft) thrust becomes infinite as speed approaches zero. This means that a Cessna 152 when standing still with the engine running has infinitely more thrust than a Boeing 747 with engines running full blast. It also has more power! What an ego boost for the private pilot!

In using the concept of power to examine aircraft performance we will do much the same thing as we did using thrust. We will speak of the intersection of the power required and power available curves determining the maximum and minimum speeds. We will find the speed for minimum power required. We will look at the variation of these with altitude. The graphs we plot will look like that below.

While the maximum and minimum straight and level flight speeds we determine from the power curves will be identical to those found from the thrust data, there will be some differences. One difference can be noted from the figure above. Unlike minimum drag, which was the same magnitude at every altitude, minimum power will be different at every altitude. This means it will be more complicated to collapse the data at all altitudes into a single curve.

4.16 Power Required

The power required plot will look very similar to that seen earlier for thrust required (drag). It is simply the drag multiplied by the velocity. If we continue to assume a parabolic drag polar with constant values of CDO and K we have the following relationship for power required:

We can plot this for given values of C DO , K, W and S (for a given aircraft) for various altitudes as shown in the following example.

We will note that the minimum values of power will not be the same at each altitude. Recalling that the minimum values of drag were the same at all altitudes and that power required is drag times velocity, it is logical that the minimum value of power increases linearly with velocity. We should be able to draw a straight line from the origin through the minimum power required points at each altitude.

The minimum power required in straight and level flight can, of course be taken from plots like the one above. We would also like to determine the values of lift and drag coefficient which result in minimum power required just as we did for minimum drag.

One might assume at first that minimum power for a given aircraft occurs at the same conditions as those for minimum drag. This is, of course, not true because of the added dependency of power on velocity. We can begin to understand the parameters which influence minimum required power by again returning to our simple force balance equations for straight and level flight:

Thus, for a given aircraft (weight and wing area) and altitude (density) the minimum required power for straight and level flight occurs when the drag coefficient divided by the lift coefficient to the two‑thirds power is at a minimum.

Assuming a parabolic drag polar, we can write an equation for the above ratio of coefficients and take its derivative with respect to the lift coefficient (since C L is linear with angle of attack this is the same as looking for a maximum over the range of angle of attack) and set it equal to zero to find a maximum.

The lift coefficient for minimum required power is higher (1.732 times) than that for minimum drag conditions.

Knowing the lift coefficient for minimum required power it is easy to find the speed at which this will occur.

Note that the velocity for minimum required power is lower than that for minimum drag.

The minimum power required and minimum drag velocities can both be found graphically from the power required plot. Minimum power is obviously at the bottom of the curve. Realizing that drag is power divided by velocity and that a line drawn from the origin to any point on the power curve is at an angle to the velocity axis whose tangent is power divided by velocity, then the line which touches the curve with the smallest angle must touch it at the minimum drag condition. From this we can graphically determine the power and velocity at minimum drag and then divide the former by the latter to get the minimum drag. Note that this graphical method works even for non­parabolic drag cases. Since we know that all altitudes give the same minimum drag, all power required curves for the various altitudes will be tangent to this same line with the point of tangency being the minimum drag point.

One further item to consider in looking at the graphical representation of power required is the condition needed to collapse the data for all altitudes to a single curve. In the case of the thrust required or drag this was accomplished by merely plotting the drag in terms of sea level equivalent velocity. That will not work in this case since the power required curve for each altitude has a different minimum. Plotting all data in terms of Ve would compress the curves with respect to velocity but not with respect to power. The result would be a plot like the following:

Knowing that power required is drag times velocity we can relate the power required at sea level to that at any altitude.

The result is that in order to collapse all power required data to a single curve we must plot power multiplied by the square root of sigma versus sea level equivalent velocity. This, therefore, will be our convention in plotting power data.

4.17 Review

In the preceding we found the following equations for the determination of minimum power required conditions:

We can also write

Thus, the drag coefficient for minimum power required conditions is twice that for minimum drag. We also can write

Since minimum power required conditions are important and will be used later to find other performance parameters it is suggested that the student write the above relationships on a special page in his or her notes for easy reference.

Later we will take a complete look at dealing with the power available. If we know the power available we can, of course, write an equation with power required equated to power available and solve for the maximum and minimum straight and level flight speeds much as we did with the thrust equations. The power equations are, however not as simple as the thrust equations because of their dependence on the cube of the velocity. Often the best solution is an itterative one.

If the power available from an engine is constant (as is usually assumed for a prop engine) the relation equating power available and power required is

For a jet engine where the thrust is modeled as a constant the equation reduces to that used in the earlier section on Thrust based performance calculations.

EXAMPLE 4.3

For the same 3000 lb airplane used in earlier examples calculate the velocity for minimum power.

• It is suggested that the student make plots of the power required for straight and level flight at sea level and at 10,000 feet altitude and graphically verify the above calculated values.
• It is also suggested that from these plots the student find the speeds for minimum drag and compare them with those found earlier.

4.18 Summary

This chapter has looked at several elements of performance in straight and level flight. A simple model for drag variation with velocity was proposed (the parabolic drag polar) and this was used to develop equations for the calculations of minimum drag flight conditions and to find maximum and minimum flight speeds at various altitudes. Graphical methods were also stressed and it should be noted again that these graphical methods will work regardless of the drag model used.

It is strongly suggested that the student get into the habit of sketching a graph of the thrust and or power versus velocity curves as a visualization aid for every problem, even if the solution used is entirely analytical. Such sketches can be a valuable tool in developing a physical feel for the problem and its solution.

1. Use the momentum theorem to find the thrust for a jet engine where the following conditions are known:

Assume steady flow and that the inlet and exit pressures are atmospheric.

2. We found that the thrust from a propeller could be described by the equation T = T 0 – aV 2 . Based on this equation, describe how you would set up a simple wind tunnel experiment to determine values for T 0 and a for a model airplane engine. Assume you have access to a wind tunnel, a pitot-static tube, a u-tube manometer, and a load cell which will measure thrust. Draw a sketch of your experiment.

Figure 4.1: Kindred Grey (2021). “Static Force Balance in Straight and Level Flight.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.1_20210804

Figure 4.2: Kindred Grey (2021). “Different Types of Stall.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.2_20210804

Figure 4.3: Kindred Grey (2021). “Part of Drag Increases With Velocity Squared.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.3_20210804

Figure 4.4: Kindred Grey (2021). “Part of Drag Decreases With Velocity Squared.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.4_20210804

Figure 4.5: Kindred Grey (2021). “Total Drag Variation With Velocity.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.5_20210804

Figure 4.6: Kindred Grey (2021). “Altitude Effect on Drag Variation.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Retrieved from https://archive.org/details/4.6_20210804

Figure 4.7: Kindred Grey (2021). “Drag Versus Sea Level Equivalent (Indicated) Velocity.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.7_20210804

Figure 4.8: Kindred Grey (2021). “Graphical Method for Determining Minimum Drag Conditions.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.8_20210805

Figure 4.9: Kindred Grey (2021). “Thrust and Drag Variation With Velocity.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.9_20210805

Figure 4.10: Kindred Grey (2021). “Minimum and Maximum Speeds for Straight & Level Flight.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.10_20210805

Figure 4.11: Kindred Grey (2021). “Thrust Variation With Altitude vs Sea Level Equivalent Speed.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.11_20210805

Figure 4.12: Kindred Grey (2021). “Straight & Level Flight Speed Envelope With Altitude.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.12_20210805

Figure 4.13: Kindred Grey (2021). “True Maximum Airspeed Versus Altitude .” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.13_20210805

Figure 4.14: Kindred Grey (2021). “Graphical Solution for Constant Thrust at Each Altitude .” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.14_20210805

Figure 4.15: Kindred Grey (2021). “Power Available Varies Linearly With Velocity.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.15_20210805

Figure 4.16: Kindred Grey (2021). “Power Required and Available Variation With Altitude.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.16_20210805

Figure 4.17: Kindred Grey (2021). “Power Required Variation With Altitude.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.17_20210805

Figure 4.18: Kindred Grey (2021). “Graphical Determination of Minimum Drag and Minimum Power Speeds.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.18_20210805

Figure 4.19: Kindred Grey (2021). “Plot of Power Required vs Sea Level Equivalent Speed.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.19_20210805

Figure 4.20: Kindred Grey (2021). “Compression of Power Data to a Single Curve.” CC BY 4.0 . Adapted from James F. Marchman (2004). CC BY 4.0 . Available from https://archive.org/details/4.20_20210805

<!– pb_fixme –><!– pb_fixme –>

• CFI Tools
• All Articles
• Aerodynamics
• Performance
• All Quizzes
• Fun Quizzes
• VFR Quizzes
• IFR Quizzes
• Aleks Udris
• Colin Cutler
• Swayne Martin
• Swag & Wallpaper
• Tips, Stories & Pitches
• Demographics & Reader Statistics
• Media Options
• Weather Reports & Forecasts
• Turns Around A Point
• Forces In A Turn
• Forces In Climbs And Descents
• CG Effects On Performance
• Terms & Policies
• Terms of Service
• Privacy Policy
• Refund Policy
• Security Policy

How To Pick The Best Cruise Altitude For Your Cross Country, In 7 Steps

• By Colin Cutler

You're planning your cross country, and the all-important question comes up: what altitude should I file? Here are seven things you need to be thinking about before you pick your cruise altitude.

1) What are the winds doing?

It's usually the first thing that comes to mind: where are the winds? After all, the last thing you want to do is buck a 40-knot headwind. That's where tools like ForeFlight's winds aloft overlay come in. By dragging up and down on the altitude selector, you can get a quick look at how strong the winds are aloft.

2) What are my available altitudes?

Next up, you need to make sure you're flying at the right altitude for your direction of flight. According to FAR 91.159, if you're more than 3,000 AGL, you need to be flying an odd-thousand MSL altitude +500 feet on a magnetic course of 0-179. And if you're flying a magnetic course of 180-359, you should fly an even-thousand altitude +500 feet. An easy way to remember this is the phrase "East is odd, West is even odder."

3) Is there anything out there I could hit?

How do you make sure you're clear of terrain and obstacles on your route? If you're flying VFR, one of the easiest ways is to open your sectional chart and look at the MEF (Maximum Elevation Figure) altitudes for your route.

The MEF is the bold blue altitude, in hundreds of feet MSL, listed in the middle of each quadrant of your sectional. That altitude guarantees you at least 100 feet (up to 300 feet, in some cases) of clearance from all terrain and obstacles in the quadrant.

As long as you pick an altitude above the MEF, and your chart is current, you can rest easy in knowing that you're not going to hit something sticking out of the ground.

4) Can my plane get to that altitude?

You need to be practical with your altitude choice. If you're flying a short distance, it doesn't make sense to spend the majority of your flight in a climb.

That's where your aircraft's Fuel, Time and Distance to climb chart comes into play. For most aircraft, your time-to-climb is pretty linear, but if you're flying a normally aspirated airplane above 10,000 feet MSL, your climb rate can start dropping off significantly. On top of that, you're burning extra fuel in a climb, and flying a slower indicated airspeed too.

But the opposite is true when it comes to true airspeed. The higher you go, the higher your true airspeed. The rule-of-thumb is that you gain 2% of true airspeed for every 1,000 feet you climb , and that can make a big difference. Consider this: if you're flying at 140 knots indicated at 5,500' MSL, your true airspeed will be roughly 154 knots. But if you fly the same indicated speed at 11,500' MSL, your true airspeed shoots up to 170 knots. That's a gain of 16 knots, which is a big difference, especially on long flights.

5) Am I going to have airspace problems?

There's controlled airspace, special use airspace, and just about every kind of airspace you can think of listed on sectional charts.

Fortunately, tools like ForeFlight can help you navigate around tower controlled airports and special use airspace along your route. But there's another way to make life easy on yourself: simply climb above it.

If you climb above 10,000 feet MSL, you've all but guaranteed yourself clearance above tower controlled airspace, even Class B. There are a few exceptions, of course, like the Denver Class B that extends up to 12,000' MSL, but those are few and far between.

Unfortunately, the same can't be said for restricted areas and other special use airspace, but a quick check on your sectional chart or ForeFlight can clear up any questions about that.

6) Where are the clouds?

On most flights, you need to contend with at least some weather. And mother nature isn't always cooperative when it comes to flying.

That's where your METARs, TAFs and PIREPs come into play. When you're checking the clouds, think about coverage and altitude. If you're looking at few or scattered clouds, climbing above them might be an option, but if there's a broken layer along your route, it's probably best to stay below it.

Also, remember that METARs and TAFs list cloud bases in AGL, not MSL . You'll need to do some math to figure out where the bases will be to maintain your VFR cloud-clearance requirements.

7) Is it going to be a smooth ride?

There's one final consideration, and it's quite possibly the most important thing: are your passengers going to be comfortable on the flight?

If you're getting bounced around because of turbulence, your passengers might not be very impressed. One place you're almost guaranteed to find turbulence is around shear layers in the winds aloft.

While you obviously want to consider your headwind or tailwind along your route, you also want to make sure you're keeping yourself clear of any significant shear layers aloft.

On this route from KGCY-KEHO, there's a 24-knot wind velocity difference between 3,000' and 6,000', with a nearly 50-degree wind direction difference. And if you're thinking things would be bumpy in that area, you're right.

Taking a look at the PIREPs confirms what you'd expect: a Cherokee pilot reported continuous moderate turbulence below 4,500' MSL.

Unless you want to pack extra sick sacks for your passengers, it's a good idea to be on the lookout for the "smooth ride" altitudes, along with the favorable winds aloft.

Picking The Best Altitude

There's a lot to consider when you're picking your cruise altitude. But if you're thinking about obstacles, your plane's performance, and the weather and winds along your route, you'll have a smooth flight, and hopefully some happy passengers as well.

Become a better pilot. Subscribe to get the latest videos, articles, and quizzes that make you a smarter, safer pilot.

Colin is a Boldmethod co-founder and lifelong pilot. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed the development of numerous commercial and military training systems. You can reach him at [email protected] .

Recommended Stories

Latest Stories

• Light Aircraft

• [email protected]
• 720-663-7754
• [email protected]
• facebook.com/boldmethod
• About Boldmethod
• © 2024 Boldmethod, LLC
• Terms and Policies

¶ Introduction

When you are an IFR pilot, you must respect minimum rules in order to select a cruise altitude or flight level. This article will help you choose one of the possible solutions.

¶ IFR altitude and level restrictions

¶ minimal altitude.

All IFR flight shall be flown except for take-off, landing or except by emission from the appropriate authority:

• At a level which is not below the minimum flight altitude established by the local regulation (published on charts).
• At a level which is a least 600m or 2000ft above the highest obstacle located within 8km of the estimated position of the aircraft, in mountainous areas , when no minimum flight altitude has been established
• At a level which is a least 300m or 1000ft above the highest obstacle located within 8km of the estimated position of the aircraft, elsewhere than the two first items , when no minimum flight altitude has been established.

Note that there is no maximal altitude in IFR flight. Only your aircraft performance and the chosen route constraints can limit the altitude. The maximum controlled flight level is usually FL660.

¶ Cruise altitude or flight level selection

¶ transition layer constraints.

No cruise flight level or cruise altitude can be chosen in the transition layer.

The transition layer is the airspace between the transition altitude and the transition level.

Consult our altimetry documentation for pilot or controller in order to have more information about transition layer, transition altitude and transition level.

¶ Available IFR levels

Except where otherwise indicated in air traffic control clearances or specified by the appropriate ATS authority, IFR flights when operated above minimum flight altitude as specified shall be conducted at a flight level/altitudes appropriate to the track as specified in the following levels:

• IFR flights use altitudes ending with the number 000: 5000ft, 6000ft, 7000ft, 8000ft, 12000ft, ...
• IFR flights use flight levels ending with the number 0: FL50, FL60, FL70, FL210, FL320 ...

The cruise altitude or cruise flight level must be chosen using this assigned rule and must follow the semi-circular rule depending on the heading of the aircraft (see next chapter).

Note that VFR flight levels end with final number 5 and not 0. This provides enough separation between VFR and IFR flights and adequately avoids possible conflicts during the cruise phase.

¶ Odd and even flight levels

For answering to the need of flight level separation between the same types of flight, flight levels have been separated in two categories, the even and the odd flight level:

• Even flight level: the last number before the final number 0 shall be even: FL80, FL120, FL260 ...
• Odd flight level: the last number before the final number 0 shall be odd: FL90, FL130, FL270 ...

This rule is applicable for flight levels below FL290. This rule is applicable only for flight levels between FL290 and FL410 in RVSM airspace.

Consult the available tables at the end of this document in order to select your flight level or altitude.

¶ Semicircular rule

¶ default worldwide semicircular rule.

The default worldwide semi-circular rule is the East/West orientation of the flight level parity:

Your aircraft has track between 0° and 179° , your flight level or altitude must be odd. Your aircraft has track between 180° and 359° , your flight level or altitude must be even.

By following the semi-circular rule, an IFR aircraft will limit possible conflicts with another aircraft coming from the opposite direction through providing 1000ft separation between opposite west/east tracks.

¶ Specific semicircular rule

In some counties due to IFR routes or special regulations set by the local administration, the semicircular rule can be the North/South orientation of the flight level parity:

• Your aircraft has track between 90° and 269°, your flight level or altitude must be odd
• Your aircraft has track between 270° and 359° & between 0° and 89°, your flight level or altitude must be even.

¶ List of available flight level and altitude

Since year 2010, the worldwide airspace respects the RVSM regulation between FL290 and FL410 except for some local areas around the world. (RVSM = Reduced Vertical Separation Minimum)

Magnetic track, or in polar areas at latitudes higher than 70 degrees and within such extensions to those areas as may be prescribed by the appropriate ATS authorities, grid tracks as determined by a network of lines parallel to the Greenwich Meridian superimposed on a polar stereographic chart in which the direction towards the North Pole is employed as the Grid North.

¶ In a RVSM airspace

In areas where feet are used for altitude and where, in accordance with regional air navigation agreements, a vertical separation minimum of 1000 feet is applied between FL290 and FL410 inclusive.

¶ In a RVSM metric airspace

In areas where metres (meter) are used for altitude and where, in accordance with regional air navigation agreements, a vertical separation minimum of 300 m is applied between 8 900 m and 12 500 m inclusive.

¶ In a non-RVSM airspace

If your airspace is a non-RVSM airspace, a vertical separation minimum of 2000 feet is applied between FL290 and FL410 inclusive.

For the level below FL290 and above FL410, you must select the flight level according the table in RVSM airspace.

¶ In a non-RVSM metric airspace

If your airspace is a non-RVSM airspace, a vertical separation minimum of 600 m is applied between 8 900 m and 12 500 m inclusive.

For the level below 8 900 m and above 12500 m, you must select the flight level according the table in RVSM airspace.

• VID 150259 - Creation
• VID 435695 - Wiki integration
• VID 496402 - Wiki.js integration

Flight Crew Guide

Search our database, cruising levels.

IFR Flights within Controlled Airspace An IFR flight operating in cruising flight in controlled airspace shall be flown at a cruising level, or, if authorized to employ cruise climb techniques, between two levels or above a level, selected from: a) the table of semicircular cruising level system; or b) a modified table of cruising levels, when so prescribed for flight above FL410; except that the correlation of levels to track prescribed therein shall not apply whenever otherwise indicated in ATC clearances or specified by the appropriate ATS authority in Aeronautical Information Publications.

IFR Flights outside Controlled Airspace An IFR flight operating in level cruising flight outside of controlled airspace shall be flown at a cruising level appropriate to its track as specified in: a) the table of semicircular cruising level system, except when otherwise specified by the appropiate ATS authority for flight at or below 3000ft (900m) AMSL; or b) a modified table of cruising levels, when so prescribed for flight above FL410.

Source: ICAO

Share this:

The Federal Register

The daily journal of the united states government, request access.

Due to aggressive automated scraping of FederalRegister.gov and eCFR.gov, programmatic access to these sites is limited to access to our extensive developer APIs.

If you are human user receiving this message, we can add your IP address to a set of IPs that can access FederalRegister.gov & eCFR.gov; complete the CAPTCHA (bot test) below and click "Request Access". This process will be necessary for each IP address you wish to access the site from, requests are valid for approximately one quarter (three months) after which the process may need to be repeated.

An official website of the United States government.

If you want to request a wider IP range, first request access for your current IP, and then use the "Site Feedback" button found in the lower left-hand side to make the request.

How to determine correct cruising altitude?

I know when you’re in the world map making a flight plan you can look a the nav log and it’ll show you the correct cruising altitude. However once you’re in the cockpit the nav log no longer shows this. Is there another way to determine the correct cruising altitude once you’re already in flight?

There is no such thing as a correct altitude. In actual flight planning it will depend on a multitude of factors. If you are looking for the even–odd rule, in many countries you‘d fly odd levels going on headings 0–179° (5000‘, 7000‘, FL310, FL330…) and even levels going 180–359° (6000‘, 8000‘, FL360, FL400…). For VFR you’d add 500‘. If you look at a suitable altitude performance or airspace wise then things get more complex.

Ah yeah, I was wondering if there was a sweet spot for performance, but it sounds like you’re saying it can change from aircraft to aircraft, right?

I guess I was leaning on my knowledge that commercial flights typically fly between 31,000 and 38,000 feet. So I was wondering if there was an optimal range for propeller planes.

The correct cruising level can get very complex in certain regions. Airlines rely on complex flight planning software with certain rules built into the navigation data. The information is taken from individual countries AIP’s. Some of these you can get access to for free and others not. Im pretty sure IVAO gives region specific info which may help.

You can either take the cruising altitude that the flight plan gives you in the sim or you can change it to reflect your personal flight planning. This is going to depend on the type of aircraft, IFR or VFR flight plan, winds aloft, and other factors.

You can look up the optimal ranges for the propeller aircraft you want to fly and go from there. If you are wanting to be realistic that is. Generally though the propeller aircraft without oxygen are not going to cruise above 10,000 ft. (ballpark). Turboprop aircraft with oxygen and/or pressurization can get into those higher altitudes but you would have to look at service ceilings and performance tables to see where the optimum altitudes would be. Then look at winds aloft forecasts and adjust from there.

The even-odd rule mentioned is correct and for VFR you add 500’ up to FL180. There is no VFR flight above FL180 (18,000 ft.)

Another option to the OP – get a simbrief account (it’s free) and it does the flight plan for you, including the altitude/speed/etc. Very useful.

Oh wow, SimBrief seems really in depth!

Depends on the plane. For the airbus and 745 there are some factors to consider.

Let’s take the Airbus A320 for example> we could say the average is 35,000 feet however i reality their are other aspects. Climbs on such planes including the 747 are gradual of a period of time. Gunning engines to get up high is not fuel economic.

Factors: Flight distance. Weight on plane including Fuel, passengers Cargo. Weather, Cloud ceiling, wind sheers.

A320 ratings: (Speed and alt) Max cruising speed 903km/h (487kt) at 28,000ft, economical cruising speed 840km/h (454kt) at 37,000ft.

Lift versus weight is important. More, you fly lower, less weight you can cruise higher.

Minimal Alt recommendations not on long trips: probably 10000, can cruise 12000-15000feet Longer trips, and can reach the optimized cruise alt. 20,000 to 35,000, 45,000 in weight displacement

Game wise for a decent trip let’s average about 35,000 feet. Short about 12000-20000, mid 15000 to 25000.

IF cloud ceilings are 36,000+ feet Don’t even attempt to fly over it. You will have to drop alt below the ceiling. Going through moisture does create drag and is not economic. IF you can get over and cruise 35,000 to 40,000 feet, that is good, and the lift will be their. if not and you are popping through alot of moisture, don’t and drop below the cloud bank.

If setting it in your plane. Set the cruise 25,000 on a decently long trip over 3-5 hours unless you know weather will be bad with high storm banks. IF going more then 6 hours Sure cruise at 35,000

Uhm, I tend to disagree with quite a lot up there, if we’re talking jets here.

First of all, of course it is fuel economic to climb as high as possible. Jet engines run a whole lot more fuel saving at altitude. Example: even on a short flight of <1hr I’d climb up to FL410 if the weight permits (talking about real life here). That might very well be the FMC calculated optimum altitude in such a case at low weight. The cruise portion might be very short then, with the T/D being not long after the T/C. Depending on the aircraft there might even be a number deep in the FMC called something like minimum cruise time to determine its optimum altitudes etc., and this might be as low as one minute. Just so you have an idea how „short“ you could realistically be in cruise. However, the level might also be as low as FL240ish, but then mostly due to airspace considerations. Lot less efficient down there.

If the flights are longer and you have payload to carry we’d be somewhere in the 350—390 range, simply because of weight.

I have never in my life heard anything about flying through moisture having an effect on flight planning, at least if we’re not talking about CBs or icing issues and maybe avoiding those areas.

And of course do we climb above clouds >FL360. In fact you’d even climb higher if performance permits for a smoother ride if you were riding the tops.

Last but not least, if you are talking about cruising at FL250 on 3—5hr flights (!) you’d burn tons, tons of fuel that is just going to waste. There are malfunctions that would dictate such a low level even with these flight durations (think of pressurization…), otherwise this is completely unrealistic.

how are you guys calculating your altitude? let a flight planner such as simbrief to determine it? or you make your own calculations and decide your appropriate altitude? (just curious to see the different options out there…)

I use Simbrief as I have no idea how to calculate it myself. If I was doing A320 flights I could just pick a low FL30x’s altitude since that’s pretty obvious based on real flights I’ve been on, but anything else I’m lost instantly other than knowing the max altitude as per the aircraft selection pages.

It should be based on aircraft performance charts, then modify for hemispheric cruising rules/airspace rules, then weather.

Could we trust Simbrief flight level when set on AUTO?

I was planning a flight LOWW - EFHK, simbrief recommend FL360 As the average heading is around 020 I was expecting FL350 or 370.

So should I trust Simbrief?

Hi, as an airline pilot, I can agree that there are tons of factors to be considered, especially flying IFR and jet engines. One is performance: on very short flights the general rule is calculating a flight level as high as possible that leaves a section of five minutes at level flight in that cruise level, but as also mentioned the optimum flight level does change with weight, prevailing winds and temperature. The heavier the aircraft, the lower you fly. If you can get your hands on the performance handbook of your aircraft you have many, many tables you can use. However, a modern jet should be able to indicate its optimum cruise level to the pilot, especially if fed with good wind data for the different flight levels and the altitude of the tropopause. Then comes the question of airspace structure. Most countries use that semicircular rule that was mentioned before, others (especially those that have a much bigger N-S extension than E-W) turn that semicircular rule by 90 degrees. Furthermore, using airways, that rule may once again be flipped on its head for certain airways. Here, you may want to consult each (!) of the overflown countries AIPs. And then of course there’s always the question of the minimum enroute, offroute altitudes as well as the minimum usable flight levels (MUFL). Modern briefing systems take care of all of these and are good in calculating a very reasonable sequence of flight levels (remember, as fuel is burnt, the weight decreases and the optimum rises), provided that the actual weight and weather match the planning conditions to a high degree. How well Simbrief does that? No idea, I use MSFS only to fly VFR. All in all: there is virtually no limit when it comes to perfecting flight planning and execution. Look for the best compromise for your needs and wishes when it comes to SIM-flying!

Simbrief has never failed me on most flights except extremely short sectors in an airliner. Sometimes it plans at altitude that is too high, so that you never actually reach cruise.

I find that on certain airliners, like the 747, it sometimes plan an altitude that is too low when it might be better optimized to climb higher given winds and fuel economy. However, thats normally not an issue since one can request higher enroute.

A good rule of thumb though, especially if you are doing short sectors, is to take your distance and base off your final altitude from that. For instance, if you are flying a 150 nm leg, then I wouldn’t go higher than 15000’. But this is mostly for shorter sectors. Longer sectors, you take into account the weight of the aircraft, winds, temperatures, and airspaces into account. I generally try to comply with westbound/eastbound altitude cruising orders as well, although certain airspaces and airways may be more specific on such matters.

This is all talking about jets of course.

I once asked this question to a 737 airline pilot who flew mostly short legs. He said as a rule of thumb, minimum 5 minutes on cruise (from TOC to TOD).

Thank you very much for your detailled answer, it helps a lot.

I’ll try to optimize on VATSIM or IVAO, in particular when my flight is going through Italy or Portugal as they have a North Sud Semi Circular rule.

14 CFR § 91.179 - IFR cruising altitude or flight level.

Unless otherwise authorized by ATC , the following rules apply—

(a) In controlled airspace. Each person operating an aircraft under IFR in level cruising flight in controlled airspace shall maintain the altitude or flight level assigned that aircraft by ATC . However, if the ATC clearance assigns “VFR conditions on-top,” that person shall maintain an altitude or flight level as prescribed by § 91.159 .

(b) In uncontrolled airspace. Except while in a holding pattern of 2 minutes or less or while turning, each person operating an aircraft under IFR in level cruising flight in uncontrolled airspace shall maintain an appropriate altitude as follows:

(1) When operating below 18,000 feet MSL and—

(i) On a magnetic course of zero degrees through 179 degrees, any odd thousand foot MSL altitude (such as 3,000, 5,000, or 7,000); or

(ii) On a magnetic course of 180 degrees through 359 degrees, any even thousand foot MSL altitude (such as 2,000, 4,000, or 6,000).

(2) When operating at or above 18,000 feet MSL but below flight level 290, and—

(i) On a magnetic course of zero degrees through 179 degrees, any odd flight level (such as 190, 210, or 230); or

(ii) On a magnetic course of 180 degrees through 359 degrees, any even flight level (such as 180, 200, or 220).

(3) When operating at flight level 290 and above in non-RVSM airspace, and—

(i) On a magnetic course of zero degrees through 179 degrees, any flight level , at 4,000-foot intervals, beginning at and including flight level 290 (such as flight level 290, 330, or 370); or

(ii) On a magnetic course of 180 degrees through 359 degrees, any flight level , at 4,000-foot intervals, beginning at and including flight level 310 (such as flight level 310, 350, or 390).

(4) When operating at flight level 290 and above in airspace designated as Reduced Vertical Separation Minimum (RVSM) airspace and—

(i) On a magnetic course of zero degrees through 179 degrees, any odd flight level , at 2,000-foot intervals beginning at and including flight level 290 (such as flight level 290, 310, 330, 350, 370, 390, 410); or

(ii) On a magnetic course of 180 degrees through 359 degrees, any even flight level , at 2000-foot intervals beginning at and including flight level 300 (such as 300, 320, 340, 360, 380, 400).

How To Book Your Cruise And Flight Together

W hen a well-deserved vacation is in order, nearly 30 million passengers across the globe choose to cruise annually. Whether you're heading to the Caribbean or looking forward to a European tour, cruising is an appealing way to travel. A cruise provides passengers access to a variety of destinations. It's often seen as a comprehensive and convenient form of travel where onboard luxuries can be as inviting as the ports of call.

Travel logistics are typically taken care of onboard, but getting to the ship can be another story. Many travelers who book a cruise are tempted to book airline tickets to the ship's point of departure after the cruise reservation is already in place. However, you might be setting yourself up for unnecessary stress and expenses when you choose this approach to planning.

The Bureau of Transportation Statistics reported its highest flight cancellation rate in a decade in 2022. Delays were also on the rise, giving travelers every reason to assume that modern flying comes with plenty of unexpected hurdles. When you're heading toward a cruise departure point and are faced with delays or flight cancellations, it can be an expensive and inevitably disappointing experience.

Fortunately, there are routes to booking your cruise and flights together that don't require plotting out time-sensitive connections on your own. There's no reason to tackle uncertain flight planning when you can eliminate the stress from the process altogether. A better option is to book your cruise and flight simultaneously directly through the cruise line.

Book Through The Cruise Line And Enjoy Peace Of Mind

When a flight you've booked separately from your cruise reservation is canceled or delayed, there's a good chance you'll be responsible for booking an entirely new flight to meet your cruise at its first port of call. This is expensive and means missing a significant portion of your first sailing days. Your cruise is cut short and money that could have gone toward fun is now directed to additional flight costs.

That's what makes booking flights and a cruise together through the cruise line so reassuring. Booking airfare and cruises together is also a refreshingly simple process. Passengers just need to call up a cruise line representative directly to make their cruise reservations and receive flight options that suit their plans.

In many cases, cruise reservations that are made alongside flights through a cruise line come with some great guarantees that provide peace of mind. For example, many cruise lines will fly passengers to the first port of call if the flight they've booked through the cruise line is canceled or delayed. Cruise lines will typically cover the cost of a hotel in those situations too.

Many major cruise lines try to make combining flights and sailing experiences as smooth as possible through specialized reservation programs. Carnival operates Fly2Fun while Princess offers EZair booking options to passengers. If you're cruising with Holland America or Royal Caribbean, start your cruise and flight search at Flight Ease and Air2Sea respectively.

Consider The Benefits Of Travel Agent Services

Another convenient option when you're ready to book a cruise and flight together is to go through a travel agent . Much like the cruise lines, travel agents who specialize in cruises often have access to special fares and flights. This is largely due to meticulously negotiated contracts with both cruise companies and airlines alike.

Many cruise travel agents have taken time to experience various cruise routes for themselves. This gives them first-hand experience with how flight times and cruise departures match up. That makes them a wonderful resource when you're looking for a stress-free booking experience that's well planned out too.

A travel agent is also a great choice when getting to your cruise requires multiple flight connections. Your agent will handle the reservations to ensure you get where you're going easily and successfully. They can adjust itineraries on your behalf to make sure you have plenty of time between flights to reach your next gate and are available to contact should problems arise.

If you ultimately remain completely committed to the idea of booking your cruise and flights on your own, there are still options for making the process more streamlined. In many cases, the reservation ID for the cruise you've booked can be used to search for corresponding flights online. This will likely lead you back to the cruise line's flight program and is a good route to making sure you reach your departure destination on time.

Read this next: 18 Best Ways To Help You Get Through Airport Security Faster