Principles of Flight: 7 Latest ATPL Questions Explained

Many Principles of Flight questions test whether you truly understand the aerodynamic principles behind them. That's why seemingly straightforward questions about Mach number, stability or lift distribution often catch students out.

In this June update, we break down seven recently reported Principles of Flight questions seen across EASA ATPL exams. This walkthrough covers a range of high-frequency POF topics, including critical Mach effects, wing planform design, asymmetric flight after engine failure, parasite drag, longitudinal stability, and high-altitude performance. These are concepts that regularly appear in the exam and are much easier to remember once you understand the aerodynamic reasoning behind them.

Prefer to watch instead? You can follow the complete walkthrough on the Airhead YouTube channel, where airline pilot Michal explains every question step by step using practical examples and visual illustrations.

Subscribe to the AirheadATPL YouTube channel and hit the notification bell for weekly ATPL revision videos. Each video helps you tackle the latest challenging questions from all 13 ATPL subjects, straight from recent EASA ATPL exams.

7 Latest Principles of Flight Questions Covered in This Blog

• AIR-251927: Critical Mach Effects — Flying Near Mcrit in a Cold Air Mass

• AIR-253333: Wing Planform Design — Lift Distribution at the Wing Root

• AIR-253252: Asymmetric Flight — Fin Angle of Attack After Critical Engine Failure

• AIR-252253: Aerofoil Contamination — Drag Effects of Dirt and Surface Roughness

• AIR-280490: Static Longitudinal Stability — The Wing's Contribution

• AIR-277325: Critical Mach Number — Definition and Aerodynamic Significance

• AIR-277684: High-Altitude Performance — Climb Above the Tropopause at Constant Mach

Unlock the secrets to success with these proven student strategies and question bank tips. 

Question 1: Critical Mach Effects in a Cold Air Mass

Question ID AIR-251927: You are in cruise flight and maintaining a constant pressure altitude (flight level) and IAS, which keeps you flying close to Mcrit. You suddenly encounter a cold air mass. What is most probable to happen?

1. Mach speed stays constant.

2. Mach speed increases or decreases, depending on the exact temperature of the cold air mass.

3. Mach speed decreases, and you could stall if you do not intervene.

4. Mach speed increases

Correct Answer: Mach speed stays constant.

Explanation

This is one of those questions where the wording can easily lead you in the wrong direction. The examiner is not testing your understanding of aircraft performance in changing weather; they are testing the relationship between IAS and Mach number.

At first glance, it seems logical that entering colder air would change the Mach number because the speed of sound decreases as the temperature falls. However, the aircraft is maintaining the same IAS and flight level, so the immediate effect is different.

Although the lower temperature reduces the True Airspeed, it also reduces the local speed of sound by almost the same proportion. Since Mach number is simply the ratio between True Airspeed and the local speed of sound, the two effects cancel each other out.

For example:

• At FL380, 250 KIAS and an OAT of -40°C:

  • TAS ≈ 470 kt

  • Mach ≈ 0.79

• If the aircraft suddenly enters air at -60°C:

  • TAS decreases to approximately 449 kt

  • The local speed of sound also decreases

  • Mach remains approximately 0.79

In reality, the aircraft would eventually require small adjustments in altitude and thrust as the colder air affects its performance. However, the word "suddenly" tells you to ignore these secondary effects and focus only on the immediate relationship between IAS and Mach.

Exam Tip

The examiner is testing the IAS–Mach relationship, not aircraft performance. Temperature changes both TAS and the local speed of sound. Because both change together, the Mach number initially remains unchanged.

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Question 2: Lift Distribution at the Wing Root

Question ID AIR-253333: Assuming zero wing twist, the wing planform that gives the highest local lift coefficient at the wing root is:

1. Rectangular

2. Tapered.

3. Elliptical.

4. Swept-back

Correct Answer: Rectangular

Explanation

This question tests your understanding of the difference between lift distribution and local lift coefficient. Two concepts that students often confuse.

A rectangular wing produces the highest local lift coefficient near the wing root. As you move towards the wing tips, the lift coefficient decreases rapidly.

By contrast, an elliptical wing is designed to produce the most efficient lift distribution across the entire span, giving a much more even loading and reducing induced drag.

The examiner's trap is that many students immediately associate the elliptical wing with "best lift" and choose it automatically. However, the question is not asking about the overall lift distribution or aerodynamic efficiency. It asks specifically where the highest local lift coefficient occurs.

Exam Tip

Read the wording carefully.

• Local lift coefficient → Rectangular wing.

• Overall lift distribution and efficiency → Elliptical wing.

Those are not the same thing.

Principles of Flight: 6 Latest ATPL Questions Explained (previous update)

Question 3: Critical Engine Failure and Fin Angle of Attack

Question ID  AIR-253252: Flying a twin engine aircraft with the critical engine failed, the fin angle of attack will (1) _____ when banking slightly towards the (2) _____ engine.

•  (1) remain the same; (2) inoperative.

•  (1) increase; (2) operative.

•  (1) decrease; (2) operative.

•  (1) decrease; (2) inoperative.

Correct Answer:  (1) decrease; (2) operative.

Explanation

After the failure of the critical engine, the remaining engine produces asymmetric thrust that causes the aircraft to yaw towards the failed engine. If the aircraft is kept wings level, it naturally enters a sideslip, with the airflow striking the fin and rudder from the side.

Although the rudder can counteract this yaw, the sideslip creates additional drag and places a high aerodynamic load on the vertical stabiliser. This reduces climb performance and requires continuous rudder input from the pilot.

To minimise these effects, pilots use the well-known "5° towards the live engine" technique. A slight bank (typically between 2° and 5°) towards the operative engine reduces the sideslip angle. As the sideslip decreases, the airflow meets the fin more directly, reducing its angle of attack.

The result is lower drag, improved climb performance, and less rudder force required to maintain directional control. Banking beyond approximately 5° is not beneficial, however, as it begins to reduce overall lift and increase drag again.

Exam Tip

Remember the phrase: "Five to the live." A slight bank towards the operative engine reduces sideslip, decreases the fin angle of attack, lowers drag, and improves one-engine-inoperative climb performance.

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Question 4: How Aerofoil Contamination Increases Drag

Question ID AIR-252253: Compared to a cleaned aerofoil, an aerofoil which has dirt and other particles increases what type of drag?

1. Parasite Drag

2. Induced Drag

3. Wave Drag

4. Material Drag

Correct Answer: Parasite Drag

Explanation

A clean aerofoil is designed to keep airflow as smooth and attached as possible. Dirt, insects, dust, ice, or other surface contamination increases the roughness of the wing, disturbing the smooth airflow over the surface.

This increases skin friction drag, which is one component of parasite drag. Unlike induced drag (which is associated with lift production), parasite drag results from the aircraft moving through the air and generally increases with airspeed.

Although the effect of small amounts of contamination may seem insignificant, it can noticeably reduce aircraft performance over time. Increased parasite drag leads to higher fuel consumption, lower cruise efficiency and, in some cases, degraded climb performance. This is one reason why airlines invest significant effort in keeping aircraft surfaces clean and free from contamination.

The examiner's trap is the reference to "friction drag". While this is technically correct, friction drag is simply one form of parasite drag. Since the question asks for the broader category, Parasite Drag is the correct answer.

Exam Tip

Remember the hierarchy:

• Skin friction drag → part of Parasite Drag

• Induced Drag → created by lift

• Wave Drag → associated with transonic and supersonic flight

If contamination makes the surface rougher, think Parasite Drag.

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Question 5: Static Longitudinal Stability Explained

Question ID AIR-280490: The contribution of the wing to the static longitudinal stability of an aeroplane:

1. Is always negative

2. Depends on the wing location relative to the fuselage. 

3. Is positive if the wing has a positive cambered aerofoil section and the aerodynamic centre is ahead of the CG

4. Depends on CG location relative to the wing aerodynamic centre. 

Correct Answer: Depends on CG location relative to the wing aerodynamic centre. 

Explanation

Static longitudinal stability is the aircraft's natural tendency to return to its original pitch attitude after a small disturbance. Whether the wing contributes positively or negatively to this stability depends primarily on the position of the centre of gravity (CG) relative to the wing's aerodynamic centre.

If the CG is located ahead of the aerodynamic centre, a nose-up disturbance creates a restoring pitching moment that tends to return the aircraft to its original attitude. This provides positive longitudinal stability. If the CG moves behind the aerodynamic centre, the restoring moment is reduced or may even reverse, making the aircraft less stable or unstable.

Although the wing contributes to longitudinal stability, it is the horizontal stabiliser that provides the greatest stabilising effect. The tailplane generates the restoring moment required to balance the aircraft, and its effectiveness depends on both its size and its distance from the centre of gravity.

The examiner's trap is that several options appear technically plausible. Wing position, aerofoil shape and tailplane design all influence stability to some extent, but the question asks specifically what determines the wing's contribution. The decisive factor is the position of the centre of gravity relative to the aerodynamic centre.

Exam Tip

Think of longitudinal stability in two steps:

• The wing creates the pitching moment.

• The tailplane provides the restoring force.

For exam questions, always check where the CG sits relative to the aerodynamic centre; this is the key to determining static longitudinal stability.

Question 6: What Is the Critical Mach Number?

Question ID AIR-277325: The critical Mach number of an aeroplane is the free-stream Mach number at which, for the first time, somewhere on the aeroplane, the following occurs:

1. Local sonic flow

2. Buffet

3. A shock wave

4. Supersonic flow

Correct Answer: local sonic flow

Explanation

The Critical Mach Number (Mcrit) is the free-stream Mach number at which the airflow first reaches Mach 1 somewhere over the aircraft, even though the aircraft itself is still flying below the speed of sound.

This happens because airflow does not travel at the same speed everywhere around the aircraft. As air passes over the wings and fuselage, it accelerates and decelerates depending on the shape of the aerofoil. At a certain flight speed, the airflow over a small part of the wing accelerates enough to reach the speed of sound before the aircraft itself does.

This marks the critical Mach number.

An important point is that Mcrit does not mean the aircraft is flying at Mach 1. It simply means that a small region of airflow has become sonic for the first time.

As speed increases beyond Mcrit, small areas of supersonic airflow begin to develop, followed by the formation of shock waves. These shock waves can eventually cause airflow separation, buffet, increased drag and a loss of control effectiveness.

The examiner's trap is the sequence of events. Many students select shock wave or supersonic flow because these are closely associated with high-speed flight. However, the question asks what occurs first.

The correct sequence is: Local sonic flow → Supersonic airflow → Shock wave → Buffet

Exam Tip

Remember the progression: Mcrit = the first appearance of local Mach 1.

Shock waves and buffeting occur after the critical Mach number has been exceeded.

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Question 7: Constant Mach Climbs Above the Tropopause

Question ID AIR-277684: Assuming ISA conditions and a climb above the tropopause at constant Mach number and aeroplane mass, the:

1. Lift coefficient decreases

2. Lift coefficient remains constant

3. TAS increases

4. IAS decreases 

Correct Answer: IAS decreases 

Explanation

This question combines several performance concepts and is often answered incorrectly because students confuse the behaviour of Mach number, True Airspeed (TAS) and Indicated Airspeed (IAS).

Above the tropopause, the outside air temperature remains essentially constant. Since the speed of sound depends only on temperature, it also remains constant.

If the aircraft maintains a constant Mach number, then its True Airspeed also remains constant, because Mach is simply the ratio between TAS and the local speed of sound.

The key change during the climb is the air density.

As altitude increases, air density decreases. IAS is not a measure of actual speed through the air—it is a measure of dynamic pressure. With the same TAS but lower air density, the dynamic pressure becomes smaller, so the indicated airspeed gradually decreases.

This is the principle the examiner is testing.

Many students assume that climbing always means a higher TAS or a changing Mach number. However, once you're above the tropopause and maintaining a constant Mach, both the speed of sound and TAS remain essentially unchanged. The only value that decreases is IAS.

Exam Tip

Above the tropopause:

• Temperature → constant

• Speed of sound → constant

• Constant Mach → constant TAS

• Increasing altitude → lower air density → lower IAS

Think of IAS as pressure, not speed.

The Airhead ATPL question bank is a great way to practise these scenarios, since you’ll see similar logic tested in real EASA exams. 

Next step: Open your Airhead ATPL question bank and practise Principles of Flight questions.