ATPL Performance & Principles of Flight Questions

During a multi-engine take-off, what does V1 represent?

• The maximum speed at which the take-off can be rejected and stopped within the accelerate-stop distance
• The speed at which the aircraft first becomes airborne
• The minimum speed for the aircraft to safely rotate
• The speed at which flaps must be retracted

Explanation: V1 is the decision speed: the maximum speed at which a rejected take-off can be initiated and the aircraft brought to a stop within the calculated accelerate-stop distance. Above V1, the take-off must be continued even after an engine failure.

Performance - Take-off Speed Definitions

What is 'ground effect' and how does it generally influence an aircraft close to the surface?

• A reduction in induced drag and increase in effective lift when flying very close to the ground
• An increase in induced drag when flying close to the ground
• A phenomenon that only affects helicopters, not fixed-wing aircraft
• An effect that only occurs above the tropopause

Explanation: Ground effect occurs when an aircraft flies within roughly a wingspan of the surface: the ground restricts the formation of wingtip vortices and downwash, reducing induced drag and effectively increasing lift for a given angle of attack - which is why aircraft can 'float' during landing flare.

Performance - Ground Effect

All else being equal, how does higher density altitude generally affect takeoff performance?

• It increases the required takeoff distance and reduces the climb rate available
• It reduces the required takeoff distance and improves climb rate
• It has no measurable effect on takeoff performance
• It only affects landing performance, not takeoff

Explanation: Higher density altitude (from high elevation, high temperature, or low pressure) means the air is less dense, reducing both the lift generated at a given airspeed and the thrust or power an engine can produce - the combined effect is a longer takeoff roll and reduced climb performance.

Performance - Density Altitude Effects

How does the best angle-of-climb speed (Vx) generally compare with the best rate-of-climb speed (Vy)?

• Vx is generally lower than Vy
• Vx is generally higher than Vy
• Vx and Vy are always the same speed
• Vx applies only to descent, not climb

Explanation: Vx (best angle of climb) is generally flown slower than Vy (best rate of climb) - Vx maximises altitude gained per unit of horizontal distance covered, useful for clearing obstacles close ahead, while Vy maximises altitude gained per unit of time, useful for climbing to altitude efficiently once obstacles are no longer a concern.

Performance - Climb Speed Definitions

In a propeller aircraft, 'torque reaction' generally describes:

• A tendency for the aircraft to roll (and/or yaw) in the direction opposite to the propeller's rotation, as a reaction to the torque driving the propeller
• A tendency for the aircraft to pitch nose-up whenever power is reduced
• An effect that only appears at speeds above the aircraft's VNE
• A phenomenon unique to jet engines with no propeller equivalent

Explanation: By Newton's third law, the engine applies a torque to spin the propeller, and an equal and opposite reaction torque acts on the airframe - tending to roll the aircraft in the direction opposite to the propeller's rotation, an effect most noticeable at high power and low airspeed.

Performance - Propeller Torque Reaction

'Spiral instability' describes a tendency for an aircraft to:

• Continue gradually increasing its bank angle (and entering a slowly tightening descending turn) if a bank is left uncorrected
• Oscillate in pitch with a long, slow period after a disturbance
• Return immediately and precisely to wings-level after any disturbance, with no tendency to diverge
• Only affect aircraft at very high Mach numbers

Explanation: An aircraft with spiral instability, if left in a slight bank with no correction, will tend to gradually increase that bank angle over time (rather than returning to wings-level or holding steady), potentially developing into a slowly tightening, descending spiral if not corrected - distinct from the phugoid, which is an oscillation rather than a one-directional divergence.

Performance - Spiral Stability

Near the top of an aircraft's service ceiling, why can even a moderate bank angle risk triggering buffet or a stall that would not occur at the same speed in level flight?

• Banking increases load factor, which raises the effective stalling speed right at an altitude where the margin to the high-speed buffet limit is already small
• Banking always reduces load factor, which has no effect on buffet margins
• Bank angle has no relationship to load factor or buffet margins at any altitude
• Buffet margins only depend on outside air temperature, not on bank angle

Explanation: Since load factor - and therefore stalling/low-speed buffet speed - increases with bank angle, and the margin between low-speed and high-speed buffet is already narrow near the top of the service ceiling (the 'coffin corner' condition), even a moderate bank at high altitude can be enough to push the aircraft into buffet from either end of that narrowed speed range.

Performance - Coffin Corner & High-Altitude Buffet Margins

What two characteristics generally combine to produce a 'superstall' (deep stall), and which aircraft configuration is it historically associated with?

• A strong nose-up pitching tendency at the stall combined with an ineffective, blanked tailplane, historically associated with rear-engined, high-T-tail, swept-wing designs
• A strong nose-down pitching tendency combined with an unusually powerful tailplane, associated with low-wing, low-tail designs
• A superstall can only occur on straight-wing, propeller-driven aircraft
• A superstall is simply another name for a normal, fully recoverable stall on any aircraft type

Explanation: A superstall (or deep stall) combines a nose-up pitching tendency at the stall - often from wingtip stalling first on a swept wing, moving the centre of pressure inboard and forward - with a tailplane rendered ineffective because it sits in the low-energy, turbulent wake shed by the stalled wing and fuselage; this combination has historically been most associated with rear-engined, high-T-tail, swept-wing designs, where the high-set tailplane can become immersed in that disturbed wake at high angles of attack.

Performance - Superstall / Deep Stall