EASA ATPL — Principles of Flight (080) Exam Prep

Correct answer: B. A normal shock wave forms where the local airflow first reaches supersonic speed

The Critical Mach Number is the free-stream Mach number at which the local airflow over the wing first reaches Mach 1.0 (usually at the point of maximum thickness on the upper surface). As the aircraft accelerates beyond MCRIT, the region of supersonic flow expands and a normal shock wave forms at the aft boundary of this supersonic region. This shock wave causes a sharp increase in pressure and can trigger boundary layer separation (shock stall/buffet), increased drag (drag divergence), and pitch changes.

Correct answer: B. To delay the onset of compressibility effects (increase MCRIT) and reduce wave drag

Wing sweepback increases the Critical Mach Number (MCRIT) by effectively reducing the component of airflow velocity perpendicular to the leading edge. Only this perpendicular component determines when the local flow reaches Mach 1.0. A 30° swept wing effectively 'sees' an airflow component that is cos(30°) ≈ 0.87 of the free-stream velocity, allowing higher cruise Mach numbers before shock waves form. The trade-offs include reduced CLmax, tip stall tendency, and reduced low-speed performance.

Correct answer: C. 2.0 g

In a coordinated level turn, the load factor n = 1/cos(bank angle). At 60° bank: n = 1/cos(60°) = 1/0.5 = 2.0 g. This means the aircraft's wings must generate twice the lift compared to straight and level flight to maintain altitude. The stall speed increases by √n = √2 ≈ 1.41, so it increases by approximately 41%. This is why steep turns significantly increase stall speed.

Correct answer: B. An uncommanded nose-down pitch tendency caused by aft movement of the centre of pressure and shock-induced changes in downwash on the tailplane

Mach tuck is an uncommanded nose-down pitching tendency at high transonic speeds. It is caused by two main factors: (1) the centre of pressure moves aft as the shock wave pattern develops on the wing, creating a nose-down pitching moment, and (2) changes in the downwash pattern behind the wing (due to shock-induced flow changes) reduce the effective angle of attack of the tailplane. The Mach trimmer automatically compensates for this by adding nose-up stabiliser trim as Mach number increases.

Correct answer: B. A coupled yaw-roll oscillation where the aircraft alternately yaws and rolls, common in swept-wing aircraft with strong dihedral effect

Dutch roll is a coupled lateral-directional oscillation involving both yawing and rolling motions that are out of phase. It is caused by an imbalance between directional (weathercock) stability and lateral (dihedral) stability — specifically when dihedral effect is strong relative to directional stability. Swept-wing aircraft are particularly susceptible because sweep itself contributes strongly to effective dihedral. Yaw dampers are fitted to suppress Dutch roll in transport aircraft.

Correct answer: C. An adverse (decelerating) pressure gradient, surface roughness, and higher Reynolds number

Transition from laminar to turbulent boundary layer flow is promoted by: adverse (decelerating/positive) pressure gradients (which occur aft of the point of minimum pressure on the aerofoil), surface roughness (including rivets, bugs, ice), higher Reynolds numbers (higher speed or larger chord), and disturbances in the free stream. A favourable (accelerating) pressure gradient helps maintain laminar flow.