EASA ATPL 081 Principles of Flight common mistakes
EASA ATPL 081 Principles of Flight · Understand the traps and how to avoid them.
Treating lift as caused by 'air travelling faster over the top so it must arrive at the trailing edge at the same time', then using that to justify equal-transit-time claims in exam items.
This is the most widely taught school-level explanation, so candidates accept it without checking it against the continuity and pressure-distribution reasoning the syllabus actually uses; it also produces wrong answers on questions about circulation, downwash and the origin of induced drag.
What to do instead
Re-derive lift from the pressure distribution around the aerofoil and from Newton's third law applied to downwash. Practise stating in one sentence why the equal-transit-time argument is invalid, and rehearse the roles of the stagnation point, the leading-edge suction peak and the adverse pressure gradient on the upper surface.
Confusing the boundary-layer behaviour at the transition point and the separation point, and predicting separation where the flow is merely turbulent.
Candidates memorise 'laminar then turbulent' as a sequence without linking it to the pressure gradient; they then assume a turbulent boundary layer separates more readily, when the opposite is true because of the higher near-wall momentum.
What to do instead
Learn the chain: favourable gradient, laminar layer, transition, turbulent layer, adverse gradient, separation. Practise explaining why vortex generators and turbulators delay separation, and why a forward movement of the transition point can delay rather than hasten the stall.
Applying the stall angle of attack as a fixed number regardless of flap setting, sweep, Mach number or contamination.
The critical angle is taught as roughly 15 degrees, so candidates treat it as a constant; exam items that vary configuration or surface condition then catch them out, especially on swept wings where the effective normal-section angle differs from the aircraft angle.
What to do instead
Build a table of what changes the stall angle and what changes the stalling speed: flaps and slats increase CLmax and lower the stall speed; sweep raises the stall speed and gives tip-first stall tendencies; ice and frost reduce CLmax sharply. For swept wings, always distinguish the wing angle of attack from the section angle normal to the leading edge.
Mixing up the roles of dihedral, sweep and fin in lateral and directional static stability, and answering as if dihedral produces a yawing moment.
The three effects are taught together as 'lateral stability', and candidates memorise keywords without tracing the moment each produces; sideslip is the common input, so the outputs get swapped.
What to do instead
For each of dihedral, sweep and fin, write out: input (sideslip), intermediate effect (change in local angle of attack or side force), output moment (roll or yaw), and sign. Then test yourself with the wing-low, ball-out and skid cases, and with why anhedral is used on some high-wing and fighter designs.
Treating load factor in a level turn as n = 1/cos(bank) but then applying the same formula to a pull-up, a push-over or a steady climb.
Only the level-turn case is drilled, so candidates use it as a general rule; they also forget that a steady climb at constant angle does not by itself increase load factor, while a pull-up from a dive does.
What to do instead
Learn the general relation n = L/W and evaluate L from the manoeuvre geometry each time. Practise the level turn, the balanced pull-up (n increases with speed squared over radius), the push-over (negative n) and the constant-angle climb (n approximately 1). Cross-check stalling speed in the turn as Vs times the square root of n.
Assuming positive static stability guarantees acceptable dynamic behaviour, and describing phugoid and short-period modes with their characteristics reversed.
Static and dynamic stability are studied as one topic, and candidates collapse them into 'stable or unstable'; the two longitudinal modes are then remembered by name only, without their period and damping characteristics.
What to do instead
Separate the concepts: static stability concerns the initial tendency, dynamic stability the subsequent motion over time. Learn that the short-period mode is fast and heavily damped and the phugoid is slow with nearly constant angle of attack, and be able to say which is more critical for handling and why.
Describing flap and slat effects only as 'more lift', without distinguishing the CLmax increase from the change in stall angle and the drag penalty, and confusing spoiler and speedbrake functions.
High-lift devices are summarised in a single line in many study notes; candidates then cannot answer items that compare a plain flap with a slotted flap or that ask what a spoiler does to lift and to pitch.
What to do instead
Tabulate each device against CLmax change, stall-angle change, drag change and pitching moment: plain, split, slotted and Fowler flaps; leading-edge flaps, slats and slots; spoilers and speedbrakes. Note that spoilers reduce lift and increase drag and are used for roll control, descent and ground lift dumping.
Confusing the critical Mach number with the drag-divergence Mach number, and treating Mach buffet as identical to the low-speed stall buffet.
Both are 'the speed at which things go wrong near Mmo', so candidates merge them; they also miss that the local speed over the wing reaches Mach 1 before the aircraft does, which is the whole point of the critical Mach number.
What to do instead
Fix the definitions: Mcrit is where local supersonic flow first appears; Mdd is where drag rise becomes rapid; Mmo is the operating limit. Learn the sequence of shock formation, shock-induced separation, buffet and Mach trim actuation, and how sweep and thin sections raise Mcrit.
Answering propeller questions with the geometric blade angle when the question concerns the angle of attack of the blade element, and mis-stating the asymmetric blade effect.
Blade angle, helix angle and angle of attack are used interchangeably in casual reading; the asymmetric (P-factor) effect is then attached to the wrong phase of flight or the wrong direction of yaw.
What to do instead
Draw the blade-element velocity triangle for a given rpm and TAS and read off the angle of attack. Learn that a constant-speed unit holds rpm by changing blade angle as TAS changes, that the descending blade takes the greater bite at high angle of attack, and hence the yaw direction on the take-off run.
Treating V-speeds as fixed numbers independent of mass, altitude and configuration, and applying wake-turbulence and windshear rules as memorised phrases.
V-speeds are listed as constants in the aircraft documents, so candidates forget which are mass-dependent (stalling and manoeuvring speeds) and which are structural or limiting; wake and windshear items then get answered by keyword rather than by the underlying mechanism.
What to do instead
Classify each V-speed as mass-dependent, configuration-dependent or fixed limit, and know the relation Vs1g to the square root of mass. For wake turbulence, reason from vortex generation and decay relative to the generating aircraft's flight path; for windshear, reason from the change in headwind or tailwind component and its effect on performance and on the indicated airspeed.