ATPL Aircraft General Knowledge common mistakes
ATPL Aircraft General Knowledge · Understand the traps and how to avoid them.
Treating the hydraulic system as a single common pressure source and assuming a pump failure drops system pressure everywhere, instead of tracking which consumers sit on which sub-system and what the isolating/check valves and priority valves do.
Candidates memorise 'system pressure 3000 psi' and pump types but never build a mental schematic, so they cannot answer questions about a leak downstream of a check valve, a failed engine-driven pump on a twin, or why the gear still lowers on the emergency accumulator.
What to do instead
For each aircraft system you study, draw one page: reservoir, filters, EDPs, electric pump, check valves, priority/relief valves, accumulators, and every consumer with its sub-system letter. Then rehearse three failure scenarios per drawing (pump loss, leak, accumulator depletion) and state exactly which services are lost and in what order.
Confusing the roles of the cabin pressure controller, the outflow valve and the safety valves, and answering pressurisation questions as if the controller directly sets cabin altitude rather than modulating the outflow valve.
The components are described in the same paragraph of most manuals, and candidates learn 'controller controls pressure' as a slogan without a closed-loop picture, so they invert cause and effect in questions about valve position, cabin rate, and the effect of a stuck-open or stuck-closed valve.
What to do instead
Write the loop explicitly: controller compares selected cabin altitude/rate with actual, drives the outflow valve actuator, valve position sets the mass outflow, inflow from engine bleed or turbocharger sets the other side. Then answer practice questions by asking 'what does the outflow valve do next?'. Also separate the negative-pressure relief and positive-pressure safety valves from the controller in your notes.
Mixing up the protection logic in ice and rain protection: which surfaces are anti-iced versus de-iced, whether the timer cycles heat on or off, and what happens to the system on the ground versus in flight.
Candidates learn 'thermal anti-ice' as one block and never distinguish the continuously heated leading edge from the cycled de-ice boots or the cycling of windshield heat, so they cannot answer questions about power demand, holdover time, or the consequence of a failed timer.
What to do instead
Build a table with columns: surface, method (bleed air, electrical, chemical), continuous or cycled, controlled by what, ground inhibition, and failure indication. Fill it for each type you study and state the failure consequence in words ('boots stay inflated' or 'heat stays on and overheats').
Answering fire detection questions by naming the detector type without linking it to the zone it protects, and confusing the meaning of a dual-loop fault with an actual fire warning.
Engine fire loops, cargo smoke detectors and toilet waste-bin detectors are taught as a list, and candidates do not learn the AND/OR logic of twin-loop systems, so they misread questions about a single-loop failure producing a fault versus a fire indication.
What to do instead
For each zone, note detector technology, how many loops, and the logic (both loops must agree for a fire warning, one loop failure gives a fault). Then practise interpreting described indications: which light, which aural warning, which extinguisher bottle discharges and whether discharge is guarded by a second action.
Treating AC and DC generation as interchangeable and failing questions on paralleling, bus ties, reverse current, and what is lost when a generator trips, because the candidate never learned which buses are essential.
Basic electrics is studied as physics (Ohm's law, series/parallel) and the aircraft distribution side is skimmed, so candidates cannot trace power from generator to essential bus or explain why a battery is on the essential DC bus and what it can carry.
What to do instead
Draw a distribution diagram per aircraft: generators, APU generator, external power, TRUs, battery, bus ties, essential buses. Mark which loads are on which bus and what sheds automatically on loss of a generator. Learn the protection terms (over/under voltage, overfrequency, reverse current, differential fault) with the relay action each causes.
Applying piston-engine mixture logic to a turbine engine, or vice versa, and answering fuel-system questions with the wrong engine model in mind.
The syllabus covers both engine types in one domain and candidates build a single blended model, so they misanswer questions on fuel/air ratio control, carburettor icing versus engine anti-ice, and what the fuel control unit actually schedules.
What to do instead
Keep two separate one-page summaries. Piston: carburettor or fuel injection, mixture control, float chamber, carb heat, detonation and pre-ignition. Turbine: fuel control unit or FADEC, hydro-mechanical versus electronic scheduling, fuel manifold and nozzles, LP/HP fuel cocks. For each, list what the pilot controls directly and what is automatic.
Confusing propeller blade angle, pitch and angle of attack, and getting the constant-speed unit logic backwards, especially the effect of an engine failure or oil pressure loss on blade angle.
The terms are used loosely in study material and candidates memorise 'fine pitch for take-off' without understanding that the governor moves oil to change blade angle, so they cannot reason about overspeed, feathering, or a counterweight versus oil-pressure-to-fine design.
What to do instead
Write the chain: governor senses RPM, ports oil, piston moves, blade angle changes, torque and drag change, RPM returns. Then state for each design (oil to fine, oil to coarse, counterweighted) what happens on oil pressure loss and on feathering. Draw the blade angle versus RPM graph and label fine, coarse, feather and reverse.
Answering flight-control questions about control surface effectiveness, hinge moment and balance without distinguishing aerodynamic balance from mass balance, and misjudging what happens with a jammed or disconnected surface.
Candidates learn surface names and the three axes but not the design features behind them, so questions about flutter prevention, control feel, or the effect of a failed autopilot servo on control authority are answered by guesswork.
What to do instead
For each primary and secondary control, note the axis, the balance method (horn, inset hinge, mass balance), the feel or artificial-feel device, and the failure case. Practise explaining in one sentence why mass balance prevents flutter and why aerodynamic balance reduces stick force.
Mixing up landing gear sequencing details: when the gear warning sounds, what the squat switch inhibits, and whether the emergency extension is mechanical, pneumatic or gravity, and confusing anti-skid with auto-brake.
Gear systems are taught as a list of components, and candidates do not connect the logic (weight-on-wheels, throttle position, flap position) to the warnings and inhibitions, so they misanswer questions about a gear-up landing warning or a gear retraction inhibited on the ground.
What to do instead
Draw the sequence as a timeline: selection, unlocking, retraction, up-lock, and the reverse for extension, marking what the WOW switch inhibits at each stage. Separately tabulate the warning triggers (throttle at idle, flaps beyond a setting, radio altimeter) and the emergency extension method with its number of pilot actions.
Answering structural and airframe questions by recalling a definition rather than identifying the load path, so questions on fail-safe versus safe-life design, stress corrosion, or the purpose of a spar web are answered from memory of a phrase.
Structures is treated as vocabulary and candidates never picture how load travels from skin through stringers and ribs to spars and frames, so they cannot reason about damage tolerance, fatigue, or why a particular inspection is required.
What to do instead
Take one wing and one fuselage and trace the load path in writing from aerodynamic pressure to the attach fittings. Then classify each element as primary or secondary structure and state whether the design philosophy is safe-life, fail-safe or damage-tolerant, with the inspection implication. Answer questions by asking 'where does this load go next?'.