Airframe, systems and powerplant
This domain tests how the airframe carries flight and ground loads and how the engine turns fuel into thrust. PPL questions are practical: identify a spar from a rib, explain why a magneto is independent of the battery, or recognise carburettor icing before it costs you the engine.
Airframe
Most light trainers are semi-monocoque: the skin carries shear and some bending, stringers and frames stiffen it, and the main spar carries bending. Ribs hold the aerofoil shape. Certificated types may be fail-safe (damage is detectable before it becomes critical) or safe-life (a fixed life). Composite structures (GRP/CFRP) have excellent fatigue resistance but can hide delamination and impact damage. Control surfaces are mass-balanced to prevent flutter; free play and incorrect rebalancing are serious.
Powerplant
A four-stroke horizontally opposed engine produces one power stroke per cylinder per two revolutions: induction, compression, combustion, exhaust. The carburettor uses a venturi; mixture leans with altitude. Carburettor icing can occur at outside air temperatures roughly between -5 °C and +20 °C with high humidity, and carb heat enriches and warms the charge, causing an RPM drop. Magnetos are engine-driven and independent of the electrical system, so a total electrical failure does not stop the engine. A fixed-pitch propeller couples RPM directly to power; manifold pressure is the relevant gauge on a constant-speed type.
Worked example
Bore 120 mm, stroke 100 mm, four cylinders. One cylinder swept volume = π/4 × 12² × 10 = 1,131 cm³. Four cylinders = 4,524 cm³, about 4.5 litres.
Traps
Assuming the engine stops when the alternator fails; treating carb heat as a winter-only control; confusing spar and rib function; ignoring internal composite damage because the surface looks intact.