Aircraft General Knowledge – Systems and Powerplant
Aircraft General Knowledge – Systems and Powerplant
This domain covers piston and turbine powerplants, fuel, oil, ignition, electrical, hydraulic, pneumatic and fire-protection systems. The CPL exam tests normal operation, limitations and failure management, with emphasis on the piston engine and constant-speed propeller used in typical training aircraft.
Core rules
- In a four-stroke engine, the sequence is intake, compression, combustion and exhaust. The mixture must be leaned as density altitude increases; over-rich mixture causes rough running, fouling and reduced power, while over-lean mixture raises cylinder temperature and can cause detonation.
- Detonation is uncontrolled explosive combustion after normal ignition; pre-ignition is ignition before the spark. Both demand reducing power, enriching the mixture, lowering temperature and landing for inspection if severe.
- Magneto ignition is independent of the electrical system. A magneto check confirms both magnetos and the dead-cutter; a large RPM drop or rough running is a reason not to fly.
- Constant-speed propellers maintain selected RPM through a governor. Oil pressure drives the pitch change; loss of oil pressure usually drives the blades to fine pitch or high RPM, depending on design.
- Fuel management includes sumping, checking quantity and type, and monitoring consumption. Electrical and hydraulic systems require checking alternator load, circuit breakers and pressure limits.
Worked example
A 180 hp engine burns fuel at a brake-specific fuel consumption of 0.50 lb/hp/h. Fuel flow is 180 × 0.50 = 90 lb/h. Aviation gasoline weighs about 6 lb/US gal, so flow is 90 ÷ 6 = 15 US gal/h. For a 2.5 h flight with 45 min reserve, fuel needed is 15 × (2.5 + 0.75) = 48.75 US gal, before taxi and contingency.
Traps
- Leaning only on the ground and not at altitude.
- Treating a magneto drop as normal if the engine still runs.
- Using car fuel or wrong grade.
- Forgetting that carburettor ice can occur at outside temperatures well above 0°C.