Aircraft General Knowledge: Electrics, Electronics and Instrumentation
This domain combines electrical power generation and distribution with the instruments that present flight data. Expect questions on DC/AC systems, busbars, pitot-static instruments, gyroscopes and EFIS.
Electrical power
- Light aircraft use 28 V DC; transport aircraft use 115 V AC at 400 Hz, with transformer-rectifier units (TRUs) producing 28 V DC.
- Generators/alternators feed busbars through generator control units (GCUs); bus ties allow one source to supply both sides.
- Batteries provide emergency DC power and start power; NiCd batteries need careful thermal monitoring.
Instrumentation
- Pitot pressure feeds the airspeed indicator; static pressure feeds the altimeter, VSI and ASI.
- A blocked pitot tube causes the ASI to behave like an altimeter; a blocked static port causes errors in all three pressure instruments.
- Gyroscopes use rigidity in space (attitude/heading) and precession (turn coordinator).
- EFIS replaces mechanical displays but the same pitot-static and gyro inputs remain.
Worked example
A 28 V DC bus supplies 100 A. Power = V × I = 28 × 100 = 2,800 W, or 2.8 kW. A 9 kW generator has ample capacity, but a 3 kW generator would be overloaded and the GCU would trip or shed loads.
Traps that cost marks
- Thinking AC and DC busbars are interchangeable without a TRU.
- Forgetting that static blockage affects the VSI and altimeter as well as the ASI.
- Confusing generator trip with busbar failure; the bus tie may still power the bus.