Auxiliary and Accessory Systems
What this domain tests
This domain covers the APU, bleed air, anti-ice and accessory gearbox drives, and how they interact with engine operation.
Core rules
- The APU is a small turbine that drives a generator and bleed-air compressor. It provides electrical power, air conditioning and bleed air for engine starting. It has altitude and bleed limits.
- Bleed air is taken from the compressor for anti-ice, pressurisation and starting; using it reduces engine thrust and increases EGT.
- The accessory gearbox drives the fuel pump, oil pump, hydraulic pump, generator, starter and tachometers. A gearbox failure can affect several systems at once.
- Anti-ice uses hot bleed air on engine intakes and wings; it must be on before icing is encountered.
Worked example
An APU supplies 40 lb/min of bleed air. Each engine start needs 25 lb/min. Therefore only one engine can be started at a time; attempting both would starve the starter and cause a hung start. Reasoning: bleed air is a shared resource, and the APU's limit is the constraint, not the starter's capability.
Traps
- Using APU bleed above its altitude limit.
- Forgetting that engine anti-ice changes thrust and EGT.
- Assuming the accessory gearbox only drives one pump.
- Leaving anti-ice on in clear air; it wastes bleed air and fuel.
Key points
- The APU supplies electrical power, air conditioning and start bleed air.
- APU bleed has altitude and flow limits.
- Bleed air for anti-ice and pressurisation reduces thrust and raises EGT.
- The accessory gearbox drives fuel, oil and hydraulic pumps, the generator and starter.
- Anti-ice must be on before icing conditions.