Auxiliary Systems and Power Off-takes
Domain 9 - Auxiliary Systems and Power Off-takes
The exam tests accessory gearbox drives, bleed air systems and the performance cost of off-takes.
Core rules
- The accessory gearbox is driven from the HP spool and drives the fuel pump, oil pumps, hydraulic pumps, IDG or CSD, starter/generator and sometimes the APU. It is a single point of failure with torque limits and shear sections.
- Power off-takes are bleed air for cabin pressurisation and anti-ice, and shaft power for generators and hydraulic pumps. Every off-take costs thrust or raises fuel flow. The FADEC schedules N1 or EPR to hold thrust, so EGT rises.
- Anti-ice uses hot bleed on the cowl and inlet. It must be selected before icing is encountered, and it reduces surge margin at low power.
- IDG cooling uses fuel/oil heat exchange. An IDG fault can appear as oil temperature or generator frequency problems.
- The APU supplies bleed for engine start and ground electrical power, with its own load compressor and gearbox limits.
Worked example
With engine anti-ice on at takeoff, EGT rises 30 °C and N1 is unchanged because EPR is being controlled. Bleed air is taken from the HP compressor, so the FADEC increases fuel to hold EPR, and the extra fuel shows as EGT. Takeoff performance must be calculated from the anti-ice-on chart, not the baseline chart.
Traps
- Assuming bleed air is free because N1 has not changed.
- Selecting anti-ice only after ice has already formed.
- Ignoring the gearbox shear section when a driven pump seizes.