15.12 and 15.13 Engine air systems, starting and ignition

Scope

• 15.12 Air systems — A 1, B1 2: engine air distribution and anti-icing control, including internal cooling and sealing and external air services.
• 15.13 Starting and ignition — A 1, B1 2: start systems and components; ignition systems and components; maintenance safety requirements.

Internal air system

A significant fraction of compressor air never passes through the combustor; it is used inside the engine:

Function	How
Turbine cooling	HP compressor air (often from the combustor casing) flows through NGVs, turbine discs and blades (convection, impingement and film cooling)
Disc cooling / rim sealing	Air purges the cavities between discs to keep hot gas out and control disc temperature
Bearing chamber sealing	Air pressurises labyrinth seals so oil cannot leak out
Thrust balance	Air pressure in balance chambers offsets part of the rotor axial load, reducing the load on the location (ball) bearing
Active clearance control (ACC)	Fan air (or compressor air) sprayed on turbine (and some compressor) casings via FADEC-controlled valves shrinks the casing in cruise, reducing blade tip clearance and improving efficiency; turned off at take-off/acceleration to avoid tip rubs
Accessory cooling	Cooling of the gearbox, generator, EEC, ignition units by fan air

Anti-icing control

• Engine (cowl) anti-ice: HP/IP compressor bleed air through a pressure-regulating shut-off valve (FADEC or electrically controlled) to the nose cowl lip; valve position indicated in the flight deck; the FADEC raises the minimum idle when anti-ice is on to ensure enough bleed pressure.
• IGVs/nose cone may be heated by bleed air or oil.
• Anti-ice use reduces thrust margin and raises EGT; the FADEC compensates the thrust setting.