Electro-mechanical fuel control:

The start fuel valve and ignition are energised as soon as rotation (3%) is sensed by an Electronic Sequence Unit (ESU). At 14% and with rising EGT the main fuel valve is opened. The acceleration rate is controlled by the acceleration schedule adjuster, however this is modified by the differential pressure regulator which uses compressor discharge pressure to vary the fuel flow to the engine. At 50% the starter cuts out. When the engine reaches 85% the start fuel valve closes and the ignition is de-energised. The engine governor then takes over and controls the engine to 100%. As the engine passes 95% plus 3 seconds, the max fuel valve energises open and bypasses the acceleration adjuster and full control of the engine is given to the governor. If the engine is shut down both the Main and Max fuel valves are closed.

The ESU has indicators that indicate which step of the start sequence the APU is at and the resets at 95% + 3sec to act as BITE indicators.