Chapter 25: Engine Starting and Auxiliary Power Units (APU)
25.1 The Engine Start Sequence
- To start a gas turbine engine, three things are required:
- 1. Rotation: The compressor must be turned by a starter motor to get air flowing through the
- engine.
- 2. Fuel: Fuel must be sprayed into the combustion chambers.
- 3. Ignition: A high-energy spark from an igniter plug must ignite the fuel/air mixture.
- The pilot monitors the EGT and the high-pressure compressor RPM (N2) during the start
- sequence.
- 1. The starter is engaged, and N2 begins to rise.
- 2. At a specific N2 (e.g., 25%), the pilot introduces fuel and ignition.
- 3. A light-up should occur, indicated by a rapid rise in EGT.
- 4. The engine continues to accelerate. At self-sustaining speed, the turbine is producing enough
- power to drive the compressor without the starter's help.
- 5. The starter cuts out, and the engine stabilizes at idle RPM (e.g., ~60% N2).
25.2 Abnormal Starts
- Hot Start: The EGT exceeds its maximum permitted limit. This is usually caused by too
- much fuel for the available airflow. The start must be aborted immediately.
- Wet Start: The fuel/air mixture fails to ignite. EGT does not rise, but fuel flows into the
- engine. The engine must be motored over (a blowout cycle) to clear the unburnt fuel before
- another start is attempted to prevent torching.
- Hung Start: The engine lights up but fails to accelerate to idle RPM, instead stabilizing at a
- lower speed with a rising EGT. This is often caused by insufficient power from the starter.
25.3 Auxiliary Power Unit (APU)
- An APU is a small, self-contained gas turbine engine, typically located in the aircraft's tail
- cone. Its purpose is to provide electrical power and pneumatic (bleed) air for the aircraft's
- systems on the ground, eliminating the need for ground support equipment. It can also serve
- as a backup source of power in flight.
25.4 Ram Air Turbine (RAT)
- A RAT is a small turbine that can be deployed from the fuselage or wing into the airstream in
- the event of a total engine failure. The airflow spins the turbine, which powers an emergency
- hydraulic pump or electrical generator to provide power for essential flight controls and
- instruments.