The Gas Turbine Engine

Chapter 25: Engine Starting and Auxiliary Power Units (APU)

Technical General for Aviators — Capt. Pankaj Pahil

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).

 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.

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.

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.