Which is the typical sequence when starting a gas turbine with an air-start system?

• A. Lockout fuel; spool to maximum; light-off; stop air start.
• B. Start sequence; supply high-pressure air to the air turbine starter; accelerate to self-sustaining speed; introduce fuel for light-off; then transition to idle and stop the air start.
• C. Ignite fuel immediately; then supply air.
• D. Engage fuel valve; start rotation; delay air start.

Answer: (B)

Starting a gas turbine with an air-start system relies on first using high-pressure air to spin the air turbine starter and bring the generator/compressor up to a speed where ignition can be sustained. Fuel is not added until the rotor is rotating fast enough to support light-off. Once light-off occurs, the engine accelerates through to self-sustaining speed, at which point it can run on fuel and air without the starter. After reaching that self-sustaining point, the air start is stopped and the engine settles into idle.

This sequence ensures safe and reliable ignition, avoids compressor surge, and smoothly transitions from air-driven rotation to self-sustained operation.