How does the starter motor disengage on a gas turbine engine?

• A. Manual by pilot switch
• B. Electrical cut-off when engine reaches speed
• C. Disengaged automatically by centrifugal force releasing the clutch
• D. Failsafe spring returns gear

Answer: (C)

When the engine starts, the starter drives the accessory gearbox through a clutch that is designed to release automatically. At low starting speeds, springs hold the clutch engaged so the starter can turn the engine. As the engine accelerates and its speed quickly surpasses the starter’s speed, centrifugal forces on the clutch weights cause the outer portion to slip relative to the inner portion, releasing the connection. This automatic disengagement prevents the running engine from driving the starter and allows the engine to take over spin-up without the starter being driven at high speeds. The cockpit switch simply provides power to the starter; the actual disengagement happens automatically because of the centrifugal clutch.