At what percentage does the speed relay disengage the electric starter in a gas turbine engine?

The disengagement of the electric starter in a gas turbine engine is typically set at a threshold speed to ensure that the engine has reached a sufficient operational speed where the combustion process can be maintained independently of the starter. The correct answer, which states that the speed relay disengages the electric starter at 95% of the engine's rated speed, reflects standard operational practices within gas turbine systems.

At 95%, the engine is generally close to its full operational capacity, allowing for a smooth transition as the starting system disengages. This percentage strikes a balance between ensuring that the engine is adequately accelerated to function without the starter while also providing some margin for variation in engine performance. Achieving this percentage implies that the engine has successfully initiated the combustion process and is self-sustaining.

The rationale behind other percentages would not typically be appropriate as disengaging the starter too early, such as at 85% or 90%, could lead to difficulties in engine operation and potentially stall the engine. Conversely, waiting until 100% would not be practical, as the starter's role is to assist the engine only during the initial phases of operation. Thus, the choice of 95% effectively accommodates the need for both reliability and efficiency in the gas turbine starting sequence.