What happens at approximately 35-40% N2 during engine start, according to the GCU's function?

At approximately 35-40% N2 during engine start, the Gas Turbine Control Unit (GCU) performs the critical function of disconnecting the starter. This is an essential step in the engine start sequence. Once the engine reaches this threshold of N2, it indicates that the engine has spooled up sufficiently and is capable of sustaining its own operation without the aid of the starter.

Disconnecting the starter is crucial because it helps prevent starter damage or failure once the engine is self-sustaining. Furthermore, this action ensures that the engine can receive full power from the fuel system without interference from the starter motor, allowing for a smooth transition to normal engine operation. The monitoring of the N2 speed is part of the automated engine control systems that enable safe and efficient engine management during start procedures.