At what N2 speed will the overspeed solenoid valve automatically activate to shut down the engine?

• A. 118%
• B. 131%
• C. 123%
• D. 140%

Answer: (C)

Overspeed protection uses the N2 (compressor) speed signal to guard the turbine from turning faster than it can safely handle. When N2 reaches the preset limit, the overspeed solenoid valve is energized to cut fuel flow, causing the engine to shut down automatically before the rotor reaches damaging speeds. The trip point is chosen to provide a safe margin below the speed at which damage could occur, and 123% N2 is a common, well-balanced setting for this protection. Values significantly lower would cause unnecessary shutdowns during normal transients, while much higher values could leave the engine exposed to damage before protection kicks in.