Which component is designed to break and disconnect the starter from the engine if the engine drives the starter beyond its design speed?

Which component is designed to break and disconnect the starter from the engine if the engine drives the starter beyond its design speed?

• A. The shear section in the air-turbine drive shaft.
• B. The starter relay.
• C. The generator circuit breaker.
• D. The impulse coupling.

Answer: (A)

When starting an engine with an air-turbine starter, there’s a mechanical safeguard in the drive shaft called a shear section (shear pin). This weak link is designed to fail at a specific torque, so if the engine accelerates the starter beyond its design speed, the shear section shears and the starter is physically disconnected from the engine. This protects both the starter and the drive system from damage due to overspeed or back-driving. The other items aren’t intended to perform that mechanical disconnection. The starter relay is just an electrical switch that energizes the starter motor. The generator circuit breaker guards the electrical power system, not the starter drive. The impulse coupling helps control the timing and impulse delivered during starting, but it isn’t a protective disconnect for overspeed.

When starting an engine with an air-turbine starter, there’s a mechanical safeguard in the drive shaft called a shear section (shear pin). This weak link is designed to fail at a specific torque, so if the engine accelerates the starter beyond its design speed, the shear section shears and the starter is physically disconnected from the engine. This protects both the starter and the drive system from damage due to overspeed or back-driving.

The other items aren’t intended to perform that mechanical disconnection. The starter relay is just an electrical switch that energizes the starter motor. The generator circuit breaker guards the electrical power system, not the starter drive. The impulse coupling helps control the timing and impulse delivered during starting, but it isn’t a protective disconnect for overspeed.