During the coastdown of a turbofan or turbojet engine, what sound should a mechanic listen for?

During the coastdown of a turbofan or turbojet engine, it is crucial for a mechanic to be attentive to any rubbing sound or unusual noises. This is because coastdown refers to the engine's deceleration as it winds down after throttling back or shut down.

If the engine is functioning properly, there should be a smooth decrease in RPM without any irregular sounds. Any rubbing or unusual noises may indicate issues such as rotor-to-stator contact, bearing failures, or other mechanical problems that could lead to serious engine damage or failure. Therefore, identifying such sounds during coastdown can help in early detection of potential faults, allowing for timely maintenance or intervention.

Other sounds such as a throttle response change or a constant hum may not provide significant information about the engine's health during this phase, while a high-pitched whirring might not be inherently problematic unless it coincides with other alarming indications. Therefore, listening for unusual noises during coastdown is vital for maintaining the overall safety and functionality of the engine.