A hung start in a jet engine is typically caused by what issue?

A hung start in a jet engine occurs when the engine fails to reach the required RPM to continue the start sequence after the starter is disengaged. This situation is often caused by the starter cutting off too soon before the engine has built sufficient speed.

In normal operation, the starter engages to bring the engine to a specific RPM, allowing fuel to ignite and the engine to stabilize at a self-sustaining speed. If the starter disengages prematurely, the engine may not achieve the necessary momentum, leading to an incomplete start, which is termed a hung start.

This scenario is distinct from the other options. Insufficient fuel flow typically leads to difficulties in the combustion process, rather than just an incomplete start. Low ambient temperature can affect engine performance but is not a direct cause of a hung start as much as the timing of the starter's disengagement. Excessive engine vibration can indicate other issues but does not specifically result in a hung start; rather, it may arise from already established problems within the engine system.