What happens during a hung start in jet engines?

In a hung start scenario in jet engines, the engine does initiate the start process, but it does not reach the expected idle RPM. This occurs when there is a failure to achieve sufficient fuel or airflow to sustain or increase the engine's speed after the start sequence begins.

During the starting process, the engine's core typically needs to achieve a certain rotational speed to allow for the combustion process to stabilize and to reach idle conditions where the engine can operate correctly. If an issue arises that prevents reaching this idle RPM—such as insufficient fuel pressure, an inadequate air supply, or other mechanical problems— the engine will not gain the rotational speed needed for normal operation, leading to a hung start.

This condition can be concerning for pilots and ground crew as it indicates potential engine issues which require assessment and may necessitate corrective actions before proceeding with the flight. The other options either describe normal operation or failures that do not align with the specific definition of a hung start, including engine shutdowns that do not align with the criteria for this particular type of failure.