Final answer:
The air turbine starter in an aircraft engine is designed to disengage at approximately 55% N2 RPM, which allows the engine to continue running independently.
Step-by-step explanation:
The air turbine starter is supposed to cut off at 55% N2 RPM. The air turbine starter assists in starting the aircraft engine by spinning the N2 rotor (the high-pressure compressor and turbine stages) up to a speed where the engine can sustain operation on its own. Once a certain rotational speed is reached, the starter disengages from the engine. This cut-off typically occurs around 55% N2 RPM for many jet engines, as it signifies that the engine's internal components have achieved sufficient speed to enable continuous combustion and self-sustained operation.