Final answer:
The specific N2 RPM at which the air turbine starter cuts off depends on the aircraft's engine design, and the exact value can be found in the engine's operation manual.
Step-by-step explanation:
The air turbine starter is supposed to cut off at a specific N2 RPM that varies depending on the design of the aircraft's engine. Generally, this cutoff occurs when the engine has reached sufficient rotational speed to sustain combustion and operate under its own power without the assistance of the starter. Since manufacturers might set different cutoff values, it's important to refer to the specific engine's operation manual for the exact N2 RPM value. For example, turboprop engines might have a cutoff around 50-60% N2 RPM, while high-bypass turbofan engines might have a cutoff at higher percentages.