What is the maximum operating limitation associated with an APU?

The maximum operating limitation associated with an APU (Auxiliary Power Unit) is primarily its duration of operation. An APU is designed to provide electrical and pneumatic power to the aircraft when the main engines are not running, primarily for operations on the ground. Each APU has a specific operational time limit to avoid overheating and to ensure reliability. Exceeding this limit can lead to mechanical failure or safety hazards.

While the other limitations such as maximum altitude, weight limits, and temperature thresholds are relevant concerns in the overall context of aircraft operation and performance, particularly for the main engines, they are not the primary limitation typically associated with an APU's use. Each APU has a recommended operating duration to manage wear and tear, and ensure that it functions effectively during periods when the engines are off.