Final answer:
An APU is shut down by turning off the control switch and allowing it to cool naturally to prevent thermal stress, maintain its longevity, and for safety. Shutting down the APU when it's not needed conserves its lifespan, saves fuel, and reduces noise and emissions.
Step-by-step explanation:
The Auxiliary Power Unit, commonly known as the APU, is a device on aircraft that provides energy for various functions while the main engines are not running. To shut down an APU, the pilot or technician must first ensure that it is no longer needed to supply power or air conditioning to the aircraft. Then they will follow the aircraft's specified shutdown procedure, which typically involves turning the APU control switch to the off position, allowing the APU to cool down and come to a stop naturally. The cool-down period is critical to prevent damage to the APU by avoiding a rapid change in temperature and ensuring the APU's intricate components aren't subjected to thermal stress.
The shutdown process is essential for maintaining the longevity of the APU and for safety reasons, ensuring that it is not operating when it's not needed, thereby reducing the risk of fire or mechanical failure. It also helps to conserve the APU's lifespan and save fuel. In addition to these benefits, shutting down the APU when it's not required also reduces noise and emissions at the airport.