Which of the following is NOT a function of the APU?

The APU, or Auxiliary Power Unit, serves several important functions primarily on the ground, aiding in the overall operation of the aircraft when the main engines are not running. One of its primary roles is to power essential systems, such as lighting and avionics, while on the ground, ensuring that the crew can operate the aircraft systems without relying on the main engines. Additionally, the APU provides bleed air to assist in starting the main engines and can help regulate cabin temperature, making it comfortable for passengers before engine startup.

However, the APU is not designed to provide emergency power during flight. While it can be crucial for start-up and ground operations, in-flight situations that require power redundancy typically rely on the main engines or the aircraft's main generators. If an engine fails during flight, the generators take over to supply power, rather than relying on the APU.

Understanding these various roles helps clarify why emergency power during flight is not a function of the APU; its operation is limited to ground support and engine start assistance, as well as certain onboard functions, but not as a primary power source in the air.