Which function is NOT typically performed by an APU?

The function that is not typically performed by an Auxiliary Power Unit (APU) is related to increasing altitude performance. An APU is primarily used for ground support functions such as providing electrical power to aircraft systems when the engines are not running, starting the main engines, and supplying air conditioning while on the ground. However, it does not have any direct impact on the aircraft's performance at altitude, including the ability to climb or maintain altitude. Altitude performance is primarily influenced by the main engines and their operational capabilities, as well as aerodynamic factors, rather than the APU. Therefore, the role of the APU is more focused on providing auxiliary support on the ground rather than enhancing flight performance at altitude.