Final answer:
The APU is normally started on the ground during the pre-engine start phase to power aircraft systems before the main engines are started, and it might also be used after landing until ground power is connected.
Step-by-step explanation:
The Auxiliary Power Unit (APU) is typically started on the ground during the pre-engine start phase of an aircraft's operation. The APU provides power to start the aircraft's main engines and to run onboard systems before engine start, when external electrical power is not connected or available. It is also used to power cabin environmental controls and other electrical systems during ground operations, ensuring passenger comfort and the functionality of critical systems. Upon landing, the APU might also be started after main engine shutdown to maintain these systems until the aircraft is connected to ground power.