Final answer:
The ON light on the START pushbutton indicates that the APU is running and providing power to essential aircraft systems.
Step-by-step explanation:
The ON light on the START pushbutton indicates that the APU (Auxiliary Power Unit) is running and providing power to essential aircraft systems. The APU is a small gas turbine engine located in the tail of the aircraft that generates electrical power and also provides compressed air for engine starting and air conditioning. When the APU is running, the ON light illuminates to indicate its operational status.
The APU is a small gas turbine engine located in the tail of the aircraft. It serves the purpose of providing electrical power, pneumatic power for air conditioning and pressurization, and sometimes hydraulic power when the main engines are not operating.
During normal operations, the APU is started before the main engines, and once it's running, it provides the necessary power for various aircraft systems while on the ground or when the main engines are not running.