Final answer:
Pressing the APU FIRE TEST PB initiates a test of the APU fire detection system in an aircraft.
Step-by-step explanation:
When you press the APU FIRE TEST PB, it initiates a test of the Auxiliary Power Unit (APU) fire detection system in an aircraft. The APU is a device that provides electrical power and compressed air to start the aircraft engines. The fire detection system is designed to detect any fire or overheat condition in the APU area.
During the test, the fire detection system is activated, and if everything is functioning correctly, the test will indicate a normal condition. This check ensures that the APU fire detection system is operational and ready to react in case of a real fire or overheat situation.