How long after an APU fire warning does the APU bottle discharge when the aircraft is on the ground?

How long after an APU fire warning does the APU bottle discharge when the aircraft is on the ground?

• A. 3 seconds
• B. 1 second
• C. 2 seconds
• D. 5 seconds

Answer: (A)

When the APU fire warning occurs on the ground, the APU extinguisher bottle is discharged automatically after a fixed three‑second delay. This short delay provides a quick, automated response to suppress the fire without waiting for manual action, while not delaying too long in case of a real fire. The other interval options would be either too fast to guarantee proper actuation reliability or too slow and would allow the fire to grow longer before suppression.

When the APU fire warning occurs on the ground, the APU extinguisher bottle is discharged automatically after a fixed three‑second delay. This short delay provides a quick, automated response to suppress the fire without waiting for manual action, while not delaying too long in case of a real fire. The other interval options would be either too fast to guarantee proper actuation reliability or too slow and would allow the fire to grow longer before suppression.