APU Fire Extinguishing
The APU fire extinguisher system consists of one APU fire extinguisher bottle, an APU fire
warning switch, an APU BOTTLE DISCHARGE light, and an EXT TEST switch. The APU
ground control panel located in the right main wheel well also contains an APU fire warning
light, an APU BOTTLE DISCHARGE switch, an APU fire control handle and APU HORN
CUTOUT switch. The APU fire warning switch is normally locked down to prevent
inadvertent shutdown of the APU. Illumination of the APU fire warning switch unlocks the
switch. The switch may also be unlocked manually.
Pulling the APU Fire Warning switch up:
- Provides backup for the automatic shutdown feature.
- Deactivates the fuel solenoid and closes the APU fuel shutoff valve.
- Closes the APU bleed air valve.
- Closes the APU air inlet door.
- Trips the APU generator control relay and breaker.
- Allows the APU fire warning switch to be rotated for discharge.
- Arms the APU fire extinguisher bottle squib.
Rotating the APU fire warning switch in either direction electrically “fires” the squib
discharging the extinguishing agent into the APU. The APU BOTTLE DISCHARGE light
illuminates after a few seconds indicating that the bottle has discharged.
