.
APU fire extinguishing powered by the hot battery bus.

APU Fire Detection
A single fire detection loop is installed on the APU. As the temperature of the detector increases to a predetermined limit, the detector senses a fire condition. The APU fire warning switch remains illuminated until the temperature of the detector has decreased below the onset temperature.
The system contains a fault monitoring circuit. If the loop fails, the APU DET INOP light illuminates indicating the APU fire detection system is inoperative.
Fire Protection -System Description 737 Operations Manual

The indications of an APU fire are:
.  
the fire warning bell sounds

.  
both master FIRE WARN lights illuminate

.  
the APU fire warning switch illuminates

.  
the APU automatically shuts down

.  
the APU fire warning horn in the main wheel well sounds, (on the ground
only), and the APU fire warning light flashes.

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 the bottle has discharged.

Main Wheel Well Fire Protection
Main wheel well fire protection consists of fire detection powered by the No. 1 AC transfer bus.
Note:  The main wheel well has no fire extinguishing system. The nose wheel well does not have a fire detection system.
Copyright . The Boeing Company. See title page for details.
8.20.4   December 1, 2000

Fire Protection -System Description 737 Operations Manual
Main Wheel Well Fire Detection
A single fire detector loop is installed in the main wheel well. As the temperature of the detector increases to a predetermined limit, the detector senses a fire condition. The WHEELWELL fire warning light remains illuminated until the temperature of the detector has decreased below the onset temperature.
The indications for a main wheel well fire are:
.
the fire warning bell sounds

.
both master FIRE WARN lights illuminate

.
the WHEEL WELL fire warning light illuminates.

Cargo Compartment Fire Protection (as installed)
Cargo fire protection consists of these systems:
.
cargo compartment smoke detection powered by DC bus 1 and DC bus 2

.
cargo compartment fire extinguishing powered by the hot battery bus.

Cargo Compartment Smoke Detection
The forward and aft cargo compartments each have smoke detectors in a dual loop configuration. Normally, both detection loops must sense smoke to cause an alert. These loops function in the same manner as the engine overheat/fire detection loops.
Cargo Compartment Fire Warning
The indications of a cargo compartment fire are:
.
the fire warning bell sounds

.
both master FIRE WARN lights illuminate

.
the FWD/AFT cargo fire warning light(s) illuminates.

Cargo Compartment Fire Extinguishing
On planes with ETOPS configuration, two fire extinguisher bottles are installed in the air conditioning mix bay on the forward wing spar. Detection of a fire in either the forward or aft compartment will cause the FWD or AFT cargo fire warning light to illuminate. The extinguishers are armed by pushing the appropriate cargo fire ARMED switch. Once armed, the first bottle is discharged by pushing the cargo fire DISCH switch. This results in the total discharge of the first bottle contents into the selected compartment. The second bottle discharge is metered to discharge at a reduced flow into the selected compartment. Discharge of the second bottle may be disabled if the system is disarmed. The cargo fire DISCH light illuminates when a bottle is discharged. It may take up to 30 seconds for the light to illuminate. On landing, if the first bottle was discharged and the system remains armed, the second bottle discharge is inhibited.
　
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