2 Heat Activated Nozzles
Flat black – normal condition.
Aluminum – indicates extinguisher has discharged.

Copyright . The Boeing Company. See title page for details.
8.10.8   February 15, 2001

737 Flight Crew Operations Manual

Introduction
There are fire detection and extinguishing systems for:
.
engines . lavatories

.
APU . cargo compartments. The engines also have overheat detection systems. The main gear wheel well has a fire detection system, but no fire extinguishing

system.
Engine Fire Protection
Engine fire protection consists of these systems:
.
engine overheat and fire detection powered by the battery bus

.
engine fire extinguishing powered by the hot battery bus.

Engine Overheat and Fire Detection
Each engine contains two overheat/fire detector loops. Each loop provides both fire and overheat detection. As the temperature of a detector increases to a predetermined limit, the detector senses an overheat condition. At higher temperatures, the detector senses a fire condition. Normally, both detector loops must sense a fire or overheat condition to cause an engine overheat or fire alert. The ENG OVERHEAT light or engine fire warning switch remains illuminated until the temperature drops below the onset temperature.
An OVHT DET switch for each engine, labeled A, B, and NORMAL, permits selection of either loop A or B, or both A and B, as the active detecting loops.
The system contains a fault monitoring circuit. If one loop fails with the OVHT DET switch in NORMAL, that loop is automatically deselected and the remaining loop functions as a single loop detector. There is no flight deck indication of single loop failure. If both loops fail on an engine, the FAULT light illuminates and the system is inoperative.
If the OVHT DET switch is positioned to A or B, the system operates as a single loop system. The non–selected loop is not monitored. If the selected loop fails, the FAULT light illuminates and the system is inoperative.
The indications of an engine overheat are:
.
both MASTER CAUTION lights illuminate

.
the OVHT/DET system annunciator light illuminates

.
the related ENG OVERHEAT light illuminates.

Copyright . The Boeing Company. See title page for details.
Fire Protection -System Description 737 Flight Crew Operations Manual

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

.  
both master FIRE WARN lights illuminate

.  
the related engine fire warning switch illuminates

.  
all related engine overheat alert indications illuminate.

Engine Fire Extinguishing
The engine fire extinguisher system consists of two engine fire extinguisher bottles, two engine fire warning switches, two BOTTLE DISCHARGE lights, and an EXT TEST switch. Either or both bottles can be discharged into either engine.
The engine fire warning switches are normally locked down to prevent inadvertent shutdown of an engine. Illumination of an engine fire warning switch or ENG OVERHEAT light unlocks the engine fire warning switch. The switches may also be unlocked manually.
Pulling the engine fire warning switch up:
.  
closes both the engine fuel shutoff valve and the spar fuel shutoff valve

.  
closes the engine bleed air valve resulting in loss of wing anti–ice to the
affected wing and closure of bleed air operated pack valve

.  
trips the generator control relay and breaker

.  
closes the hydraulic fluid shutoff valve. The engine driven hydraulic
pump LOW PRESSURE light is deactivated

.  
disables thrust reverser for the related engine.

.  
allows the engine fire warning switch to be rotated for discharge

.  
arms one discharge squib on each engine fire extinguisher bottle.

Rotating the engine fire warning switch electrically “fires” a squib, discharging the extinguishing agent into the related engine. Rotating the switch the other way discharges the remaining bottle.
The L or R BOTTLE DISCHARGE light illuminates a few seconds after the engine fire warning switch is rotated, indicating the bottle has discharged.
Copyright . The Boeing Company. See title page for details.
8.20.2   July 28, 2000

Fire Protection -System Description
Engine Fire Extinguisher Schematic

Copyright . The Boeing Company. See title page for details.
April 28, 2000  8.20.3
Fire Protection -System Description 737 Flight Crew Operations Manual

APU Fire Protection
APU fire protection consists of these systems:
.  
APU fire detection powered by the battery bus.
　
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