OVERHEAT/FIRE Test Detection  . . . . . . . . . . . . . . . . . . . . . 8.20.7 Extinguisher Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.20.8
Cargo Fire System Tests (as installed) . . . . . . . . . . . . . . . . . . . . . 8.20.8 Cargo Fire TEST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.20.8 Cargo Fire Extinguisher Test . . . . . . . . . . . . . . . . . . . . . . . . . . 8.20.8
Copyright . The Boeing Company. See title page for details.
8.TOC.0.2  June 4, 2004

Overheat/Fire Protection Panel Switches

Overheat Detector (OVHT DET) Switch
NORMAL – detection loop A and loop B are active.
A – detection loop A is active.
B – detection loop B is active.

Fire Warning BELL CUTOUT Switch
Push –
.
extinguishes both master FIRE WARN lights

.
silences the fire warning bell

.
silences the remote APU fire warning horn (on the ground only)

.
resets the system for additional warnings.

Extinguisher (EXT) TEST Switch
(spring–loaded to center)
1 or 2 – tests bottle discharge circuits for all three extinguisher bottles and on some
airplanes the three amber BOTTLE DISCHARGE lights.

FAULT/Inoperative (INOP) and Overheat (OVHT)/FIRE TEST Switch
(spring–loaded to center)
FAULT/INOP – tests fault detection circuits for both engines and the APU.

Fire Protection -Controls and Indicators

737 Operations Manual
OVHT/FIRE – tests overheat and fire detection loops on both engines and APU, and wheel well fire detector
Note: See Fire and Overheat Detection System Fault Test in Section 20.

APU Fire Warning Switch
Illuminated (red) –
.
indicates fire in APU

.
unlocks APU fire warning switch. Note: Master FIRE WARN lights illuminate, fire warning bell sounds, APU fire warning horn in main wheel well sounds (on ground only), and APU fire

warning light flashes. In – normal position, mechanically locked if no fire signal. Up –
.
arms APU extinguisher circuit

.
closes APU fuel shutoff valve, APU bleed air valve, and APU inlet door

.
trips generator control relay and breaker

.
allows APU fire warning switch to rotate. Rotate (left or right) –

.
discharges APU fire bottle.

Engine Fire Warning Switch
Illuminated (red) –
.
indicates fire in related engine

.
unlocks related engine fire warning switch. Note: Master FIRE WARN lights illuminate and fire warning bell sounds. In – normal position, mechanically locked if no fire signal. Up –

.
arms one discharge squib on each engine fire extinguisher

.
closes fuel, hydraulic shutoff and engine bleed air valves

.
disables thrust reverser

.
trips generator control relay and breaker

.
deactivates engine driven hydraulic pump LOW PRESSURE light

.
allows engine fire warning switch to rotate. Rotate (left or right) – discharges related fire bottle.

Copyright . The Boeing Company. See title page for details.
8.10.2  December 1, 2000

Fire Protection -Controls and Indicators
Fire Warning Switch Override

Fire Warning Switch Override
Push – unlocks fire warning switch.
Overheat/Fire Protection Panel Lights

Engine (ENG) OVERHEAT Light
Illuminated (amber) – indicates overheat in related engine.
Note: MASTER CAUTION and OVHT/DET system annunciator lights illuminate.

FAULT Light
Illuminated (amber) – with the overheat detector switch in NORMAL – indicates both detector loops for an engine have failed.
Fire Protection -Controls and Indicators

737 Operations Manual
Illuminated (amber) – with the overheat detector switch in A or B – indicates the related loop for an engine has failed.
Note:  MASTER CAUTION and OVHT/DET system annunciator lights do not illuminate.

WHEEL WELL Fire Warning Light
Illuminated (red) – indicates fire in main gear wheel well
Note: Master FIRE WARN lights illuminate and fire warning bell sounds.

Engine BOTTLE DISCHARGE Light
Illuminated (amber) – indicates related fire extinguisher bottle has discharged.

APU Detector Inoperative (DET INOP) Light
Illuminated (amber) – indicates APU detector loop has failed.
Note:  MASTER CAUTION and OVHT/DET system annunciator lights illuminate.

APU BOTTLE DISCHARGE Light
Illuminated (amber) – indicates APU extinguisher bottle has discharged.
　
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