FIRE PROTECTION

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FIRE PROTECTION - DESCRIPTION AND OPERATION
1. General
A. Fire protection consists of a detection system and an extinguishing system. The detection
system provides the means to detect a fire and/or overheat condition and alert the crew by
aural and visual indications. The extinguisher system provides the means to extinguish a fire.
B. The detection systems give warning of fire and/or overheat in the following areas: each engine,
auxiliary power unit (APU), main wheel well, wings and lower aft body. A smoke detection
system is also installed on Passenger/Cargo Convertible Airplanes. The smoke detection
system monitors the electronic equipment compartment, and the main cabin for the presence of
smoke. Fire and smoke warning is indicated by warning lights on the fire protection system
module, master warning lights on the lightshield module and an alarm bell in the control cabin.
The APU also has a fire warning light located on the APU remote control panel and a warning
horn adjacent to the panel in the right main wheel well. Overheat warning is indicated by lights
on the air conditioning module and master caution lights on the lightshield module.
(1) On 737-200 series passenger airplanes incorporating Service Bulletin 26A1081, there are
smoke detectors and Cargo Electronic Units (CEU) installed in the forward and aft lower
cargo compartments (Fig. 1).
C. The extinguishing systems include a fixed engine fire-extinguishing system, a fixed APU fireextinguishing
system, a fixed lavatory fire-extinguishing system, and portable fire extinguishers.
The fixed engine fire-extinguishing system consists of two fire extinguisher bottles, providing a
two-shot extinguishing capability at either engine. The bottles are connected to each engine by
manifolds and tubing. A fire switch for either engine provides the means of selecting either
extinguisher bottle for discharge. The fixed APU fire-extinguishing system consists of a single
extinguisher bottle connected by tubing to the APU shroud. The engine and APU discharge
switches are located on the fire protection module. The APU bottle may be discharged by
actuating a discharge switch on the remote APU fire control panel (Fig. 1). The fixed lavatory
fire-extinguishing system consists of a heat activated fire extinguisher bottle located in each
lavatory. The bottle is provided to extinguish fires under the sink counter and in the towel chute.
(1) On 737-200 series passenger airplanes incorporating Service Bulletin 26A1081, there is a
fire-extinguishing bottle located in the mix manifold bay. Fire-extinguishing tubing and
discharge nozzles are installed in the forward and aft lower cargo compartments that
direct the extinguishing agent into the cargo compartments (Fig. 1).
D. Portable fire extinguishers are located at various positions in the crew and passenger cabins.
They provide fire-extinguishing capability in these cabins and other areas accessible from them.

2. Fire Protection System Module
A. The fire protection system module (P8-1) contains the components necessary to monitor the
fire detection system; smoke detection system and wheel well overheat detection system. The
fire protection module also has facilities to control and test the fire extinguishing system and
smoke detection system. The fire protection system module is located on the aft control stand
electronic panel (P8). The front of the module contains the following components for fire and
overheat detection: red fire warning lights for each engine and the APU, amber engine overheat
lights for each engine, a red wheel well overheat light, an overheat detector switch for each
engine, an APU detector inoperative light, a bell cutout switch and a fire test switch. On
Passenger/Cargo Convertible Airplanes, the fire protection module also has two red cargo
smoke lights. The front of the module also contains the following components for fire
extinguishing control: a fire switch for each engine, left, right and APU amber bottle discharge
lights, a fire extinguisher test switch, and three green fire extinguisher test circuit lights. Master
caution light control circuits and fire extinguisher bottle test circuits are on cards within the
module. For detailed coverage of components, refer to applicable system coverage.
(1) On 737-200 series passenger airplanes incorporating Service Bulletin 26A1081, there is
also a cargo fire control panel located on the aft control stand electronic panel (P8), that
indicates fire in the cargo areas, and from which fire extinguishing agent can be dispensed
to the forward or aft lower cargo area (Fig. 1).
3. Lightshield Module
A. The lightshield module (P7) contains master fire warning and master caution lights and the
annunciator lights necessary to warn the captain and first officer of a fire or overheat condition.
For further information on the module, refer to system coverage and Chapter 33, Master
Caution and Warning Lights.
4. Remote APU Fire Control Panel
A. The remote APU fire control panel (P28) contains the components necessary to visually check
for a fire warning in the APU, to control the remote horn and alarm bell and to control the APU
fire extinguishing system from the ground. The fire panel is located on the aft bulkhead of the
right main wheel well. For detailed coverage of components, refer to applicable system
coverage.

DETECTION - DESCRIPTION AND OPERATION
1. General
A. Detection is separated into two categories; fire and overheat. The detection systems include
the following subsystems; engine fire detection system, auxiliary power unit (APU) fire detection
system, wheel wel1 overheat detection system, and wing and lower aft body overheat detection
system. The fire detection systems are basically dual loop detectors (engines) and single loop
detector (APU) circuits with the sensing elements spirally wrapped about a rigid support tube
and connected to the engine and APU fire detection accessory unit. The overheat detection
systems are single loop detector circuits connected to the compartment overheat detection
accessory unit.
2. Engine and APU Fire Detection Accessory Unit (Control Module)
A. The engine and APU fire detection accessory unit contains all components necessary for fire
detection, except the sensing elements, system control switches and indicators. The accessory
unit is located on electronic equipment rack E3 in the electronic compartment. The accessory
unit contains a detector control card for each sensing loop in the engine and APU areas, relays
for fire warning and short circuit discrimination tests, aural warning and master warning light
control circuits, a detector test switch and four amber detector inoperative lights (Fig. 1).
3. Compartment Overheat Detection Accessory Unit (Control Module)
A. The compartment overheat detection accessory unit contains all components necessary for
overheat detection, except the sensing elements, system control switches and indicators. The
accessory unit is located on electronic equipment rack E3 in the electronic compartment. The
accessory unit contains three overheat control units; an alarm cutout control card; a wheel well
overheat test relay; an overheat test relay; test switches and a test light. The overheat control
units are connected to sensing loops in the wheel well area, the right overheat section and the
left overheat section (Fig. 1).
4. Bleed Air Duct Overheat Test and Indication Section
A. The bleed air duct overheat test and indication section is part of the air conditioning module
P5-10 on overhead panel P5 and contains the left and right overheat detection system lights to
provide a visual means of monitoring overheat conditions along the air conditioning bleed air
ducts. A test switch provides the means to test the overheat detection system (Fig. 1). For
further information on the module, refer to system coverage and Chapter 21, Air Conditioning.

ENGINE AND APU FIRE DETECTION ACCESSORY UNIT - REMOVAL/INSTALLATION
1. General
A. The following procedure assures proper mating of electrical connectors while removing and
installing the fire detection accessory unit (M279).
CAUTION: IMPROPER ADJUSTMENT OF ASSEMBLIES ON EQUIPMENT SHELVES MAY
CAUSE CIRCUIT MALFUNCTION OR EQUIPMENT DAMAGE.
B. Lever latches are used for movement and retention of accessory units mounted on racks. This
unit consists mainly of a handle and trigger (Fig. 401). Accessory units are held in position by
engagement of a hook on the lever-latch handle with a fork-type catch on the rack. Correct
adjustment of the lever-latch fork is necessary to assure proper connector engagement.
2. Remove Engine and APU Fire Detection Accessory Unit (Fig. 401)
CAUTION: SOME E/E BOXES ARE STATIC SENSITIVE. DO NOT HANDLE BEFORE READING
PROCEDURE FOR HANDLING ELECTROSTATIC DISCHARGE SENSITIVE
DEVICES (REF 20-40-12). BOXES CONTAIN DEVICES THAT CAN BE DAMAGED
BY STATIC DISCHARGE.
A. Open these circuit breakers on the P6 panel:
(1) FIRE PROTECTION DETECTION - ENG NO. 1
(2) FIRE PROTECTION DETECTION - ENG NO. 2
(3) FIRE PROTECTION DETECTION - APU
B. Press on the latch to let the levers move away from handle.
C. Move the levers down to force the unit away from the shelf-mounted connector.
D. Remove the unit from the shelf.
NOTE: To prevent foreign object contamination or damage, cover all ports, openings and
electrical connectors of unit or shelf with plastic caps or bags.

3. Install Engine and APU Fire Detection Accessory Unit (Fig. 401)
CAUTION: ELECTRICAL PINS ON E/E BOX CONNECTOR AND TRAY CONNECTOR MUST
NOT BE BENT OR DAMAGED. INSTALLATION OF E/E BOX WITH DAMAGED PINS
COULD RESULT IN DAMAGE TO E/E BOX, TRAY ELECTRICAL CONNECTOR, OR
SYSTEM COMPONENTS.
A. Slide the unit into shelf with the levers in the open position until the levers engage in the shelfmounted
fork.
NOTE: Remove any protective caps or bags from unit or shelf prior to installation.
B. Move lever-latch to its locked position.
(1) The connectors shall mate to the dimensions of detail B (Fig.\401).
(2) If necessary, adjust the fork so that with connector properly mated and the lever open 0.50
to 0.60 inch from the locked position, the fork just puts pressure on the lever
(Ref 20-10-111).
C. Do the engine fire detection system test (Ref Engine Fire Detection System - Adjustment/Test).
D. Do the APU fire detection system test (Ref APU Fire Detection System - Adjustment/Test).
E. Restore airplane to normal.

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FIRE PROTECTION