(2)  
The APU fire switch gives a visual indication of a fire warning. The fire warning signal will also actuate the solenoid in the APU fire switch. Actuation of the solenoid will permit the shaft and handle to be pulled. When the fire condition is confirmed the fire switch is pulled, shutting down the APU and arming the fire extinguishing system for subsequent action. The arming action moves a cam into position to actuate the rotary switch.

C.  Bottle Discharge Light 
(1)  
An amber bottle discharge light is provided to indicate that the extinguisher bottle has been bled off. When the bottle has been discharged or the safety relief valve has relieved and bottle pressure is bled off, the associated bottle discharge light will illuminate.

(2)  
The warning lights may be tested individually by a press-to-test feature in the light assembly or the lights may all be tested simultaneously when the master test switch is closed.

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D.  Test Switch
(1)  
A three-pole, double-throw, pushbutton switch is provided to permit testing of the fire extinguishing system through a test circuit connected to the bottle discharge squib. The system may be tested either while the airplane is in flight or on the ground.

(2)  
The extinguisher test circuit provides the means to check the bottle discharge squib for continuity without actually discharging the squib.

(3)  
When the system is being tested, press the EXT TEST switch; if satisfactory, an indication should occur almost immediately. The test switch sends a very small current through the APU bottle discharge squib. If the circuits are complete, a transistor in the circuit is biased. The biased transistors will conduct a signal to illuminate the APU extinguisher test light. The APU extinguisher test light is green press-to-test lights located on the face of the fire protection system module (P8-l).

3.  Fire Extinguisher Bottle Assembly
A.  The fire extinguisher bottle assembly consists of a spherical steel container fitted with a filling port plug, pressure switch, metal sealing disk in the discharge port and discharge plug. The spherical steel container (extinguisher bottle) is about 5 inches in diameter and has a volume of approximately 78 cubic inches. The bottle is used to store the extinguishing agent under pressure until released by fire extinguishing discharge action. The bottle is filled with 1.1 pounds of freon, and charged with dry nitrogen to 600-675 psi at 70°F.
B.  The APU fire extinguisher bottle is mounted in brackets on the lower right side of the
empennage section at approximately station 1064 (Fig. 1).

C.  The filling port plug is a metal plug with a fusible metal core which acts as a safety relief valve. From the safety relief valve on the bottle a tube leads to the lower empennage skin near the bottle, where it is capped with a red indicator disk. If the bottle temperature rises to or above approximately 266°F the fusible core will melt and the bottle pressure will rupture the indicator disk dumping the bottle gas charge overboard.
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D.  The pressure switch will close and activate the bottle discharge lights when the bottles are discharged or pressure drops.
E.  The discharge port is sealed by a frangible restraining diaphragm to retain the pressurized extinguishing agent. The discharge plug is installed at the discharge port and serves as an adapter between the bottle and manifold, and provides the means to discharge the bottle. When the squib receives a signal from the applicable fire switch, it is detonated (Fig. 2). The detonation ruptures the diaphragm allowing the extinguishing agent to escape.
NOTE:  The fire extinguisher bottle is not designed for recharging while installed in the airplane. A low pressure or discharged bottle should be replaced with a charged bottle.
4.  Remote APU Fire Control Panel Components
A.  The remote APU fire panel components described in the following paragraphs will be only those associated with the APU fire extinguishing system. The remote APU fire panel (P28) is located on the aft bulkhead, right side of main wheel well. For further information of the remote APU fire panel, refer to 26-00, Fire Protection.
B.  APU Remote Fire Switch
(1)  The remote fire switch is a multi-pole, double-throw, pushbutton switch actuated by a pul1-type handle on the front of the panel (P28). When the fire handle is pulled down, the fire switch shuts down the APU and arms the fire extinguishing system for subsequent action.
　
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