(3)  
The extinguishing occurs when the APU fire handle is pulled out and rotated in either direction. After rotation of the handle, an electrical signal is applied to squib cartridge of the extinguisher bottle. The electrical signal detonates the squib producing a rupture in the retaining disc allowing the extinguishing agent to be released. The extinguishing agent runs to an outlet nozzle at the bottom of the APU fire wall and is discharged into APU compartment.

 (4)  
When the airplane is on the ground and there is a fire warning, the APU fire extinguishing system automatically releases the extinguishing agent after ten seconds.

 (5)  
Decreasing bottle pressure, by discharge or leakage, activates the bottle pressure switch. The following occurs when a decrease in pressure is detected: the APU BTL DISCH light on the fire control module comes on, and an advisory EICAS message appears.

 B. APU Fire Extinguishing System Test
 (1)  
Squib Test

 (a)  The squib test module checks the integrity of extinguisher bottle squibs. Pressing the TEST button sends a signal to the squib cartridge. If the squib and circuit continuity are good, the green light on squib test module comes on.

 (2)  
Pressure Switch Test

 (a)  Manually activating the bottle pressure switch tests discharge light circuit. When the hex key is inserted and rotated, the the amber APU DISCH light comes on and an advisory message appears on the EICAS display indicating a successful test.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

M7326 FIRE CONTROL MODULE M7353 EIU-L
APU Fire Extinguishing System Simplified Schematic Figure 2
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 FAULT ISOLATION/MAINT MANUAL
APU FIRE EXTINGUISHING SYSTEM
COMPONENT  FIG. 102 SHT  QTY  ACCESS/AREA  REFERENCE 
BOTTLE - APU FIRE EXT, M103  1  1 313AL, SERVICE DOOR 26-22-01
CIRCUIT BREAKERS - FLT COMPT, P6
 FIRE EXTINGUISHER APU, C132  1  6H5  * 
FLIGHT DECK LIGHT CAPT, C358  1 6L4 * 
LIGHT - APU BTL DISCH, L9  1  1  FLT COMPT, P5, FIRE CONTROL  * 
MODULE, M7326
LIGHT - APU SQUIB OK, L8  1  1  FLT COMPT, P5, SQUIB TEST * 
APU FIRE SHUTDOWN, M869 MODULE - (REF 26-21-00, FIG. 101) MODULE - (REF 26-15-00, FIG. 101) FIRE CONTROL, M7326 SQUIB TEST, M7327  MODULE, M7327 
SQUIB (DISCHARGE CARTRIDGE)  2  1  APU FIRE EXT BOTTLE, M103  26-22-02 
SWITCH - APU DISCH, S246  2  1  FLT COMPT, P5, FIRE CONTROL * 
MODULE, M7326 
SWITCH TEST 1, S1  2  1 FLT COMPT, P5, FIRE CONTROL * 
MODULE, M7327 
SWITCH TEST 2, S2  2  1 FLT COMPT, P5, FIRE CONTROL * 
UNIT - (REF 31-61-00, FIG. 101) EFIS/EICAS INTERFACE C, M7352 EFIS/EICAS INTERFACE L, M7353 EFIS/EICAS INTERFACE R, M7351  MODULE, M7327

* SEE THE WDM EQUIPMENT LIST
APU Fire Extinguishing System - Component Index
Figure 101

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 FAULT ISOLATION/MAINT MANUAL

SQUIB TEST MODULE, M7327

APU Fire Extinguishing System - Component Location Figure 102 (Sheet 1)
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