(2)  
As temperature increases, the detector resistance decreases. When detector element resistance drops to 450 ohms, a timer on the control card starts. If the resistance drops to 250 ohms or below before the timer has run out, a fault indication is passed on to the AFOLTS card. If the timer has already expired before the resistance drops to 250 ohms, a fire indication will be given when the detector drops below 250 ohms.

 (3)  
The APU fire detection can be tested by a test button on P5 pilot's overhead panel. The test button initiates tests for APU, engines, wing leading edge, and lower cargo compartments. The test button initiates a fire or overheat conditions for each system. The test results indications for APU are fire warning light, aural warning, the light on APU discharge handle, and EICAS messages.

 (4)  
A fire condition sensed on either the Loop A or Loop B fire detection control card will provide two identical signals to the AFOLTS card. The AFOLTS card operates in "OR" logic so that either signal will result in an alarm.

 (5)  
A cockpit APU fire indication is given by the following:

 (a)  
The red APU fire switch handle, on the P5 pilot's overhead panel, illuminate and unlocks.

 (b)  
The fire warning message is displayed on EICAS (Engine Indication and Crew Alerting System - P2 panel).

 (c)  
The red master warning lights, on P72 main instrument light shield, illuminate.

 (d)  
The fire bell sounds, on the flight deck aural warning speakers.

 (6)
An APU remote fire indication is given on M869 by the following:

 (a)  
The red APU FIRE light comes on.

 (b)  
When the airplane is on the ground, with the engines off - the APU fire warning horn sounds.

 (7)  
When a fire is detected, the fire alarm signal is also provided to the APU fire shutdown module M869, which causes warning horn to go off. The warning horn shuts off when the APU FIRE CONTROL handle is pulled down. Pulling down the APU FIRE CONTROL handle shuts down APU and provide continuous circuit to the APU fire extinguisher bottle squib. After the APU FIRE CONTROL handle is pulled down, the APU EXTINGUISHER DISHCARGE button can be pushed to discharge the extinguishing agent.

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 ALL  ú ú 06 Page 8 ú Jun 10/96
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (8)  
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.

 (9)  
A dual-loop fault indication is given by the following:

 (a)  The advisory messages are displayed on EICAS (P2).

 (10)
 A single-loop fault or alarm signal indication is given by the following:

 (a)  The status messages are displayed on EICAS (P2).

 (11)
 Detector, card, or associated wiring failures may not provide a valid fault output at the moment of failure. These inoperative loops can only be detected during a system test.

 B.  The CMC can initiate the fire detection system ground test if either of the ground tests switches is enabled and the CMC test relays are closed. The CMC initiates a ground test to determine the integrity of the fire detection card file and its inputs/outputs using the LRU's built-in test equipment.
 C.  When the ground test is initiated, the CMC outputs five simultaneous test initiating signals which activate five test relays. These test relays send various discrete signals to the fire detection card file. From these inputs, the fire detection card file generates fire, overheat, and fault detection discretes which it sends to the EIUs and MAWEA. The MAWEA sends signals received from the fire detection card file to the CMC on an ARINC 429 bus, and the MAWEA also sends signals received from the fire detecteion card file to the EIUs or two ARINC 429 buses. While the test is in progress, the CMC monitors all fire detection system interf aces to the EIUs and the MAWEA. If the CMC detects an inoperative interface, it displays an appropriate fault message.
 D.  The test relays can also be activated by pressing FIRE/OVHT test switch on the M7317 (P5) miscellaneous switching panel, but the test initiated by the CMC is more effective at isolating failures. The FIRE/OVHT test switch on M7317 tests critical wiring paths, while the CMC tests all wiring paths and isolates to the failed interface with a text message.
　
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