APU FIRE DETECTION -APU COMPONENT LOCATIONS

Component Locations
These are the APU fire detection system components in the APU compartment:
*
APU fire detector -upper forward

*
APU fire detector -upper aft

*
APU fire detector -lower.

The upper forward detector is on the forward support and vibration isolation mount. The upper aft detector is on the left side of the APU compartment. The lower detector is on the left APU access door.
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APU FIRE DETECTION -APU COMPONENT LOCATIONS

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APU FIRE DETECTION -APU FIRE DETECTORS

Purpose
The APU fire detectors monitor the temperature in the APU compartment.
Physical Description
Each detector has two elements attached to a support tube. The elements connect to make loop 1 and loop 2.
The element is an inconel tube filled with a thermistor core material. Two electrical conductors go through the length of the core. One conductor has a ground connection to the tube. The other conductor connects to the APU fire detection card.
Functional Description
As the temperature of the core increases, electrical resistance to ground decreases. If the resistance decreases to the set point, a fire alarm occurs. When the fire is gone, the resistance increases to the reset point and the alarm signal goes away.
The tables show approximate temperatures in degrees F (and degrees C) which cause a fire alarm. Reset temperatures are also shown.
Training Information Point
Each element uses a #10 screw wire-termination on one end and a # 8 screw wire-termination on the other end to prevent crossed wiring. The #10 terminal and the mating wire terminal have a black stripe around the insulation.
The ends of the element attach to the support tube bracket with retaining nuts. Quick-release clamps and bushings support the element along its length.
DETECTOR  FIRE TEMP  RESET TEMP 
UPPER FWD  479 (248)  468 (242) 
UPPER AFT  511 (266)  499 (259) 
LOWER  488 (253)  478 (248) 

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BOEING PROPRIETARY -Copyright # -Unpublished Work -See title page for details. 

777 AIRCRAFT MAINTENANCE MANUAL

APU FIRE DETECTION -APU FIRE DETECTORS

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APU FIRE DETECTION -FIRE DETECTION CARD -APU

Purpose
The APU fire detection card has these purposes:
*
Operate in attended or unattended mode

*
Monitor the detectors for a fire condition

*
Monitor the detectors for failures and send the data to AIMS

*
Set the correct logic for the loops (and/or logic)

*
Do tests of the APU fire detection system.

Location
The APU fire detection card is in the P85 system card file in the A9 slot. The cardfile slots have labels.
Fire Detection Circuits
The fire detection circuit monitors the resistance of the detector element thermistor core. The resistance of the core is normally high. If the resistance goes low, the card supplies a FIRE output.
A short circuit in the loop also cause low resistance. The rate at which the resistance changes identifies an electrical short or a fire. A short circuit causes the resistance to decreases quickly. A fire causes the resistance to decrease more slowly.
If the resistance decreases quickly, the circuit supplies a SHORT output for the life of the short circuit. If the short circuit stays for more than 0.2 seconds, the circuit supplies a DESELECT output. A deselect output also occurs if a power-up BIT finds an open circuit in the loop.
Program Logic Device
The program logic device gives attended or unattended mode of operation. A signal from the ARINC 429 receiver puts the device in the correct mode.
The device sets the correct logic for the fire detection circuits. Dual-loop operation is normal. If a detection circuit sends a short, deselect, or power loss signal, the logic device changes to single-loop operation.
The program logic device supplies the fire alarm signals to the other circuits which have interfaces with the card.
Attended Mode Operation
The program logic device operates with AND logic. Loop 1 and loop 2 both must have a fire to cause the alarms. The table shows alarm and fault conditions:
ATTENDED MODE -AND LOGIC 
LOOP 1  LOOP 2  CARD OUTPUT 
　
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