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ENGINE FIRE DETECTION -ENGINE FIRE DETECTORS
Purpose
The engine fire detectors monitor the temperature in the engine nacelles.
Physical Description
Each detector has two elements that attach 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 fire detection card.
Functional Description
As the temperature of the core increases, electrical resistance to ground decreases. The fire detection card monitors this resistance. If the resistance decreases to the overheat set point, an overheat indication occurs in the flight deck. There is a 9.6 second time delay for the overheat indication. If the resistance decreases more to the fire set point, a fire warning occurs. When the fire or overheat condition is gone, the resistance increases to the reset point and the flight deck indications go away.
The tables show temperatures in degrees F (and degrees C) which cause overheat and fire indications. Reset temperatures are also shown.
EFFECTIVITY
BEJ ALL
DETECTOR OVERHEAT TEMP RESET TEMP
UPPER 840 (449) 821 (438)
LOWER 783 (417) 764 (407)
LOWER FWD 580 (304) 562 (294)
DETECTOR FIRE TEMP RESET TEMP
UPPER 937 (503) 921 (494)
LOWER 881 (472) 865 (463)
LOWER FWD 669 (354) 654 (346)
The rate of change of resistance identifies an electrical short or a fire. The resistance decreases more quickly with an electrical short than with a fire.
In addition to fire and overheat detection, the engine fire detectors continuously supply nacelle temperature data to the airplane condition monitoring function of the AIMS.
Training Information Point
Each element uses a #10 screw wire-termination on one end and a #8 screw wire-termination on the other end. The elements are installed so each end of the detector has a #8 terminal and a #10 terminal. This prevents crossed wiring. The #10 terminal and the mating wire terminal have a black stripe around the insulation.
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ENGINE FIRE DETECTION -ENGINE FIRE DETECTORS
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.
EFFECTIVITY BEJ ALL 26-11-00
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ENGINE FIRE DETECTION -ENGINE FIRE DETECTORS
EFFECTIVITY BEJ ALL 26-11-00
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777 AIRCRAFT MAINTENANCE MANUAL
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ENGINE FIRE DETECTION -FIRE DETECTION CARDS
Purpose
The fire detection card has these purposes:
*
Measures the nacelle temperature and sends it to the airplane condition monitoring system (ACMS)
*
Monitors the fire detectors for a fire or overheat condition and supplies alarm signals
*
Monitors the fire detectors for defects and sends fault data to AIMS
*
Sets the logic (and/or logic) for the detector loops
*
Does tests of the engine fire detection system.
Fire Detection Circuits
The fire detection circuits monitor the resistance of the fire detector element thermistor core. The resistance of the core is normally high. If the resistance goes low, the circuit supplies a FIRE output.
A short circuit in the loop also causes low resistance. The rate at which the resistance changes identifies an electrical short or a fire. A short circuit causes the resistance to decreases very 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.
EFFECTIVITY
BEJ ALL
Program Logic Device
Output signals from the fire detection circuits go to the program logic device. The device normally causes dual-loop operation. Both Loop 1 and loop 2 must have a fire to cause fire alarms. If a detection circuit sends a FAULT signal, the logic device causes single-loop operation. A FAULT is a short, deselect, or power loss. The table shows alarm and fault conditions:
LOOP 1 LOOP 2 CARD OUTPUT
NORMAL NORMAL -
FIRE NORMAL FAULT (SEE TEXT)
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