ENG/APU FIRE Panel (1WD)
Figure 006
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B. Fire Detectors Each fire and overheat detector has a sensing element and responder assembly.
(1) Sensing element
(Ref. Fig. 007)
A sensing element is a tube 0.063 in. (1.6 mm) in outer diameter and
0.018 in. (0.46 mm) in thickness.
It contains a hydrogen-charged titanium core with a spiral wound
around it. This spiral is made of an inert material which has a
special property: it can give off and absorb a gas.
The gap between the sensing-element outer-tube wall and the core is
filled with helium.
The initial pressure of the helium is related to the pre-set
temperature threshold selected for each sensing element.
The sensing element reacts according to the ideal gas law.
One end of the sensing element is hermetically soldered and the other one is connected to a 1 in. (25.4 mm) diameter stainless steel body called responder.
(2) Responder assembly
(Ref. Fig. 008)
The responder contains a chamber connected to two pressure switches :
an ALARM switch and a MONITOR switch. The free end of the responder
is connected to the aircraft electrical circuit.
The detector has two sensing functions. It responds to an overall
"average" temperature threshold or to a highly localized "discrete"
temperature caused by impinging flame or hot gases. This results in
the ALARM switch closure. the "average" and discrete temperatures
cannot be adjusted.
In addition, the averaging and discrete functions are reversible.
When the sensor tube has cooled, the average gas pressure decreases
and the core material absorbs the discrete hydrogen gas.
If the detector leaks, the gas pressure decrease causes the MONITOR
switch to open and generates a detector fault signal.
The system then does not operate during test.
Fire Detector - Schematic
Figure 007
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Fire Detector - ALARM and FAULT States
Figure 008
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(3)
Location
The fire detection loop has three fire detectors connected in parallel. They are installed at the engine-to-pylon forward mount, in the fan compartment close to the accessories gearbox and vertically in the aft part of the core compartment.
(4)
Installation
The fire detectors are installed in pairs on pre-formed, stainless steel supports on the engine. The sensing elements are installed with quick-release clamps which have teflon bushings and are designed for continuous high temperature operation.
Temperature Chart:
------------------------------------------------------------------------------Compartment Discrete Temperature Average Temperature
------------------------------------------------------------------------------FAN 500 deg.C (932 deg.F) 221 deg.C (430 deg.F) CORE 621 deg.C (1150 deg.F) 375 deg.C (707 deg.F) PYLON 675 deg.C (1247 deg.F) 400 deg.C (752 deg.F)
C. Fire Detection Unit (Ref. Fig. 009)
The Fire Detection Unit (FDU) processes the signals received from the fire detection loops.
There are three functional modules:
-two independent channels (1 for each detection loop)
- one monitoring circuitry (for maintenance purpose only).
(1)
The channels Each channel has its own power supply. The two channels normally operate together, with an AND logic, for the fire detection. However, if one loop is inoperative, the other loop can operate independently.
(a)
Input signals Each channel receives and analyzes continuously the signal from its related detection loop. Three comparators are used for this analysis:
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