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MAINTENANCE MANUAL
(2)
The inert gas in a sensing element expands as a function of average gas temperature. The gas-emitting core material expells gas due to high localized temperatures. Both actions cause an increase in pressure in the element, which causes an alarm pressure switch (in the responder) to activate. Both actions are completely reversible
-as the temperature decreases, the pressure decreases, and the alarm switch deactivates. When a sensing element is damaged allowing the inert gas to leak out, an integrity pressure switch deactivates. The pressure switches provide alarm and integrity signals to the appropriate detector control card.
(3)
The fire detectors are installed above and below the APU. One detector forms a 180 degree arc around the air inlet above the APU and the other detector is below the APU, on an access door.
C. Fire and Overheat Detection Cards
_________________________________
(1)
Each detection loop has a corresponding detection control card. These cards constantly monitor and process signals produced by each loop. The cards output alarm or fault signals to the fire/overheat logic/test card.
(2)
The detection control cards are in the P54 fire detection card file. The card file is in the electrical/electronic equipment compartment, along the right side of the fuselage.
D. Fire/Overheat Logic/Test Card
_____________________________
(1)
The AFOLTS (Automatic Fire/Overheat Logic/Test System) card interprets detection control card signals, provides a system functional test, and outputs fire warning and fault indication signals.
(2)
The AFOLTS card is located in the P54 card file.
E. Fire/Overheat Test Panel
________________________
(1)
The fire/overheat test panel contains a momentary pushbutton switch - ENG/APU/CARGO. This switch initiates a test of the fire, overheat, and smoke detection systems for the engines, APU, and cargo compartments. The test results in cockpit fire and overheat indications.
(2)
The test panel also has an amber SYS FAIL - FAIL P-RESET switchlight. This switchlight indicates a fire, overheat, or smoke detection system failure. The switchlight is pressed to reset.
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757
MAINTENANCE MANUAL
(3) The fire/overheat test panel is on aft pilots' control stand P8.
F. LTS/APU/INTERPHONE Panel
________________________
(1)
LTS/APU/INTERPHONE panel P62 contains the APU FIRE light, and the APU fire warning horn. In addition, the panel contains switches that allow the APU to be shutdown (when a fire indication occurs), and the APU fire extinguisher to be discharged from the nose landing gear area.
(2)
P62 is located on the right side of the nose landing gear.
3. Operation (Fig. 2)_________
A. Functional Description (Graviner)
_________________________________
(1)
When temperature increases, the resistance of the detector element drops, and the detector card senses the voltage across the capacitance, triggering a fire indication. If the detector is shorted, contaminated, or has opened up, only the voltage across the detector resistance changes. The detector card interprets this as a fault, and provides a fault indication.
(2)
The AFOLTS card configures the control card signals in AND logic, and determines the appropriate output to flight and ground crews. The APU Fire Detection Logic table shows the normal operating mode.
NOTE: APU FIRE LP 1 and APU FIRE LP 2 are EICAS maintenance
____ messages visible only when "ECS/MSG" switch is pushed.
~........................................................ | APU Fire Detection Logic | | Table 1 | .......................................................... | INPUTS TO AFOLTS | EICAS MESSAGE | | FROM APU FIRE | | | DETECTION CARDS | | .......................................................... | LOOP 1 | LOOP 2 | GROUND | AIR | .......................................................... | FIRE | FIRE | APU FIRE | APU FIRE | .......................................................... | FIRE | ----| APU FIRE | APU FIRE LP 1 | .......................................................... | ----| FIRE | APU FIRE | APU FIRE LP 2 | .......................................................... | FIRE | FAULT | APU FIRE | APU FIRE | .......................................................... | FAULT | FIRE | APU FIRE | APU FIRE | .......................................................... | FAULT | FAULT | APU FIRE | APU FIRE | .......................................................... | FAULT | ----| APU FIRE LP 1 | APU FIRE LP 1 | .......................................................... | ----| FAULT | APU FIRE LP 2 | APU FIRE LP 2 | 2..........…..........…................….................1
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