(c)
Integrity Detection The pressure of the ambient inert gas holds the integrity switch in the closed position. In the event of gas pressure loss, the switch opens and generates a loop fault signal.
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(2) Fire Detection Unit (Ref. Fig. 005)
(a)
Dual Loop Detection During normal conditions (no fault, no fire, no tests) the input monitoring voltage is higher than the integrity fault threshold. It is less than the contamination fault and fire thresholds. Any fault at the sensing element/responder level (integrity switch opens) or a loss of the electrical signals increases the resistance. The voltage decreases and makes a loop inop signal. A responder fire detection closes the alarm switch. This causes a voltage higher than the threshold of contamination fault and fire comparators. When both inputs to the exclusive OR gate are high, its output is low and stops the fault output. Fire output is only energized, if the condition from the second loop are correct. The conditions are:
-
fire,
-
fire within 5 s.
-second loop inop.
(b)
Input/Output circuit The FDU creates a fire warning signal in channel A and B, if one of these conditions appear:
-
Fire A and Fire B,
-
Fire A and Fault B,
-
Fault A and Fire B,
-
Fault A and Fault B within 5 s.
The FDU creates discrete inop signals in the respective channel
(A and B) if one of these conditions appear:
-electrical failure (loss of power, plug not connected),
-fault in a detector,
-failure in the detection circuit,
-detection of a single loop fire for greater than 15 s. while the other loop is is normal.
(c)
Controller circuit The software monitors both detection loops and reports all failures to the CFDU. The software functions are:
-perform self test whenever power is first energized,
-isolate all detector and loop circuit failures and memorize in non volatile memory,
-continuously transmit current and/or previous system status on the ARINC 429 bus,
-perform built in test and transmit the test results on ARINC 429 bus,
-verify the fire tests circuitry when it is activated.
FDU - Detection Logic
Figure 005
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R 6. Test
____ A TEST pushbutton allows a check of the two detection loops and the fire warning circuit. A voltage across both loops simulates a fire signal and gives an alarm output. When the fire warnings (Ref. Para 5.A (1) fire warnings) come on, the result of this test is correct. The fire detection is operative.
Fire Detection - Functional Schematic
Figure 006
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APU FIRE AND OVERHEAT DETECTION - ADJUSTMENT/TEST
_________________________________________________ TASK 26-13-00-710-001 Operational Test of the APU Fire and Overheat Detection System
1.
Reason for the Job
__________________
Refer to the MPD TASK: 261300-01
2.
Job Set-up Information
______________________
A. Fixtures, Tools, Test and Support Equipment
-------------------------------------------------------------------------------REFERENCE QTY DESIGNATION -------------------------------------------------------------------------------
No specific circuit breaker(s) safety clip(s)
B. Referenced Information
-------------------------------------------------------------------------------REFERENCE DESIGNATION -------------------------------------------------------------------------------
24-41-00-861-002 Energize the Aircraft Electrical Circuits from the External Power 24-41-00-862-002 De-energize the Aircraft Electrical Circuits Supplied
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