-
the FIRE comparator
-
the CONTAMINATION comparator
-
the INTEGRITY comparator
(b)
Output signals The output signals are generated via discrete signals and/or the ARINC 429 bus. The fire warning signals (aural and/or visual) thus generated are transmitted to the cockpit.
(2)
Monitoring circuitry The monitoring circuitry analyses and monitors continuously the fire detection system.
In case of failure of the system the monitoring circuitry:
-memorizes the fault in a non-volatile memory,
-isolates the faulty channel,
-generates the appropriate discrete signals,
(i.e. LOOP A(B) INOP ENG 1 (2) to the FWC 1(2))
-transmits continuously a system status message to the CFDIU 1(2) via the ARINC 429 bus.
Fire Detection Unit (FDU)
Figure 009
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7. Operation/Control and Indicating
________________________________
R A. Fire Detection Unit R (Ref. Fig. 010)
(1) Operation of the channels
Channels A and B are identical and particular to each fire detection loop.
The input section of each channel comprises a bridge circuit with:
-
a reference voltage,
-
a variable loop voltage made of the three fire detectors in parallel.
The reference voltage is sent to the three comparators (FIRE, INTEGRITY and CONTAMINATION) and forms the thresholds values. A resistor, upstream of each comparator, adjusts this reference voltage which becomes the comparator threshold value.
Each fire detector resistance is 4.5 Kohms, thus the equivalent resistance of each fire detection loop is 1.5 Kohms.
The equivalent resistance varies when a change of state occurs in the monitored areas of the engine. This generates a variable loop voltage at the three comparators.
The comparator outputs are sent on the FIRE and FAULT logic gates. The logic gates generate the alarm outputs.
(a) Normal conditions
In normal conditions (no failure, no fire, no test), the variable voltage of the loop is:
-higher than the threshold 1 of the integrity comparator
-lower than the threshold 2 of the fire and contamination thresholds.
Fire Detection Unit (FDU) - Logic Diagram
Figure 010
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(b)
FAULT Circuit (Ref. Fig. 010)
(c)
FIRE circuit (Ref. Fig. 010) The detection of a fire by one of the responders causes the closure of the corresponding ALARM switch.
1_ INTEGRITY Fault
Any failure of the fire detector (responder/sensing element)
causes an increase of the equivalent resistance of the three
other fire detectors.
A detector can be unserviceable because of:
. the opening of a MONITOR switch installed in series with an
integral resistance,
. or the loss of the electrical signal. The loop voltage
decreases and falls under the threshold 1 of the INTEGRITY
comparator: this generates a LOOP A(B) INOP signal.
This failure description is also applicable in case of
accidental grounding of the responder.
2_ CONTAMINATION Fault
If the responder or the connectors of FDU are contaminated:
. the equivalent resistance decreases
. the loop voltage is higher than threshold 2 and lower than 3
. the CONTAMINATION comparator supplies the logic gates and
generates a FAULT signal.
This generates a voltage higher than the threshold of the INTEGRITY and CONTAMINATION comparators. The FIRE comparator supplies the logic gates and transmits a FIRE signal.
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(2)
Controller circuit The controller circuit operating software:
-monitors the two detection loops,
-isolates the failed detector and loop circuit and memorizes the failures in a non-volatile memory,
-
does a check of the fire test circuity when it is activated,
-
does the self-test at the first power-up of the FDU,
-
does the built-in test and transmits the test results on the ARINC 429 bus,
-transmits the failure signals to the CFDIU via the ARINC 429 bus,
-continuously transmits current and/or previous system status on the ARINC 429 bus,
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