(1)
With 28-volts dc power applied to the system, current is sent from each control card to the detectors in the corresponding loop. Each detector element has a resistance value, and each loop has an equivalent resistance value. The control card senses a loop voltage potential.
(2)
As temperature increases, the detector resistance decreases. When detector element resistance drops to 450 ohms, a timer on the control card starts. If the resistance drops to 250 ohms or below before the timer has run out, a fault indication is passed on to the AFOLTS card. If the timer has already expired before the resistance drops to 250 ohms, a fire indication will be given when the detector drops below 250 ohms.
(3)
A ground in the circuit between control cards and detector elements will cause an immediate fault whereas an open circuit will cause a fault when a test is performed.
(4)
A ground or an open in the circuit between control cards and detector elements will cause an immediate fault.
B.
An engine fire indication is given by the following:
(1)
The appropriate red (1, 2, 3, and 4) fire switch handle light, on the P5 pilot's overhead panel, illuminates and unlocks.
(2)
The appropriate red (1, 2, 3, or 4) fuel control switches, on P8 pilot's control stand illuminate.
(3)
The appropriate fire warning messages are displayed on EICAS.
(4)
The red master warning lights on the P72 main instrument light shield panel illuminate.
(5)
The fire bell sounds on the flight deck aural warning speakers.
C.
An engine overheat indication is given by the following:
(1)
The appropriate overheat caution messages are displayed on EICAS.
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(2)
The amber caution lights on the P72 main instrument light shield panel illuminate.
(3)
The caution annunciation tone sounds on the flight deck aural warning speakers.
D.
A dual-loop fault indication is given by the following:
(1)
The advisory messages are displayed on EICAS.
E.
Detector, card or associated wiring features may not provide a valid fault output at the moment of failure. These inoperative loops can only be detected during a operational test.
F.
The CMC can initiate the fire detection system ground test if either of the ground tests switches is enabled and the CMC test relays are closed. The CMC initiates a ground test to determine the integrity of the fire detection card file and its inputs/outputs using the LRU's built-in test equipment.
G.
When the ground test is initiated, the CMC outputs five simultaneous test initiating signals which activate five test relays. These test relays send various discrete signals to the fire detection card file. From these inputs, the fire detection card file generates fire, overheat, and fault detection discretes which it sends to the EIUs and MAWEA. The MAWEA sends signals received from the fire detection card file to the CMC on an ARINC 429 bus, and the MAWEA also sends signals received from the fire detecteion card file to the EIUs or two ARINC 429 buses. While the test is in progress, the CMC monitors all fire detection system interfaces to the EIUs and the MAWEA. If the CMC detects an inoperative interface, it displays an appropriate fault message.
H.
The test relays can also be activated by pressing FIRE/OVHT test switch on the M7317 (P5) miscellaneous switching panel, but the test initiated by the CMC is more effective at isolating failures. The FIRE/OVHT test switch on M7317 tests critical wiring paths, while the CMC tests all wiring paths and isolates to the failed interface with a text message.
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