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APU FIRE DETECTION -FIRE DETECTION CARD -APU
ATTENDED MODE -AND LOGIC
LOOP 1 LOOP 2 CARD OUTPUT
FAULT FIRE ALARM
FAULT FAULT FAULT -BOTH LOOPS
If one loop reports a fire and the other loop is normal, a 15 second timer starts. During this time, if the loop status changes from fire to normal, the timer stops. If the status of the normal loop changes to fire, the card sends a fire alarm.
If neither loop changes status in 15 seconds, the card does a test of the normal loop. If the test fails, the card removes the selection of that loop and sends a fire alarm. If the test passes, the card removes the selection of the other loop and reports a fault.
Unattended Mode Operation
The program logic device operates with OR logic. A fire at loop 1 OR loop 2 causes fire alarms. The table shows alarm and fault conditions:
UNATTENDED MODE -OR LOGIC
LOOP 1 LOOP 2 CARD OUTPUT
NORMAL NORMAL -
FIRE NORMAL ALARM
FIRE FIRE ALARM
NORMAL FIRE ALARM
EFFECTIVITY BEJ ALL
UNATTENDED MODE -OR LOGIC
LOOP 1 LOOP 2 CARD OUTPUT
FAULT NORMAL FAULT -LOOP 1
NORMAL FAULT FAULT -LOOP 2
FIRE FAULT ALARM
FAULT FIRE ALARM
FAULT FAULT FAULT -BOTH LOOPS
The unattended fire alarm signal causes the fire extinguishing system to operate automatically.
Built-In-Test Circuit
BIT continuously monitors the fire detection circuits. The ARINC 429 transmitter sends failure data to the AIMS as soon as BIT finds the condition.
BIT also does a test of the full fire detection system:
*
At power-up
*
After power interrupt
*
Every five minutes of operation.
BIT signals a fire on both loops at the same time and does a check of the system. The ARINC 429 transmitter sends a fire alarm signal to AIMS. The central maintenance computing system (CMCS) uses this signal to make sure the fire detection system transmits ARINC data. Fire warnings and APU shutdown do not occur during the tests.
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APU FIRE DETECTION -FIRE DETECTION CARD -APU
If there is a failure, the ARINC 429 transmitter sends the data to the AIMS.
The system changes to single loop operation if necessary.
The fire/overheat test switch starts a similar test. If there is a failure, the ARINC 429 transmitter sends the data to the AIMS. Fire warnings occur during this test.
Training Information Point
The engine and APU fire detection cards function differently but are interchangeable. Program pins in the cardfile have an interface with the channel/slot selection logic circuit. This circuit tells the card whether to function as an APU or engine fire detection card.
The fire detection cards are electrostatic discharge sensitive (ESDS) devices. Be careful to obey the procedures in part II of the maintenance manual (Standard Practices -Airframe, Electrostatic Discharge Sensitive Devices -Maintenance Practices) before you touch the cards.
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777 AIRCRAFT MAINTENANCE MANUAL
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APU FIRE DETECTION -FUNCTIONAL DESCRIPTION -FLIGHT DECK INDICATION CIRCUIT
Attended Mode
If there is an APU fire, the APU fire control card supplies fire alarm 1 and fire alarm 2 outputs. Alarm 1 makes a ground for these:
*
The fire switch unlocking solenoid
*
The APU fire warning light
*
The nose gear electrical service panel circuit.
Alarm 2 supplies a ground signal to these:
*
The APU controller
*
The ELMS
*
The warning electronics system (WES)
*
The AIMS.
The APU controller and ELMS shut down the APU. The WES turns on the master warning light and the fire warning aural. The AIMS shows an EICAS warning message.
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