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.
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ENGINE FIRE DETECTION -FIRE DETECTION CARDS
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.
Temperature Monitor Circuit
This circuit measures the average temperature of the fire detection loops. The ARINC 429 transmitter sends temperature data to AIMS.
The circuit also compares average temperature to an overheat set point kept in memory. If the temperature is more than the set point for 9.6 seconds, the temperature monitor circuit sends an overheat alarm through the ARINC 429 transmitter to AIMS.
Software in the temperature monitor circuit selects single or dual loop logic as applicable.
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 complete engine fire detection system at these times:
* 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 CMCS uses this signal to make sure the fire detection system transmits ARINC data. Fire warnings do not occur during the tests.
If there is a failure, the ARINC 429 transmitter sends fault data to the AIMS. The system changes to single loop operation if necessary.
The fire/ovht test switch in the flight deck starts a similar test. If there is a fault, the ARINC 429 transmitter sends fault 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 interface with the channel/slot selection logic circuit. That circuit tells the card whether to function as an APU fire detection card or as a left or right 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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ENGINE FIRE DETECTION -FIRE DETECTION CARDS
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ENGINE FIRE DETECTION -FUNCTIONAL DESCRIPTION
Fire Detection
If there is a fire in an engine nacelle, the fire detection card supplies three fire alarm outputs. Alarm 1 supplies a ground for these components:
*
The fire switch unlocking solenoid
*
The engine fire warning lights
*
The fuel control switch fire warning light.
Alarm 2 supplies a signal to AIMS and to the warning electronics unit. An EICAS warning message shows. The master warning lights and fire warning aural come on.
A third fire alarm output goes to the AIMS through the ASG (ARINC signal gateway) cards. The CMCS uses this signal to make sure the engine fire detection system transmits ARINC data.
The indications go away when the fire is out.
Overheat Detection
If the fire detection loops find an overheat condition, the fire detection card sends a signal to AIMS through the ASG cards. An EICAS caution message shows. The master caution light and caution aural come on. When the overheat condition is gone, the indications go away.
Nacelle Temperature Recording
The fire detection card measures the average temperature of the engine fire detectors. This data is sent to AIMS through the ASG cards. The ACMF (airplane condition monitoring function) of the AIMS puts this data in memory.
Single/Dual Loop Operation
The fire detection card monitors the loops for faults. In normal (dual loop) operation, both loops must have a fire or overheat condition to cause the flight deck indications.
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