The engine fire-detection system senses and gives indications in the flight compartment when a fire occurs in the left or right power plants.

The engine fire-detection system has dual-loop sensing elements that operate independently. Each loop of the dual-loop sensing elements is a full fire detection system. They are installed in parallel on three rails at different areas of each power plant.

The resistance between the inner wire of the sensing elements and the outer casing decreases as the temperature increases. The fire detection and extinguishing (FIDEEX) control unit monitors the resistance of each loop. When the resistance is at a preset value, the control unit sends a signal to the data acquisition unit (DAU). The DAU causes the engine indicating and crew alerting system (EICAS) to show the applicable warnings in the flight compartment. The EICAS also energizes the applicable fire handle lamp, gives an aural warning and a voice warning. When the fire is extinguished, the system removes the warning and sets itself again.

When there is a fire, the two loops give a fire warning (AND logic). If one loop is unserviceable, the FIDEEX control unit sends an error signal to the EICAS in the flight compartment. The FIDEEX control unit then sets the engine fire-detection system to a one loop operation. The control unit also sends the information to the centralized aircraft information maintenance system (CAIMS) for troubleshooting.

Engine Fire Zones

The engine is divided into two fire zones: Zone 1 and Zone 2.

Zone 1 is the area between the outer cowlings and the fan structural bypass duct. The VFGs, fuel pump/metering unit, oil pump, hydraulic pump and their pipes are located in Zone 1, with the associated potential for fires.

Zone 2 is the area between the engine outer case and the engine core fairings. Fuel leakage from the VSV actuator, OSU, fuel manifolds and nozzles are the potential fire hazards in Zone 2.

Locations of Engine Fire-Detection Elements

There are two separate parallel engine fire detection "loops" per engine, called Loop A and Loop B. Each loop is made up of three detectors which are located on each engine as follows:

• Gearbox-Mounted Fire-Detection Rails
• Fixed Cowl-Mounted Fire-Detection Rails
• Core Engine Fire-Detection Sensors

Gearbox-Mounted Fire-Detection Rails

Each gearbox-mounted fire-detection rail has a section of the two fire detection loops attached in parallel to it. The rail is then attached behind the engine gearbox. One end of the elements connects to the fixed cowl elements through wires. The other end connects to the engine core sensing elements through wires.

Fixed Cowl-Mounted Fire-Detection Rails

Each fixed cowl-mounted fire-detection rail has a section of the two fire detection loops attached in parallel on a rail. The rail is then attached to the aprons. One end of the elements connects to the gearbox mounted elements through wires. The other end connects to the control unit through wires.

Core Engine Fire-Detection Sensors

Each engine core has a dual-loop sensing elements installed on rails. The rails are installed on the bottom of the high pressure turbine casing of each engine. For access, you must remove the bypass-duct access panels and then the combustion section fairings. One end of the elements connects to the gearbox mounted elements through wires. The other end connects to the control unit through wires.

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Engine Fire-Detection System

The fire detection systems of the two engines are not connected to each other in any way. On each loop of each engine, the core wires of all three elements that form the loop are connected in series.

The free ends of the first and the last of the three core wires (in each loop) are connected separately to the aircraft wiring harnesses at the pylon firewall. Rails are used to provide structural support to two of the elements on each engine, the third element being directly attached by rubber clamps to brackets on the core engine.

Approximate trip points of the engine core fire detectors are respectively 432 °C/809 °F (LOOP A) and 416 °C/780 °F (LOOP B), 239 °C/462 °F for the accessory gear box and 232 °C/449 °F for the APRON.

In normal operation, the fire warnings of an engine will only be triggered when both detectors, loops A and B, of that engine sense a fire condition.

Note that one or more shorted or open detectors in loop A or loop B will make that loop unserviceable. In such a case, the control unit will automatically:

• Reschedule the warnings to be triggered by the one remaining loop
• Isolate the shorted loop from the warning systems and therefore
• Prevent the shorted loop from triggering a false alarm, and
• Post an advisory CAS (cyan) message of L/R ENG FIRE FAULT

An open circuit in loops A or B will render that loop functionally inoperative. Therefore, the Fire Detection and Extinguishing Control Unit (FIDEEX CU) will isolate the loop from triggering the warnings. However, it will post an advisory (cyan) message reading L/R ENG FIRE FAULT on the EICAS primary page.

A short/open in both of the loops A and B will render the fire detection system on that engine entirely inoperative. The control unit will then post the associated amber CAS caution message L/R ENG FIRE FAIL.

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The following common points must be observed during maintenance/troubleshooting with respect to the engine, APU and wheel well fire detection systems:

• Particular care should be taken during inspection of firewire loops to ensure that the insulating medium does not seep out of the wire, nor should foreign matter be allowed to contaminate the medium, since the trip points of the elements can become downgraded in either of these conditions 
  
  
• The element at each location is specially tailored to maintain a specific resistance value to ensure that its trip point is within acceptable limits. Therefore, replacement wires must match the specification for the particular location where it is installed 
  
  
• The NVM of the FIDEEX CU records all significant faults for future retrieval by the airplane's CAIMS system, and applicable maintenance action as prescribed in the AMM must be undertaken as specified, within time limits imposed

Caution:
In order to preclude the failure of firewires in service, it is important to handle all detector elements with extreme care and to clamp them properly to prevent their vibration or chafing with other components.

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The various warnings and the associated detection systems can be function-tested using either one of the two EMS CDUs (Electrical Management System Control Display Units) in the cockpit, by first selecting FIRE PROTECTION System and then TEST.

Note that the sensor elements are not physically heated to verify their operation during this test. Instead, a resistance value (equivalent to the trip value of the detectors) is used by the control unit to simulate a "trip" of the loops. However, a check for continuity in the core wires, and for an absence of shorts in the elements, along with an integrity check of the entire circuits is performed by the FIDEEX CU during this test.

Serviceability of each system is confirmed if the related CAS and aural warnings, as well as the red lamps in the three fire handles, are activated during the test.

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Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
-	GEARBOX-MOUNTED FIRE-DETECTION RAILS	ZONE(S) 430/440	26-11-01 [ GX ] [ GXRS ] [ G5000 ]
-	FIXED-COWL-MOUNTED FIRE-DETECTION RAILS	ZONE(S) 430/440	26-11-05 [ GX ] [ GXRS ] [ G5000 ]
-	CORE-ENGINE FIRE-DETECTION SENSORS	ZONE(S) 430/440	26-11-17 [ GX ] [ GXRS ] [ G5000 ]

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