The fire detection and extinguishing (FIREX) system monitors and provides indications for the FIREX system. The FIREX system provides fire, overheat and smoke detection, monitoring, built-in test (BIT), health and status indication, and fire extinguishing functions.

---

The fire detection and extinguishing control unit (FIREX-CU) is installed on mounting brackets in the aft equipment compartment. The FIREX-CU is attached to the mounting brackets with six fasteners. The FIREX-CU electronic components are put in an aluminum enclosure. There are three connectors on the FIREX-CU that are different from one to the other to prevent incorrect installation. There is a smaller connector and two other larger connectors of the same dimension. The two larger connectors have a different keyway for installation in the correct position.

The FIREX-CU receives 28 VDC power from the aircraft L ESS, R ESS, L BATT, and R BATT buses for detection and monitoring functions.

The FIREX-CU monitors and reports the condition and status data for the full FIREX system. It monitors and supplies status of all sensor inputs from the fire, overheat, and smoke detectors. It monitors the health status of internal circuits (channel A and B) inputs, processes, and outputs signals.

It monitors and reports the status of the engine and auxiliary power unit (APU) fire extinguishing containers. It transmits engine hydraulic and bleed-air shutoff commands from the flight compartment pushbutton annunciators (PBAs) to the related shutoff valves (SOVs). It operates the fire extinguishing system for the APU in unattended mode.

---

The FIREX-CU has two modes of operation, dual-loop and single-loop logic detection. In the dual-loop logic detection mode, the A and B loop signals are necessary to the FIREX-CU to cause a zone fire alarm. In the single-loop logic detection mode, only one fire signal from an element is necessary to cause a fire alarm. The FIREX-CU automatically changes to the single-loop logic detection mode in failure conditions (loss of a single channel or loop).

The engine and APU fire detection loops are usually monitored in dual loop logic. The main wheel well (MWW) overheat detection loop is monitored in single-loop logic mode.

The smoke detection system is monitored by the FIREX-CU for unusual conditions.

When zone fire conditions are met, the FIREX-CU transmits the applicable fire warning to the EICAS through the ARINC 429 and hardwired discretes. Also, faults on the detector loops are identified by the FIREX-CU with indications supplied to the EICAS through the DCU.

Functions

The FIREX-CU has the following functions:

• Monitors fire/overheat loops and smoke detectors for alarm and fault indications.
  
  
• Monitors fire extinguishing container pressure and cartridge bridgewire continuity.
  
  
• Tests system integrity through an automatically and manually initiated BIT and gives system health status to the EICAS and maintenance diagnostic computer (MDC).
  
  
• Receives test commands from the SYSTEMS TEST control panel and does a pilot-initiated test (PIT) on all fire detection loops and smoke detectors.
  
  
• Receives engine hydraulic SOVs and bleed-air (high-pressure valves and intermediate-pressure valves) shutdown commands from the left and right ENG FIRE PBAs and outputs shutdown signals to these devices.
  
  
• Receives APU shutdown command from the APU PBA and outputs shutdown command to the APU ECU.
• When the aircraft is on the ground (only) and there is a fire sensed, it automatically shutdown the APU and after a five second delay, it discharges the APU fire extinguishing container.
  
  
• Transmits ARINC 429 bus fire warning, caution and advisory labels, and discrete indications to the aircraft DCU for display to the flight crew by EICAS.

Interface

The FIREX-CU has interfaces with the data concentrator unit (DCU) to transmit flight compartment indications for alarm and status reporting functions and maintenance data via an ARINC 429 data bus from the FIREX-CU. These interfaces control the following FIREX system indications:

• Display of engine indicating and crew alerting (EICAS) messages
• Illumination of the arming and discharge PBAs and the master warning and master caution lamps using the lamp driver unit (LDU)
• Annunciation of aural warnings and activation of chimes.

The FIREX-CU has interfaces with the aircraft hydraulics, bleed air, and APU ECU to provide shutoff signals. The FIREX-CU has interfaces with the proximity sensor electronic unit (PSEU) for a weight-on-wheels (WOW) signal.

---

The PBAs and switches for the FIREX system are in the flight compartment on different control panels.

ENGINE Control Panel

L (R) ENG FIRE PBA

When the L (R) ENG FIRE is pushed, the steps that follow occur:

• Closes the left/right engine fuel SOV
• Arms the left/right engine cartridge on the container No. 1 and No. 2
• Closes the fuel metering valve and inhibits re-ignition of the left/right engine through the FADEC system
• Closes the hydraulic SOV
• Closes the IPV and HPV
• The FIRE EXT 1 (2) ARMED lamp is on, this gives information that the container has pressure and at least one functional bridgewire to the L (R) engine cartridge on container No. 1 and container No. 2.

APU FIRE PBA

When the APU FIRE is pushed, the FIREX CU:

• Closes the APU fuel SOV and shuts down the APU
• Turns off the APU generator
• FIRE EXT 2 ARMED is on
• Arms APU cartridge on container No 2.

FIRE EXT 1 (2) ARMED PBA

A switch that applies discharge current to the selected cartridge. For the APU, only the FIRE EXT 2 ARMED is used.

SYSTEMS TEST Control Panel

FIRE DET Position

When the test switch is set to the FIRE DET position and is momentarily pushed (approximately 1 second), it starts a pilot initiated test (PIT).

Displays

The APU FIRE warning message will show when one of the fire detection loops on the APU has sensed a fire.

The CARGO SMOKE warning message will show when smoke has been sensed in the cargo (baggage) area.

The L (R) ENGINE FIRE warning message will show when one of fire detection loops on the left (right) engine has sensed a fire.

The GEAR BAY OVHT warning message will show when an overheat condition has been sensed in the main wheel well (MWW).

The APU FIRE DET FAIL caution message will show when the APU fire detection system has failed.

The CARGO SMOKE DET FAIL caution message will show when the baggage compartment smoke detection system has failed.

The L (R) FIRE DET FAIL caution message will show when the left (right) engine fire detection system has failed.

The FIRE SYS FAULT caution message will show when the fire detection and extinguishing system has detected a fault.

The FIREX APU SQUIB FAIL caution message will show when the APU fire extinguishing container squib on bottle No. 2 has failed (no low pressure indication).

The GEAR BAY DET FAIL caution message will show when the MWW overheat detection has failed.

The FIRE SYS FAULT advisory message will show when the FIREX-CU has sensed a fault. The FIREX BTL 1 (2) FAULT advisory message will show when the left (right) container squib for the left or right engine has failed.

The FIREX BTL 1 (2) LOW advisory message will show when the fhe fire extinguishing container No. 1 (2) pressure is low.

The FIRE SYS IN TEST status message will show when the fire detection system is in pilot initiated test.

The FIRE SYS TEST OK status message will show when the fire detection system test is satisfactory.

The EICAS messages that follow are related to the FIREX system:

EICAS MESSAGE(S)	LEVEL (COLOR)
APU FIRE	WARNING (red)
CARGO SMOKE	WARNING (red)
L ENGINE FIRE	WARNING (red)
R ENGINE FIRE	WARNING (red)
GEAR BAY OVHT	WARNING (red)
APU FIRE DET FAIL	CAUTION (amber)
CARGO SMOKE DET FAIL	CAUTION (amber)
L FIRE DET FAIL	CAUTION (amber)
R FIRE DET FAIL	CAUTION (amber)
FIRE SYS FAULT	CAUTION (amber)
FIREX APU SQUIB FAIL	CAUTION (amber)
GEAR BAY DET FAIL	CAUTION (amber)
FIRE SYS FAULT	ADVISORY (cyan)
FIREX BTL 1 FAULT	ADVISORY (cyan)
FIREX BTL 2 FAULT	ADVISORY (cyan)
FIREX BTL 1 LOW	ADVISORY (cyan)
FIREX BTL 2 LOW	ADVISORY (cyan)
FIRE SYS IN TEST	STATUS (white)
FIRE SYS TEST OK	STATUS (white)

---

The FIREX-CU monitors the FIREX system through cold start BIT, continuous BIT, or periodic BIT. The FIREX-CU monitors the following external circuits:

• Container pressure switch status
• Cartridge bridgewire continuity
• WOW status
• PIT switch status
• Fire, overheat, and smoke detection systems status

The FIREX-CU does not monitor the following external circuits:

• Engine and APU fuel SOVs
• Engine hydraulic SOVs, HPV, and IPV status

The FIREX-CU monitors the following internal functions:

• Power supply
• Discrete driver
• Fire detector monitor
• Alarm processing

The FIREX-CU BIT circuits process these inputs to supply flight compartment indications or MDC fault messages. The FIREX-CU BIT monitors the health status of internal circuits (channel A and B) used to input, process, and output signals via the ARINC 429 bus, and discrete data line connections with the DCU. The FIREX-CU continuously monitors internal and external power supply status.

Cold Start BIT

Cold start BIT tests are performed when the microprocessor and associated circuits and power supplies are functional, there are no valid fire/smoke conditions, the software power senses a power outage on FIRE DET A for longer than 200 msec., and the aircraft is WOW. Sensed faults give an EICAS message if fault or condition is caused by a loss of detection or extinguishing function or if the condition is caused by a loss of redundancy. All failures are isolated and transmitted to the MDC.

Continuous BIT

The FIREX-CU continuously monitors the extinguishing system for the following conditions:

• Open circuit conditions on container No. 1 and No. 2 pressure switches that shows loss of extinguishant
• Open circuits conditions on each of the 10 bridgewire circuits used to discharge to the engines and APU
• Loss of power to the discharge cartridges

All these conditions give EICAS messages and give fault data to the MDC. The FIREX-CU continuously monitors the engine and APU fire and the MLG overheat detection systems for the following conditions:

• Loss of one of the two FIREX-CU channel B and FIRE DET B power used to monitor and test the B fire loops
• Loss of one of the two FIREX-CU channel A and FIRE DET A power used to monitor and test the A fire loops
• Fire loop short circuits
• False fire on loop

All these conditions give EICAS messages and give fault data to the MDC.

Periodic BIT

Fire and Overheat Detection

The FIREX-CU does an automatic periodic BIT of the fire and overheat detection systems every 5 minutes. The periodic BIT is equivalent to the PIT in term of functions examined, but not all the feedback is sent to the DCU. The faults display on the EICAS, and fault data is sent to the MDC. During the periodic BIT, the FIREX-CU tests:

• Loop continuity
• Short circuit
• Output discrete drivers functionality
• Loop processing

Smoke Detection

The FIREX-CU performs an automatic periodic BIT of the smoke detection system every 15 minutes. During this test, the FIREX-CU transmits a test command signal to the smoke detector. The smoke detector performs a test and transmits an alarm signal to the FIREX-CU if the test is successful. The smoke detector remains in test while the FIREX-CU transmits the test command signal. If the detector does not give a result to the test command, a failure message is shown on the EICAS.

---

Pilot Initiated Test

A pilot initiated test of the fire detection system can be done with the rotary test switch on the SYSTEMS TEST control panel set to the FIRE DET position. When the test switch is momentarily pushed (approximately 1 second), it starts a test of the entire fire, overheat, and smoke detection system.

The PIT is equivalent to the periodic BIT in terms of functions examined, but during the PIT, all the feedbacks are sent to the DCU. This interface with the DCU gives visual and aural indications in the flight compartment. These indications indicate if the PIT is successful or has failed.

The faults are displayed on the EICAS and fault data is sent to the MDC. During the PIT, the FIREX-CU tests:

• Loop continuity
• Short circuit
• Output discrete drivers functionality
• Loop processing

During this test, the FIREX-CU transmits a test command signal to the smoke detector. The smoke detector performs a test and transmits an alarm signal to the FIREX-CU if the test is successful. The smoke detector remains in test while the FIREX-CU transmits the test command signal. If the detector does not supply a result to the test command, a failure message is shown on the EICAS.

During a PIT, the EICAS messages that follow are shown:

• L ENGINE FIRE (warning)
• R ENGINE FIRE (warning)
• APU FIRE (warning)
• GEAR BAY OVHT (warning)
• CARGO SMOKE (warning)
• FIRE SYS IN TEST (status)

The related visual indications and alarm warnings occur during this test. When all systems have passed the test, the EICAS shows FIRE SYS OK.

---

Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
A91	FIRE-DETECTION-AND-EXTINGUISHING  (FIREX) CONTROL-UNIT	ZONE(S) 310	26-11-01

---