Overview
The function of the fire extinguishing system is to extinguish an engine or auxiliary power unit (APU) fire. The fire extinguishing system has containers that hold the fire extinguishant and has the applicable components to send the agent to the correct location.
07/02/20
Fire Extinguisher Containers
There are two containers in the aft equipment compartment. Each container stores the Halon 1301 fire extinguishant. The left container is the container No. 1 and the right container is the container No. 2. The two containers are the same in dimension and in capacity.
Each container has three outlets sealed by a frangible disk. The three outlets of the container No. 2 have a discharge head and a discharge cartridge installed. Two outlets of the container No. 1 have a discharge head and a discharge cartridge installed. The third outlet of the container No. 1 has a cap because it is not used.
Fire extinguishing for the left and right engines is performed by two fire extinguisher containers. Fire extinguishing for the APU is performed by a third outlet on the container No. 2. The extinguishant quantity released when the disk breaks is made to supply the full content of a container to extinguish the fire.
Note:
The Halon 1301 is a gaseous flooding agent and it does not produce any residue after discharge and therefore does not cause secondary damage.
Discharge Heads
The discharge heads are on the fire extinguisher containers. They connect the discharge lines and discharge cartridges to the fire extinguisher containers. The discharge heads are installed with swivel nuts and can turn to align with the discharge lines. Also, the discharge head has a contamination screen to keep the frangible disk pieces out of the lines.
Discharge Cartridges
The discharge cartridges are in the discharge heads. Each discharge cartridge has two bridge wires (A and B). The bridge wires start an explosive device in the discharge cartridge that breaks the frangible disk and releases the content of extinguishant into one of the two engine compartments or the APU compartment for fire extinguishing.
Cap
A cap is installed on the unused outlet of the fire extinguishing container No. 1. This protects the frangible disk from damage.
Two-Way Check Valve
The two-way check valves are in the engine discharge lines. These prevent the extinguishant from going from one container to the other container when the fire extinguishant of a container is released.
Engine Discharge Lines
The engine discharge lines come from two outlets of the two containers. For the left engine, one outlet of each container connects to a two-way check valve. From this check valve, the discharge line goes to the left engine discharge tee. For the right engine, one outlet of each container connects to a two-way check valve. From this check valve, the discharge line goes to the right engine discharge tee. The lines are pressurized only when the extinguishant is released. The lines ensure that condensation cannot collect in them.
Engine Discharge Tee
The engine discharge tee is at the end of the discharge lines that go from the containers to each nacelle. The discharge line goes through the pylon firewall forward of the forward engine mount on each side of the aircraft. The discharge tee is installed vertically to supply the extinguishant up and down into the forward area of the nacelle.
APU Discharge Lines
The APU discharge lines come from the third outlet of the container No. 2. The line divides through a tee fitting located forward of the APU forward firewall. From this tee fitting, two discharge lines attach to the firewall at two positions. There are nozzles on the other side of the firewall connected to these lines. The lines are pressurized only when the extinguishant is released. The lines are made to make sure that condensation cannot collect in them.
APU Discharge Nozzles
The APU discharge nozzles are at the end of the APU discharge lines on the APU forward firewall. There is a bottom and a top nozzle. The bottom nozzle supplies extinguishant to the bottom right side of the APU. The top nozzle supplies the extinguishant along the top left side of the APU compartment. This configuration gives a balanced flow of extinguishant into the fire zone around the APU.
11/13/15
Operation
When there is a fire in one of the engines or the APU, a signal goes to the FIREX-CU. The FIREX-CU sends visual and aural warning indications to the EICAS, ENGINE control panel, WARNING CAUTION panel and aural warning system. Then, the flight crew selects the applicable location to release the extinguishant and which source supplies the extinguishant The discharge of the two fire containers to the engines is controlled exclusively through the flight compartment pushbutton annunciators (PBAs) located on the ENGINE control panel on the center pedestal. For the two engines, two PBAs are necessary to release the container. The first PBA, called L (R) ENG FIRE, is a guarded PBA which shuts down the engine and arms the related discharge cartridges on the container. The second PBA, called FIRE EXT 1 (2) ARMED, is a momentarily switch that applies discharge current to the selected cartridge.
When the L (R) ENG FIRE PBA is pushed, the following steps occur:
- L (R) engine fuel shutoff valve (SOV) closes
- L (R) engine cartridge on the container No. 1 and No. 2 arms
- Fuel metering valve closes and re-ignition of L (R) engine is inhibited
- Hydraulic SOV closes
- Bleed valve and high-pressure valve closes
- The FIRE EXT 1 (2) ARMED PBA 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.
These steps revert to their usual condition when the L (R) ENG FIRE PBA is back to the standby position. The discharge of the container No. 2 to the APU compartment is controlled exclusively by the ENGINE control panel PBAs during flight. The discharge to the APU on the ground can be controlled by one or the other flight compartment PBA or automatically by the FIREX-CU during APU unattended mode operation. For the APU, two PBAs are necessary to release the container.
The first PBA, called APU FIRE, is a PBA with a guard which shuts down the APU and arms the discharge cartridge on the container No. 2. The second PBA, called FIRE EXT 2 ARMED, is a momentary switch that applies discharge current to the selected cartridge. On the ground, discharge to the APU can be manually started from the ENGINE control panel with the use of the PBA or automatically by the FIREX control unit in APU unattended mode. In the APU unattended mode, the APU is automatically shut down. After a 5-second delay, it discharges the APU fire extinguishing container. Activation of the FIRE EXT 2 ARMED PBA or loss of the WOW signal overrides the APU unattended mode and gives control back to the flight compartment. When the APU FIRE PBA is pushed, the following steps occur:
- APU fuel SOV closes and APU shut down
- APU generator turns off
- FIRE EXT 2 ARMED PBA is on
- APU cartridge on container No. 2 is armed
These steps revert to their usual condition when the APU FIRE PBA is back to the standby position.
Power Supply
Each discharge cartridge has two bridgewires (A and B). Only one of the two bridgewires is necessary for the ignition of the discharge cartridge explosive device. The L BATT BUS and R BATT BUS supply 28 VDC electrical power to the discharge cartridge. Each power supply is connected in series with two PBAs to arm and release the container. For the engine containers, one end of each bridgewire is connected to the discharge switch. The other end of the bridgewire is connected to ground. When the system is armed, discharge switch activation sends 28 VDC electrical signals to the bridgewires. The bridgewires then ignite the discharge cartridge to release the applicable FIREX container.
Displays And Alarms
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 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 fire extinguishing container No. 1 (2) pressure is low.
The EICAS messages that follow are related to the fire extinguishing system:
| EICAS MESSAGE(S) | LEVEL (COLOR) |
|---|---|
| FIREX APU SQUIB 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) |
System Monitoring
The FIREX-CU performs cold start built-in test and continuous BIT. This monitors the health and status of the engine/APU fire extinguishers. All FIREX containers are monitored through the FIREX-CU with an alarm gauge, using an internal pressure switch. The normally open switch is closed when the container is pressurized. A monitor circuit in the FIREX-CU checks for continuity of the pressure switch. When the monitor circuit senses an open circuit (container release or leakage), the FIREX-CU gives fault signals to the data concentrator unit (DCU) to supply EICAS messages. This signal shows a container released or a low-pressure condition.
A current-limited monitor circuit in the FIREX-CU checks for continuity of the bridgewires. When the monitor circuit detects an open in the two bridgewires in a single cartridge, it sends signals to the DCU to supply EICAS messages.
10/15/20