Engine Fire Extinguishing and the ENG FIRE Panel

Once a fire is detected (previous article) the aircraft does two things: extinguish and prevent spread. The ENG FIRE panel is the master switch for both. This article takes apart the action chain after detection — what the guarded red pushbutton isolates, how the two Halon bottles are fired and spaced, and why the first bottle is preceded by a ten-second wait. The full airmanship of when to fire, ground versus flight, and evacuation is in the abnormal article.

1. Two functions — and half the work is isolation, not extinguishing

"The system has two main functions: - to extinguish any fire in the protected zones of the nacelle as soon as possible... - to prevent engine fire extension: the engine is isolated from the rest of the aircraft ; also the different supplies such as hot air, fuel, hydraulics, electrical power are cut off."

[!warning]- Half the extinguishing system does not extinguish — it starves The Halon puts the fire out, but "prevent extension" is achieved by cutting the engine's fuel, hydraulics, bleed air and electrical power. Starving the fire matters more than dousing it: with a live fuel source, no amount of Halon holds. The core value of the ENG FIRE pushbutton is therefore to cut off everything that can burn; Halon is the finishing blow. Understand this and the eight linkages below are all logic, not memorisation.

2. The bottle — 7.3 kg of Halon 1301 on a nitrogen spring, twin-filament cartridge

Each engine has two bottles in the aft pylon; the bottle is a high-rate discharge type, comprising a spherical container, a discharge head and a cartridge.

"(a) Fire extinguishing agent - type: Halon 1301 (bromotrifluoromethane, CBrF3) - quantity: 16.00 to 16.10 lbs (7.26 to 7.30 kg) - pressurized by nitrogen (N2), quantity 1.05 lb. (0,48 kg maximum)"

The Halon is the agent, the nitrogen is the propellant: 800 psi of nitrogen behaves as a spring, and when the cartridge fires it hurls all 7.3 kg out at once — hence "high-rate discharge." The cartridge itself is deliberately redundant:

"The electro-pyrotechnic cartridge contains 300 mg of explosive powder. The powder is fired by two filaments supplied with 28VDC. Each filament is connected to a ground wire and can supply the electrical power necessary to the firing if the other filament fails. This design ensures redundancy of the fire extinguisher bottle percussion system."

[!warning]- One cartridge, two filaments, two supplies Extinguishing is a one-shot event — Halon released cannot be recalled — so the firing path must not have a single point of failure. Two filaments on separate supplies mean a burned wire still fires the cartridge. This is the "dual electric supply per squib" seen in the overview.

The frangible disc serves twice:

"Firing of the cartridge cause rupture of the frangible disc (calibrated metallic diaphragm)... The frangible disc also functions as an overpressure device in case of excessive pressure in the fire extinguisher bottle, i.e. between 2058 and 2100 psi, which corresponds to a temperature of 205° F approximately."

Normally the cartridge ruptures it (agent out); abnormally, an overheated over-pressured bottle ruptures it itself (relief). The pressure switch reports discharge or leak:

"The pressure switch monitors when the bottle is discharged or has a leakage. It is calibrated to 325 psi increasing pressure (filling) and to 200 psi decreasing pressure (discharge or leakage)."

[!warning]- An amber DISCH light does not necessarily mean "fully discharged" The switch trips on bottle pressure dropping below 200 psi — which happens on discharge (normal) and on a slow leak (fault). So DISCH-amber has two causes: you fired the AGENT, or the bottle has quietly lost its charge and should have been caught on the walk-round. On the ground the ENG FIRE panel DISCH light should be out; if it is on, a bottle has lost pressure.

3. The ENG FIRE pushbutton — one press, eight linkages

The single item worth committing to memory: releasing the guarded red button does eight things for that engine.

"When the flight crew pushes it, the PB-SW is released and sends an electrical signal that performs the following for the corresponding engine: - Silences the aural fire warning - Arms the fire extinguisher squibs - Closes the low-pressure fuel valve - Closes the hydraulic fire shut off valve - Closes the engine bleed valve - Closes the pack flow control valve - Cuts off the FADEC power supply - Deactivates the IDG."

Grouped by purpose, they confirm that half the job is starving the fire:

[!warning]- The ENG FIRE pushbutton also closes the air-conditioning pack Closing the pack flow control valve is unexpected — most would not link "engine fire" to "air conditioning." The logic: the affected engine's bleed feeds the pack, so if the pack is not closed, hot air or smoke could travel bleed → pack → cabin. Closing the bleed valve and the pack flow valve is a double block on the fire → cabin path.

The red light follows the warning, not the pushbutton:

"The red lights come on, regardless of the PB-SW position, whenever the fire warning of the corresponding engine is activated."

A second, same-source red light sits on the ENG MASTER panel (125VU):

"This light identifies the engine to be shutdown in the case of fire. The light comes on red when an engine fire warning is triggered."

The two red lights — 255VU pushbutton and 125VU ENG MASTER — cross-confirm which engine, guarding against shutting down the wrong one in a symmetric layout.

4. AGENT, SQUIB, DISCH — and the two-bottle timing

The AGENTs only become live after the ENG FIRE pushbutton:

"Both AGENT pbs of an affected engine become active when the flight crew releases the ENG FIRE pb-sw. A brief push on the PB discharges the corresponding fire agent. - 'SQUIB' comes on white when the flight crew releases the associated ENG FIRE pb-sw to help the flight crew identify the AGENT pb to be activated - 'DISCH' comes on amber when the corresponding fire extinguisher bottle has lost pressure."

[!warning]- No ENG FIRE pushbutton, no AGENT The AGENT is inert until the pushbutton is released — a hardware interlock enforcing "starve first, then douse." The white SQUIB legend is the system actively pointing to the AGENT to press: another dark-cockpit exception, built to stop a wrong press in a rushed moment.

The two-bottle sequence is spelled out in the AMM system description and the ECAM procedure:

"During the extinguishing procedure, the extinguishing agent flows in the rigid pipes and sprays the fan, compressor and core compartments. Thirty seconds later (after discharge of the first bottle), if the fire is still present, the pilot fires the second bottle."

Before the first bottle there is a ten-second wait, from the ECAM procedure:

"AGENT 1 AFT 10 S ... DISCH — Wait 10 seconds: this is the time required for the engine to reach the windmilling rate in flight. At this rating the efficiency of the fire extinguishing procedure is optimum."

[!warning]- The sequence is 10 s then AGENT 1, then 30 s then AGENT 2 — not "30/30" Ten seconds before bottle 1 lets the engine spin down to windmilling rate: at high rpm the nacelle airflow is too strong and would blow the Halon away; at windmilling rate the airflow is weak enough for the agent to build concentration — which is why ENG MASTER OFF precedes AGENT. Thirty seconds from bottle 1 to bottle 2 is an observation window: the first bottle is usually enough, and the second is fired only if the fire persists. The second bottle is conditional, not automatic.

5. Operating the panel — order, and the self-test

The ECAM ENG FIRE skeleton (decision-making in the abnormal article):

  ENG x FIRE (red) + CRC + MASTER WARN + 255/125VU red
   1  THR LEVER ......... IDLE
   2  ENG MASTER ........ OFF     close HP+LP fuel, shut down (FADEC cut)
   3  ENG FIRE P/B ...... PUSH    8 linkages: starve + isolate + arm AGENT + silence CRC
   4  AGENT 1 AFT 10 S .. DISCH   wait 10 s (windmilling) -> fire bottle 1
   5  ATC ............... NOTIFY
   6  IF FIRE AFTER 30 S: AGENT 2  DISCH

The order "thrust → master → fire pb → agent" is deliberate: THR IDLE + MASTER OFF spin the engine down to windmilling (so the Halon will work ten seconds later) and cut the biggest fuel source; the FIRE pushbutton then isolates everything else; only then does Halon finish the job. Starve always precedes douse.

The TEST pushbutton drives a full annunciation, on the ground or in flight:

"This PB tests the operation of the fire detection and extinguishing system for the engines. When pressed: - A CRC sounds - The MASTER WARNING lights flash - The ENG FIRE warning appears on ECAM - On the ENG FIRE panel: The ENG FIRE pb-sws light up red / The SQUIB lights come on white if discharge supplies are available / The DISCH lights come on amber - On the ENG MASTER panel: The FIRE lights come on red."

The key read is "SQUIB lights come on white if discharge supplies are available": a SQUIB that fails to light during test means a firing-circuit fault — the point the test article develops.

Self-test

[!note]- Q1. Which of the two functions is not really "extinguishing", and why does starving matter more? Preventing spread by isolating the engine (cut fuel/hydraulics/bleed/electrical). A live fuel source defeats any amount of Halon, so cutting the supplies is primary; Halon finishes.

[!note]- Q2. What do the Halon and the nitrogen do? Why two cartridge filaments? Halon 1301 is the agent; nitrogen (800 psi) is the propellant for high-rate discharge. Two filaments on separate supplies give single-fault-tolerant firing of a one-shot bottle.

[!note]- Q3. What eight things does the ENG FIRE pushbutton do, and why close the pack? Silence warning; arm squibs; close LP fuel valve; close hydraulic fire shut-off; close engine bleed valve; close pack flow control valve; cut FADEC power; deactivate IDG. The pack is closed to block hot air/smoke travelling bleed → pack → cabin.

[!note]- Q4. What is the 10 s before AGENT 1 for? What is the 30 s? Is bottle 2 automatic? 10 s lets the engine reach windmilling rate so airflow does not blow the Halon away. 30 s is the observation window before firing bottle 2. Bottle 2 is conditional — fired only if the fire persists.

[!note]- Q5. Give the two reasons a DISCH light can be amber. What should the panel look like on the walk-round? Fired the AGENT (normal), or a bottle has leaked below 200 psi (fault). Pre-flight the DISCH light should be out; on means a bottle has lost pressure.

Key takeaways

References

• FCOM DSC-26-20-20 — ENG FIRE pushbutton eight linkages, AGENT/SQUIB/DISCH, red-light logic, ENG MASTER light, TEST pushbutton.
• AMM 26-21-00 — two functions (extinguish + isolate), bottle (Halon 1301, nitrogen, twin-filament cartridge, frangible disc, pressure switch 325/200 psi), two-bottle sequence, ECAM procedure with 10 s windmilling wait.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.