Fire Protection Overview and Philosophy

Fire protection (ATA-26) is the aircraft's immune system. It is silent for the whole of a normal flight, but the moment a compartment overheats or fills with smoke it must detect, locate and neutralise the threat within seconds, and it must force that fact to the crew at the highest priority the aircraft can raise — a red warning and a continuous chime. Unlike the pneumatic or hydraulic systems, fire protection is mostly a self-contained set of detection and extinguishing loops; but when it fires, its reach is among the widest on the aircraft. One engine-fire pushbutton isolates eight systems; a cargo discharge drives the ventilation isolation valves; and on a freighter, fighting a main-deck fire means depressurising the whole aeroplane and reaching into the oxygen system.

This first article lays out the four things worth fixing before any of the detail: what is protected, how it is detected, how it is extinguished, and how the warning reaches you.

1. Active versus passive — this chapter is the moving half

The maintenance manual draws the boundary in its first sentence.

"There are two types of fire protection: - the active fire protection, which detects the start of a fire or smoke, locates and neutralizes it rapidly. - the passive fire protection which is built in the design of each aircraft compartment. NOTE: This chapter describes only the active fire protection."

Passive protection — fire-resistant cargo liners, the engine-pylon firewall, flame-rated structural materials — is the aircraft's true first line, but it lives in structure and design, not in ATA-26. This chapter is the electronic-plus-bottles half that actively senses and acts. When a student asks why a cargo wall does not burn through, that is passive protection; what follows here is "detect the smoke, release the Halon."

2. What is protected — the task list

The FCOM gives the crew-level summary of what the system covers.

"Aircraft Fire Protection Systems Include: - Fire and overheat detection and extinguishing systems for the: Engines, APU - Smoke detection and extinguishing for the: Cargo compartments, Lavatories - Smoke detection for the: Avionic bay - Portable fire extinguishers for the: Flight compartment, Passenger cabin"

The AMM expands the same coverage into an engineering function list:

"The active fire protection system has these functions: - detect and extinguish a fire in each engine nacelle and in the Auxiliary Power Unit (APU) compartment, - protect the engine pylon against torching flames from the combustion chamber, - detect smoke in the avionics compartment, - detect smoke and extinguish fire in the forward, aft and bulk cargo compartments, - detect smoke and extinguish fire in the lavatories, - detect leaks from the hot air ducts, - extinguish fire in the cockpit, passenger compartment and other areas accessible in flight,"

Two of these are worth flagging.

[!warning]- "Detect leaks from the hot air ducts" is a fire function owned by the pneumatic system Bleed-leak detection appears in the fire-protection function list because a burst hot-air duct is itself a fire hazard — but its hardware (the eutectic-salt dual loop) and its teaching live in ATA-36, not ATA-26. This is a "function belongs to fire, hardware belongs to pneumatic" boundary: an exam item asking "which of these is a fire-protection function?" makes leak detection a trap — correct by function, but not ATA-26 hardware.

The pylon function ("protect the engine pylon against torching flames from the combustion chamber") is the reason the pylon and turbine zones carry the highest detection thresholds — covered in the engine-detection article.

3. Detection is a dichotomy — heat loops versus optical smoke

Across the whole aircraft, fire is sensed by exactly two physical principles. The first measures temperature.

"Thermo-sensitive loops detect a fire or overheat condition. When the temperature in the monitored area reaches the preset threshold, the loops trigger warnings via the FDU."

"Two independent loops are installed in each engine nacelle. They are connected in parallel and operate according to an AND logic. This logic prevents spurious FIRE warnings."

Heat loops are used in the hot, sealed, unmanned zones — the engine nacelles and the APU compartment — where the baseline temperature is already high and the signature of a fire is a temperature crossing a threshold. The second principle measures smoke particles.

"Smoke detectors detect the visible and invisible combustion particles. When the preset threshold is reached, the smoke detector triggers a warning via the SDCU."

"Ambient smoke detectors are installed in the cargo compartments... The detectors operate in pairs to prevent spurious smoke warnings."

Optical smoke detection is used in the cool, occupied, smoulder-before-flame zones — cargo holds, lavatories, the avionics bay, crew-rest areas — where a fire tends to smoulder first and measuring temperature would be too slow.

[!warning]- Detection is allocated by temperature environment, not by "cabin versus cargo" The rule is hot zones sense heat, cool zones sense smoke. An avionics-bay fire is therefore found by smoke, not temperature — which is why the avionics procedure is about ventilation and power isolation rather than cooling. Paired detectors and dual-loop AND logic are the anti-spurious defence on both sides.

4. Extinguishing is a dichotomy too — fixed versus portable, Halon throughout

The AMM sets the philosophy: the method depends on where the fire is and whether the aircraft is airborne.

"There are several different fire-extinguishing methods. The methods depend on: - the area in which the fire occurs - the fact that the aircraft is in flight or on the ground. For each method one or two fixed fire-extinguisher bottle(s) or portable fire extinguisher(s) are used. They are operated either automatically and manually or manually only."

The fixed equipment covers four areas, and the mix of manual and automatic is worth reading closely:

"(1) Engine ... Each circuit includes two bottles. The percussion of the bottles is controlled from the cockpit. (2) APU ... includes one bottle. The percussion of the bottle on the ground can be controlled manually or automatically. In flight, the percussion is manually activated... (3) Cargo ... includes two bottles. (4) Lavatories A bottle above the waste bin ... is squibbed automatically by a thermal fuse and sprays the extinguishing agent directly into the waste bin."

Portable equipment closes the list:

"The operation of the portable extinguishers is manual ; they are used when there is a fire in the cockpit or the cabin."

The agent is Halon, but the fixed and portable bottles use two grades:

"The portable fire extinguisher is filled with halon 1211 agent and is pressurized with nitrogen."

The fixed bottles (engine, APU, cargo) are charged with Halon 1301, likewise nitrogen-pressurised — the nitrogen is the propellant, the Halon is the agent. Halon works by chemically interrupting the combustion chain reaction (breaking the free-radical chain), not by smothering oxygen or cooling — which is why it is fast, non-conductive and leaves no residue, at the cost of ozone depletion.

[!warning]- Halon 1301 versus 1211, and the "three states of extinguishing" 1301 (CBrF₃) has a low boiling point and suits total-flooding of a sealed compartment; 1211 (CBrClF₂, "BCF") is a longer-throw liquid stream and suits hand-held spot attack — so the bottle in the engine is not the same Halon as the one in your hand. Note also that engine, APU and cargo are discharged manually (the crew presses AGENT), the lavatory waste-bin bottle is fully automatic (a fusible link, the crew never knows), and the avionics bay has no bottle at all. Remember: engine/APU/cargo by hand, lavatory by fuse, avionics by ventilation.

5. The warning path — detector, processor, your red light

Between the detector and the cockpit sits a processing unit — the FDU for fire/overheat, an SDCU or CIDS-SDF for smoke.

For fire and overheat (engines and APU) the FDU raises a full set of annunciations:

"If a fire or overheat detection is confirmed by the engine Fire Detection Unit (FDU), these warnings occur: - warnings on the ENG/FIRE control panel (255VU), - warnings on the ENG MASTER control panel (125VU), - the MASTER WARN light, - messages on the Engine/Warning Display (EWD), - symbology on the System Display (SD), - the Continuous Repetitive Chime (CRC)."

An APU fire adds a ground-facing warning: in the cockpit via the APU/FIRE panel (231VU), and on the ground via the APU FIRE light on the external power panel (925VU) with a horn — so ground personnel under the belly also see an APU fire.

For smoke, the processor is an SDCU or its integrated equivalent, the CIDS-SDF, driving the VENTILATION panel (avionics) or the CARGO SMOKE panel (cargo) at 212VU, plus the cabin attendant panels for a lavatory fire.

[!warning]- The smoke processor comes in two build standards — the chapter's biggest configuration trap Earlier-build aircraft use a standalone SDCU (Smoke Detection Control Unit); later-build aircraft integrate the same function into the cabin data system as the CIDS-SDF (Smoke Detection Function). They are functionally equivalent, but the middleware between "detector" and "FWC" differs. Where this series writes "SDCU/CIDS-SDF", the later-build passenger configuration is the CIDS-SDF. See the smoke-detection article.

[!warning]- Fire protection is the counter-example to "dark-cockpit" philosophy Pneumatic and hydraulic overhead panels run lights-out — normal means no light. Fire panels deliberately do the opposite: an ENG FIRE or APU FIRE pushbutton lights red whenever the warning is active, regardless of pushbutton position, and a SQUIB legend actively points to the AGENT to press. Because a fire demands immediate hands-on action, the design is built to make sure you cannot miss it — the dark cockpit suppresses noise in normal operation, it does not mean "never light up."

6. Power and configuration — a survival system on the hardest buses

Fire protection is a survival system, so its detection and discharge circuits hang on the hot and essential buses: even with all AC lost and only batteries available, fire detection and discharge must survive. One APU-test note proves the point indirectly:

"If the fire test is performed on ground with only batteries to supply the electrical network... The red APU FIRE pb-sw light partially comes on, as one of the sets of bulbs is not electrically available."

The pushbutton light coming on "partially" shows the red bulbs are dual-fed and split across buses — one supply lost, the other keeps the fire indication alive. Supply-splitting detail is developed in the engine and APU articles.

Configuration map (the split that drives half the chapter):

                    Fire protection configuration
        ┌──────────────────────────┬──────────────────────────┐
     Passenger configuration        Freighter (A330-243F)
     - smoke processor: CIDS-SDF     - smoke processor: CIDS-SDF
     - holds: FWD / AFT / BULK       - Main Deck Cargo Compartment
       lower-deck                      + lower-deck holds
     - cargo extinguishing: 2 Halon  - MDCC: NO extinguishing system
       bottles                         -> depressurisation firefighting

Self-test

[!note]- Q1. What are the four protected families, and which one is detect-only while another is fully automatic? Engines/APU (fire + overheat, detect + extinguish); cargo + lavatories (smoke, detect + extinguish); avionics bay (detect only — no bottle); portable (cockpit/cabin). The lavatory waste-bin bottle is fully automatic (thermal fuse, no crew action).

[!note]- Q2. What are the two detection principles, and how are they allocated? Thermo-sensitive heat loops (engines, APU) and optical smoke detectors (everything else). Allocation is by temperature environment — hot sealed zones sense heat, cool occupied zones sense smoke.

[!note]- Q3. Halon 1301 versus 1211 — where and why? What is the nitrogen for? 1301 for fixed total-flooding (engine/APU/cargo); 1211 for hand-held spot attack (cockpit/cabin). Nitrogen is the propellant that expels the agent.

[!note]- Q4. Fire/overheat is processed by the FDU; smoke by what? Why does the configuration matter? Smoke by the SDCU or, on later-build aircraft, the CIDS-SDF. It matters because the detector-to-FWC middleware differs by build standard.

[!note]- Q5. Is "hot-air-duct leak detection" a fire-protection function, and where is its hardware? A fire-protection function per the AMM, but its hardware is in ATA-36 (pneumatic leak detection) — a classic "function here, hardware there" trap.

Key takeaways

References

• FCOM DSC-26-10 — fire-protection systems summary (four protected families).
• FCOM DSC-26-20-20 — battery-only test, dual-fed red light.
• AMM 26-00-00 — active vs passive, main functions, warning architecture (VU panels), build-standard smoke processor.
• AMM 26-10-00 — detection dichotomy: thermo-sensitive loops vs optical smoke, paired anti-spurious logic.
• AMM 26-20-00 — extinguishing methods: fixed (2/1/2/waste-bin auto) and portable.
• AMM 26-24-00 — portable Halon 1211, nitrogen pressurisation.

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.