Fire basics and fire zones
Aircraft carry three kinds of fire protection: detection (so the crew knows), fixed extinguishing systems in the places nobody can reach in flight, and portable extinguishers in the cabin and flight deck. CS-25 defines designated fire zones — the engine power and accessory sections, the APU compartment and combustion heaters — which must have fire detection, fixed extinguishing, fireproof firewalls (Chapter 5) and drainage. Other areas get what their risk demands: smoke detection in cargo holds, lavatories and avionics bays, overheat detection along bleed-air ducts and in wheel wells, and built-in extinguishers in cargo holds and lavatory waste bins.
Fire and overheat detection
Continuous-loop detectors
- Resistance (eutectic salt) type — an Inconel tube with a centre wire embedded in a eutectic salt or ceramic. When any part of the element is heated, its resistance falls and current flows between wire and tube, signalling fire. It cools and resets after the fire.
- Thermistor / capacitance types — similar elements in which the core's resistance or capacitance changes with temperature, allowing the control unit to distinguish an average overheat from a local fire.
- Pneumatic (gas) type — a sealed tube filled with helium and a hydride core. A general rise in temperature makes the gas expand and closes an alarm switch (overheat); a local fire makes the core release hydrogen, raising the pressure sharply (fire). A separate integrity switch, held closed by the normal gas pressure, opens and signals a fault if the tube is cut or leaks.
Other detectors
- Thermal switches (spot detectors) — bimetallic switches that close at a set temperature, wired in parallel so any one gives the warning.
- Thermocouple (rate-of-rise) systems — exposed active junctions and protected reference junctions; a rapid temperature rise creates a voltage difference that triggers the alarm, while a slow rise (normal warm-up) does not.
- Optical flame detectors — infrared or ultraviolet sensors on some installations.
Two loops (A and B) follow the same route. A fire warning needs both to detect (AND logic), so a single chafed or wet loop cannot give a false warning. If a loop fails, the control unit reports a fault and switches to single-loop operation on the other. The warning is a fire bell, the red light in the fire handle and the master warning.
Smoke detection
- Photoelectric (light-scattering) detectors — air is drawn or flows through a chamber; smoke particles scatter a light beam onto a photocell. The most common type in cargo holds, lavatories and avionics bays.
- Ionisation detectors — a small radioactive source ionises the air in a chamber; smoke reduces the ion current.
- Cargo-hold systems use several detectors (often in pairs with AND logic, as for loops) and must detect a fire within one minute (CS 25.858). A warning shuts off the hold's ventilation and prompts the crew to discharge the cargo extinguisher.
- Light aircraft may have carbon-monoxide detectors (Chapter 6).
Fixed fire-extinguishing systems
The agent in fixed systems is still mainly Halon 1301 (bromotrifluoromethane): effective in small concentrations, non-corrosive, leaves no residue and has low toxicity. Its production has stopped because it damages the ozone layer, so stocks are recycled and replacement agents are being introduced, first for portable and lavatory extinguishers.
- Bottles (containers) — usually spherical, stainless steel or titanium, holding liquid Halon pressurised with nitrogen. Each has a pressure gauge and/or pressure switch (pressure varies with temperature, so readings are checked against a chart) and a temperature/pressure-sensitive safety relief diaphragm.
- Discharge heads — fired by an explosive cartridge (squib) that ruptures a frangible disc; high-rate-discharge bottles empty in about 1–2 seconds. Cartridges have a limited life and are tracked.
- Two-shot systems — two bottles connected through two-way check valves so either can be discharged into either engine: if the fire is still burning after the first shot (typically 30 seconds later), the second bottle is fired.
- Discharge indicators — a red disc on the fuselage skin is blown out if a bottle has discharged overboard through its relief because of overheating; a yellow disc is blown out by a normal discharge commanded by the crew. A missing disc must be investigated on the walk-round.
- APU — one bottle; on the ground many APUs shut down and discharge automatically on fire, sound an external horn, and can be controlled from an external panel (often in the main-gear well).
- Cargo holds (Class C) — a high-rate discharge knocks the fire down, then a metered (low-rate) discharge maintains the Halon concentration for the rest of the flight to the nearest suitable airport (one hour or more on extended-range aircraft). Ventilation to the hold is shut off and the liners keep the agent in (Chapter 10).
- Lavatory waste bins — a small automatic extinguisher with heat-sensitive (fusible) nozzles that discharge into the bin when they melt.
The engine fire handle
Pulling (or unlocking) the engine fire handle or switch isolates the engine: it closes the low-pressure fuel shut-off (spar) valve, the hydraulic shut-off valve and the bleed-air valves, trips the generator, silences the bell and arms the squibs. Turning or pushing it then fires the selected bottle. Fire handles are lit red by a fire warning and can be tested from the flight deck.
Portable fire extinguishers
| Agent | Use on aircraft | Notes |
|---|---|---|
| Halon 1211 (and approved replacements) | Flight deck and cabin — classes A, B and C | Vaporising liquid; interrupts the chain reaction; no residue; check by weight and pressure gauge |
| Water (often with antifreeze) | Cabin — class A fires (paper, fabric, seats) | Never on electrical or fuel fires |
| Carbon dioxide (CO₂) | Older flight-deck and ground use | Checked by weight; can cause cold burns and displace oxygen in confined spaces |
| Dry powder | Ground equipment, wheel and metal fires | Not used in cabins: powder obscures vision and is corrosive to equipment |
Checks: correct type in each marked location, secure in its bracket, safety pin and tamper seal intact, gauge in the green, weight within limits, inspection label and test dates current (periodic hydrostatic test of the cylinder). Discharged or doubtful extinguishers are replaced.
System tests and maintenance
- Fire/overheat detection test — the flight-deck test switch checks loop continuity and the warning circuits (bell, lights, master warning) for each zone; a fault test checks the fault-monitoring logic.
- Squib (cartridge) circuit test — a low-current continuity test of the discharge circuits, shown by lights; it cannot fire the cartridge.
- Detector elements — inspect for kinks, dents, chafing, crushed sections and loose clamps; keep the minimum bend radius and the grommets in their clamps; damaged elements are replaced, never repaired by straightening.
- Bottles — pressure checked against the temperature/pressure chart, weight checks, cartridge life and expiry, discharge-indicator discs in place; after a discharge the bottle is replaced and the lines cleared as the AMM requires.
- Safety — squibs are explosive devices: short-circuit caps, isolated circuits and correct handling are mandatory; Halon and CO₂ displace oxygen, so ventilate after any discharge in an enclosed area.