7.4 Airframe Fire Protection Detection, Extinguishing, and Squib Safety

Detection: Spot, Thermocouple, and Continuous-Loop

Aircraft fire protection is organized around fire zones — engine, APU, cargo, lavatory, wheel well, and heater compartments — classified by airflow. The system must both detect a fire or overheat and extinguish it, and detection comes in three families the test names.

Spot thermal-switch (overheat) systems use bimetallic switches at chosen locations. The switches are wired in parallel with each other and in series with the warning light, so any one switch reaching its set temperature completes the circuit and lights the warning. They are simple but only sense at their fixed points.

Thermocouple systems detect by rate of rise: a thermocouple produces voltage only when a temperature difference exists between its hot (sensing) junction and a reference junction shielded from rapid heating. A slow, general temperature climb (like a hot day) produces little differential and no alarm; a rapid fire-driven rise creates a large differential. The system alarms when current exceeds about 4 milliamperes (0.004 A) through a sensitive relay, and total circuit resistance is kept low (typically not over ~5 ohms).

Continuous-loop systems give the most complete coverage by running a sensing element along the entire fire zone. Two common types: the Kidde system uses two conductors in an Inconel tube packed with a temperature-sensitive ceramic (thermistor) core — as temperature rises, core resistance falls, and the control unit reads the change, often providing both an overheat and a higher fire threshold.

The Fenwal system uses a single nickel wire in an Inconel tube packed with eutectic salt; the salt's resistance drops sharply at its set temperature, letting current flow between the wire and the tube. Continuous-loop systems are resistant to a single break and self-restore on cooling.

Smoke and toxic-gas detection use other principles: photoelectric or ionization smoke detectors in cargo/lavatory zones and carbon-monoxide detectors for cabin-heat leaks. A test light illuminating does not by itself prove the whole system is within limits.

Extinguishing Agents, Bottles, and Squib Safety

The stored-agent extinguishing system uses pressurized bottles (containers) plumbed to discharge nozzles in each zone. Halon 1301 has long been the standard aircraft agent: it interrupts combustion chemically, has relatively low toxicity, is noncorrosive, and leaves no residue. Halon 1211 is used in hand-held cabin extinguishers.

Because halons deplete ozone and are no longer produced new, clean-agent/HFC replacements (such as 2-BTP and HFC-based agents) are being certified and installed; the mechanic must use the agent and procedure the aircraft is certified for, plus its safety data, because some agents displace oxygen or form toxic byproducts in heat. Bottles are stored under pressure (often boosted with nitrogen) and may be High-Rate-Discharge (HRD) units that dump the full charge in about one second.

A squib (discharge cartridge) is an electrically initiated explosive device that ruptures the bottle seal to release the agent. Squib safety is a core exam theme. Maintenance must prevent accidental firing: follow procedures for power removal, connector handling, and static precautions, and never check a cartridge or its circuit with an ordinary ohmmeter — the meter's test current can detonate the squib. Use only the manufacturer-approved low-current tester or the prescribed method, and isolate the circuit before working.

Two discharge indicator discs on the fuselage skin tell maintenance how a bottle emptied. A blown yellow disc indicates a normal discharge — the crew fired the bottle electrically through the squib. A missing red disc indicates a thermal (overpressure) discharge — heat raised bottle pressure until a relief plug ruptured and dumped the agent overboard, signaling the bottle was overheated. Both conditions require investigation and bottle servicing.

Fire Zones, Classes, and System Inspection

The fire zone concept ties detection and extinguishing to airflow. Powerplant zones are commonly designated Zone 1 (the upper engine/accessory area) and Zone 2 (the lower area), each with its own detection coverage and discharge nozzles. The FAA classifies fire zones by airflow characteristics (Class A through Class X); a Class X zone, where airflow patterns make agent distribution difficult, requires roughly double the agent concentration of a Class A zone. The mechanic does not design these but must understand why nozzle placement and agent quantity are not arbitrary.

Inspection of an installed fire system covers detector security and routing (continuous-loop elements must be supported to avoid chafing and not kinked below their minimum bend radius), bottle pressure or weight against a temperature-corrected chart, bottle hydrostatic-test dates, cartridge expiration, plumbing security and blockage, and discharge-indicator status. A two-engine transport often has two-shot capability — a single bottle (or pair) that can be discharged into either engine, or a main and reserve shot.

Functional testing uses the cockpit test switch and approved procedure; a test that lights the warning verifies continuity and the lamp but does not prove the entire installation is within limits.

Troubleshoot by dividing the system into detection, warning, power, control, and extinguishing boundaries. A failed warning test could be a detector loop, control unit, lamp, power, or ground fault — not necessarily the bottle. A low bottle-pressure indication may be temperature, a slow leak, gauge error, or a true discharge. Fire-system maintenance always uses approved data because it protects crew and aircraft in time-critical emergencies, and the safest exam answer preserves the chain: correct zone, correct detector, correct bottle status, correct circuit isolation, and correct operational check.