Engine Fire Protection Systems
Detection
Fire protection splits into detection and extinguishing.
Detection types include spot detectors (thermal switches that close at a set temperature), thermocouple units, thermistor units, and continuous-loop systems. The Kidde-type loop uses a discrete wire; the Fenwal-type uses material whose resistance changes with temperature. Continuous loops respond both to a localized hot spot and to an overall average temperature rise; rate-of-rise units trigger on a rapid temperature increase.
Extinguishing
Halon 1301 is preferred for engine compartments because it leaves no residue and interrupts the combustion chain. CO2 displaces oxygen, and dry chemical is messy and rarely used in engines. Know the HRD bottle components: bottle, pressure gauge, discharge cartridge (squib), discharge indicator (red disc), check valve, and plumbing to nozzles aimed at the accessory section and combustor area.
Traps
Do not test a squib with an ohmmeter that supplies more than the rated no-fire current - use an approved tester only. A missing or blown red disc means the bottle has discharged. Do not reset a fire-detection circuit breaker in flight, and do not assume the fire is out because the warning light went out - the bottle may be empty while the fire still burns.
Rule of thumb
For an in-flight engine fire: retard the throttle, shut off the fuel, pull the firewall shutoff valve, discharge the bottle, and land as soon as possible. Extinguishing agent is sized for a few seconds of discharge, not minutes - one bottle, one shot.