Leaking fuel, hydraulic, deicing, or lubricating fluids can be sources of fire in an aircraft. Minute pressure leaks of these fluids are particularly dangerous for they quickly produce an explosive atmospheric condition. Fire protection systems are installed on aircraft to detect and protect against an outbreak of fire. These systems monitor the conditions which could lead to a fire and are comprised of smoke detectors, heat sensors near engines or hydraulic systems and visible audible warnings in the cockpit. The types of detectors most commonly used for fast detection of fires are the rate-of-rise, optical sensor, pneumatic loop, and electric resistance systems. A fire detection system should signal the presence of a fire. Units of the system are installed in locations where there are greater possibilities of a fire. Three detector system types in common use are the thermal switch, thermocouple, and the continuous loop.
Airplanes that have large number of passenger capacity must are equipped with a smoke detector system that monitors the lavatories for smoke. Smoke indications provide a warning light in the cockpit or provide a warning light or audible warning at the lavatory and at flight attendant stations that would be readily detected by a flight attendant. A smoke detection system is used where the type of fire anticipated is expected to generate a substantial amount of smoke before temperature changes are sufficient to actuate a heat detection system.