To detect fires or overheat conditions, detectors are placed  in the various zones to be monitored. Fires are detected in  reciprocating engine and small turboprop aircraft using one  or more of the following: In addition to these methods, other types of detectors are  used in aircraft fire protection systems but are seldom used  to detect engine fires. For example, smoke detectors are  better suited to monitor areas where materials burn slowly  or smolder, such as cargo and baggage compartments.  Other types of detectors in this category include carbon  monoxide detectors and chemical sampling equipment  capable of detecting combustible mixtures that can lead to  accumulations of explosive gases. The complete aircraft fire protection systems of most large  turbine-engine aircraft incorporate several of these different  detection methods. 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.  The following classes of fires that are likely to occur onboard  aircraft, as defined in the U.S. National Fire Protection  Association (NFPA) Standard 10, Standard for Portable Fire  Extinguishers, 2007 Edition, are:  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. A number of detectors, or sensing devices, are available.  Many older-model aircraft still operating have some type of  thermal switch system or thermocouple system. A thermal  switch system has one or more lights energized by the aircraft  17-2 Fire protection systems on current-production aircraft do not  rely on observation by crew members as a primary method  of fire detection. An ideal fire detector system includes as  many of the following features as possible: 3. Class C—fires involving energized electrical  equipment in which the use of an extinguishing media  that is electrically nonconductive is important. 4. Class D—fires involving combustible metals, such  as magnesium, titanium, zirconium, sodium, lithium,  and potassium.  1.  No false warnings under any flight or ground  condition. 2.  Rapid indication of a fire and accurate location of the  fire. 3.  Accurate indication that a fire is out. 4.  Indication that a fire has re-ignited. 5.  Continuous indication for duration of a fire. 6.  Means for electrically testing the detector system from  the aircraft cockpit. 7.  Resists damage from exposure to oil, water, vibration,  extreme temperatures, or handling. 8.  Light in weight and easily adaptable to any mounting  position. 9.  Circuitry that operates directly from the aircraft power  system without inverters. 10. Minimum electrical current requirements when not  indicating a fire. 11. Cockpit light that illuminates, indicating the location  of the fire, and with an audible alarm system. 12. A separate detector system for each engine. 2. Class B—fires involving flammable liquids, petroleum  oils, greases, tars, oil-based paints, lacquers, solvents,  alcohols, and flammable gases.  1.  Class A—fires involving ordinary combustible  materials, such as wood, cloth, paper, rubber, and  plastics.  8.  Observation by crew or passengers 7.  Optical detectors 6.  Combustible mixture detectors 5.  Carbon monoxide detectors 4.  Overheat detectors 3.  Smoke detectors 2.  Radiation sensing detectors 1.  Rate-of-temperature-rise detectors 1.  Overheat detectors 2.  Rate-of-temperature-rise detectors 3.  Flame detectors 4.  Observation by crewmembers Requirements for Overheat and Fire Protection  Systems Fire Detection/Overheat Systems Thermal Switch System Classes of Fires