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Aviation Maintenance Technician Handbook–Airframe

FAA-H-8083-31B Version 2023
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Chapter 17

Fire Protection Systems

Introduction

Because fire is one of the most dangerous threats to an aircraft, the potential fire zones of modern multiengine aircraft are protected by a fixed fire protection system. A fire zone is an area, or region, of an aircraft designed by the manufacturer to require fire detection and/or fire extinguishing equipment and a high degree of inherent fire resistance. The term “fixed” describes a permanently installed system in contrast to any type of portable fire extinguishing equipment, such as a hand-held Halon or water fire extinguisher. A complete fire protection system on modern aircraft, and on many older aircraft, includes a fire detection system and a fire extinguishing system. Typical zones on aircraft that have a fixed fire detection and/or fire extinguisher system are:

  1. Engines and auxiliary power unit (APU).
  2. Cargo and baggage compartments.
  3. Lavatories on transport aircraft.
  4. Electronic bays.
  5. Wheel wells.
  6. Bleed air ducts.

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:

  1. Overheat detectors.
  2. Rate-of-temperature-rise detectors.
  3. Flame detectors.
  4. Observation by crewmembers.

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.

  1. Rate-of-temperature-rise detectors.
  2. Radiation sensing detectors.
  3. Smoke detectors.
  4. Overheat detectors.
  5. Carbon monoxide detectors.
  6. Combustible mixture detectors.
  7. Optical detectors.
  8. Observation by crew or passengers.

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.

Classes of Fires

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:

  1. Class A—fires involving ordinary combustible materials, such as wood, cloth, paper, rubber, and plastics.
  2. Class B—fires involving flammable liquids, petroleum oils, greases, tars, oil-based paints, lacquers, solvents, alcohols, and flammable gases.
  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.

Requirements for Overheat & Fire Protection Systems

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:

  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.
  1. Indication that a fire has re-ignited.
  2. Continuous indication for duration of a fire.
  3. Means for electrically testing the detector system from the aircraft flight deck.
  4. Resists damage from exposure to oil, water, vibration, extreme temperatures, or handling.
  5. Light in weight and easily adaptable to any mounting position.
  6. Circuitry that operates directly from the aircraft power system without inverters.
  7. Minimum electrical current requirements when not indicating a fire.
  8. Flight deck light that illuminates, indicating the location of the fire, and with an audible alarm system.
  9. A separate detector system for each engine.

Fire Detection/Overheat 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.

Thermal Switch System

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 power system and thermal switches that control operation of the light(s). These thermal switches are heat-sensitive units that complete electrical circuits at a certain temperature. They are connected in parallel with each other but in series with the indicator lights. [Figure 17-1] If the temperature rises above a set value in any one section of the circuit, the thermal switch closes, completing the light circuit to indicate a fire or overheat condition. No set number of thermal switches is required; the exact number is usually determined by the aircraft manufacturer. On some installations, all the thermal detectors are connected to one light; on others, there may be one thermal switch for each indicator light.