Fire detection methods were introduced as fire is the most dangerous threats to the aircraft. A fixed fire protection system is installed in the potential fire zones. Here, ‘fixed’ refers to permanent installation of a system.

Typical Fire Zones on Aircraft

  1. Cargo compartment
  2. Electronic bays
  3. Engine and auxiliary power unit
  4. Wheel wells
  5. Lavatories on transport airplanes
  6. Bleed air ducts

Different Types of Fire Detectors

The entire aircraft fire protection system comprises of the following different types of detectors.

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

A fire detection system should detect the presence of a fire anywhere in the entire aircraft. Locations with greater possibilities are installed with the fire detection system unit. There are several fire detection/overheat systems. This article discusses the exclusively used methods.

Fire Detection/Overheat Methods

Thermal Switch System

Aircraft Fire Detection 
Spot Fire detection Systems
thermal switch system

Thermal switch system is used in old model aircraft. Various detectors and sensing devices are available. The aircraft power system energizes the lights of a thermal switch system and thermal switches control the operation of the lights. Thermal switches complete the electrical circuits at a certain temperature as these are heat-sensitive units.

These thermal switches connect in series with the indicator light, but parallel with each other. When, there is rise in temperature above a set value in any of the sections of the circuit, the thermal switch closes. And the light circuit is complete, indicating a fire or overheat condition.

The exact number of thermal switches is determined by the manufacturer of the aircraft. In some circuits, all the thermal switches are connected to one light whereas in others every thermal switch has a different light indicator.

Some warning lights used are push-to-test lights. A bulb is tested in these circuits. Some installations include a test and a dimming relay which are the additional safety features. The test relay when energized competes the entire series circuit checking all the wirings and the light bulb. Dimming relay when energized alters the circuit to include a resistor in series with the light.

Thermocouple System

A thermocouple is dependent on the rate of temperature rise. A thermocouple system is entirely different from a thermal switch system as it doesn’t warn right away during engine overheat or a short circuit developed condition. The size of fire zones and total circuit resistance decides the number of thermocouples in a system. A thermocouple system comprises of the following –

  1. Warning lights
  2. A relay box
  3. Thermocouples

The thermocouple unit consists of the following wiring system –

  1. Alarm circuit
  2. Detector circuit
  3. Test circuit
Fire Detection Systems - 
thermocouple system

The relay box consists of a thermal test unit and two relays – sensitive relay and slave relay. Depending on the number of potential fire zones, this box has from one to eight identical circuits. The relay takes care of the warning lights. The thermocouples control the relays operation. The circuit of the system consists of many thermocouples in series with each other and the sensitive relay.

Thermocouple material

Generally, thermocouple is made from two different metals constantan and chrome. These metals connect at the hot junction and bear the heat of fire. Reference junction is the point between two insulation blocks. A metal cage protects the thermocouple and this allows the movement of air freely to the hot junction.

When the rate of temperature rise is same in both the junctions, no voltage results. If there is a normal temperature rise in both the junctions due to the engine operation. The increase in temperature is gradual and there is no warning signal. If the temperature rises rapidly, the temperature difference in both the junctions produces voltage.

So, when there is a fire, hot junction temperature rises rapidly then the reference junction which produces voltage and the current flows within the detector circuit. If the current flow is above 4 milliamperes, this closes the sensitive relay, completing the circuit from the aircraft power system to the coil of the slave relay. The slave relay then closes to complete the circuit to the light indicator for visual fire warning.  

The circuit also contains two resistors. The contact of the sensitive relay is fragile enough to burn or weld due to the arcing. Therefore, a resistor is connected across the slave relay so as to absorb the self-induced voltage of the coil by preventing the arcing across the points.

Continuous-Loop Systems

This system provides the best detection performance. It offers complete coverage of fire zones. The two widely used continuous-loop systems are – Thermistor type detectors and pneumatic pressure detector. Transport aircraft use this system exclusively.

Why Do We Need Fire Detection Method?

The primary method of fire detection the modern aircrafts isn’t the observation by crew members. A fire detector serves the following purposes –

  1. Accurate location of a fire
  2. Rapid fire indication
  3. No false warnings
  4. Accurate fire-out indication
  5. Accurate fire re-ignition indication
  6. Continuous fire indication
  7. Electrically tested fire detection means
  8. Lightweight
  9. Adaptable to any mounting position
  10. Minimum current requirement when not in use

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