Fire Detector Typs

For fire warning and overheat warning purposes, the detectors in use are,

·         Thermal switch type.

·         Continuous wire loop type.

·         Sensor responder type.

·         Infra-red type.

·         Smoke detectors.

Detectors may be used separately, or together in a combined fire warning and engine overheat system.  Special detectors are used for detecting smoke, particularly in compartments which are not accessible in flight.

Thermal Switch Fire Detection System

These detectors are normally situated at points most likely to be affected by fire,such as an engine breather outlet or hot air ducting.  The type most commonly used is a switch, the contacts of which are actuated by the differential expansion of dissimilar metals.  The thermal switch is a unit type and called a spot or point detector.

A thermal switch fire detection system is a circuit in which one or more thermal switches are connected in an electrical circuit which also has a warning light and an aural alarm to warn the flight crew that an overheat condition exists in a particular area.  If more than one thermal switch is in the circuit, the switches are connected in parallel.

This makes sure that if any one switch closes a warning is given.  In some circuits the detectors are connected between two wiring loops, either of which may be supplied through a magnetic circuit breaker.  A short circuit in the energized loop results in operation of the magnetic circuit breaker and the supply is then routed to the second loop.  This prevents a false indication of fire.  The system can withstand one fault, either an electrical open circuit or a short to ground without sounding a false alarm.  A double fault must exist before a false fire warning can occur.   

Switches operating at different temperature settings may look identical but, when doing maintenance or replacing the switches its vital to identify by part number in the Illustrated Parts Catalogue (IPC).  The unit type is more suitable for localized overheat detection and the ‘continuous loop’ type better for fire detection.

Fire protection systems

Fire protection systems on aircraft usually consist of two separate operating systems with associated controls and indicators.  One system is for fire or overheat detection and the other is for fire suppression or extinguishing.  In some cases the systems can be interconnected so extinguishing takes place automatically when a fire is detected whereas the extinguishing was manual in early days.

Followings are some main facts that a Fire Detection and Extinguishing system must ideally be capable of,

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•  The fire warning system must accurately indicate that a fire has been extinguished and indicate if a fire re-ignites even after the first attempts of extinguishing.

• Since the system is exposed to all the critical areas of aircraft that must be durable and resistant to damage from any oil, water or other fluids that may be present in the area where it is installed.

• The system must include an accurate and effective method of testing so that the flight crew and engineers can check the integrity of the system.

• The system must be easily inspected, removed and installed.

• The system and its components must be designed so that the possibility of false indications is unlikely.

• The system must require a minimum of electrical power and must operate from the aircraft electrical system without the use of inverters or other special equipment to ensure safe operation even in some electrical system failures.

Detection systems must be capable of providing rapid detection of fire or overheat conditions and warn the crew by means of a red light and an audible signal on the flight deck indicating the area where some corrective action is required. However Fire detection systems will not automatically operate the main power unit extinguishers which may cause the main power plants(engines) to shutdown all of a sudden . But, in some installations the fire detection system may shut down APU and may operate on APU fire extinguisher automatically.