Continuous Loop Type Detectors

These detectors are designed to provide maximum coverage in the particular firezone.  They are used mainly for engine and APU installations.  They may also be installed in landing gear wheel bays and adjacent to hot air ducting.

These detectors operate on either of two principles:

•  The resistance type.

• The capacitance type.

The method of operation depends on the type of control unit fitted to the system.

Detector elements are manufactured in various lengths and are joined together to form a continuous detector loop.  This is routed round the installations as required.  An element consists of a stainless steel or inconel tube, with one or two center electrodes insulated from the tube by a temperature sensitive material.  When two conductors are provided within the tube, one of the connectors is earthed to the outer shell of the connector at the end of the tube.  Electrical connectors are provided at both ends of the tube.  Sometimes the elements are enclosed in a sheath which gives protection from damage.

 Resistance Type

The resistance of the insulating material decreases with an increase in temperature(NTC material).  At the warning temperature, sufficient current passes to operate a warning circuit.  The element is supplied with a current which is passed through a control box to operate the warning system.

In a typical system the detector is a nickel wire embedded in a temperature sensitive material called a di-electric.  This is contained within a small diameter stainless steel or inconel tube joined together with special couplings to form a loop.  The loop is routed and clamped around a firezone as required.

The inner wire and the tube form an inner and outer electrode and are connected to the aircraft power supply through a control unit.  Some systems use 115V ac and 28v dc, other systems use 28v dc only.

Under normal conditions only a very small standing current passes through the separating di-electric.  The current passed is insufficient to operate the control system.  As the temperature increases the resistance of the filling material decreases as it has a negative co-efficient of resistance.  This will allow more current to pass to the control unit.  When the temperature has risen above a pre-determined level, sufficient current will pass to operate the warning circuit.

When the temperature drops, the di-electric (the separating material) will return to its previous characteristic, the current will fall and the warning circuit will switch off.  Thus the system is now re-set and awaiting a further input.

The capacitance type will be posted in next post.

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.