Extinguishing System

These systems are provided for power plants, APUs and baggage compartments.  A system generally consists of:

·         A number of metal containers or bottles, containing an extinguishant eg. methylbromide, bromotrifluoromethane or bromochlorodifluoromethane (BCF) also known as Halon 1211.

·         Tubing to carry the extinguishing agent to areas that require protection.

·         Control valves.

·         Indicators.

·         Control circuitry.

Systems vary considerably on different aircraft but the basic elements are similar.  HRD or high rate discharge is the term applied to most systems in common use.

The extinguishant is pressurised with an inert gas and sealed in the container by means of a discharge or operating head.  When operated, either by selector switches  (fire handles) on the flight deck or crash switches, an electrically fired cartridge or squib ruptures a metal diaphragm within the discharge head and the extinguishant is released.  It then flows through spray pipes, spray rings or discharge nozzles into the appropriate firezone.  The electrical power is 28 volts dc and is supplied from an essential services busbar.

Most aircraft use a ‘two shot’ extinguishing system for the power plants.  This uses connections between the individual power plant systems.  In this system the fire extinguishers for each power plant are interconnected.  This allows two separate discharges of extinguishant into any one power plant.  On many aircraft two fire bottles are installed in each engine nacelle.  This allows two separate discharges of extinguishant into each power plant.  Indication that a fire extinguishing circuit has been operated is indicated by a warning light.

In some installations special switches are incorporated to automatically operate the extinguishers in the event of a crash.  These switches also connect cabin emergency lights to the aircraft battery power supply.  Two types of crash switch are in common use:

·         The inertia control type

·         The frangible type.

An inertia controlled switch generally consists of a heavy piston supported on its own spring and so arranged that at the required degree of deceleration (a typical value is 3g), it compresses the spring and causes a bow spring to snap over thereby bridging contacts connected in the extinguishing system circuit.  To allow resetting of the switch after operation or rough handling during transit, a reset plunger is incorporated.

Frangible switches consist of two electrical contacts mounted in a hermetically sealed glass envelope.  The contacts are prevented from closing by a spring, but in the event of the glass envelope being shattered, the contacts will close and complete the circuit to the extinguishing system.  The switches are located in positions such as the wing tips, the underside of engine nacelles, at various points on the underside of the fuselage, etc.  This is so that in the event of a crash, at least one of the switches will be shattered.

Types of Fire Extinguishants

Methyl Bromide

This extinguisher boils at 4.6°C and was used for the protection of power plants in older aircraft.  It is toxic and should not be used in confined spaces, flight crew compartments or passenger cabins.  This extinguisher is no longer listed.  Existing bottles may be maintained but refills will not be made with Methyl Bromide.

Bromochlorodifluoromethane (BCF)

This semi-toxic extinguisher is particular effective against electrical and flammable liquid fires.  It is used in power plant systems, and for the protection of auxiliary power units in some aircraft.  It is also used in certain types of portable extinguisher.  It becomes gaseous at normal temperatures and condenses to liquid at -4°C (25°F), and can be stored and discharged at moderate pressures.  It has little or no corrosive effect, although halogen acids will be formed if its products which have been decomposed by fire comes into contact with water, eg. condensation caused by fire.  In contact with fire, BCF volatilizes instantly, giving rapid flame extinction with little or no harmful effect on metallic, wooden, plastic or fabric materials.

Also known as Halon 1211.

Bromotrifluoromethane (BTM)

This semi-toxic extinguishant is used for the protection of power plant and APUs.  It is also widely used in cargo compartment fire suppression systems of some types of aircraft.

Also known as Halon 1301.  It has a boiling point of -58°C.

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.

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,

·         

•  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.