Fire Protection SystemsChapter 12

Aim

State principals of operation of the aircraft fire protection system

Objectives

1.State the use of fire zones2.Explain the use of fire detection

systems3.Describe smoke and gas detection

systems4.Describe fire extinguishing systems

1. Fire ZonesFire Zones

The aircraft is divided into zones (engine, equipment, passengers, cargo, etc.), each zone is isolated by heat shields or bulkheadsThe piston engine is normally contained in one zone and in the case of a single engine aircraft is isolated from the cabin by use of a fire wallSome ancillary aircraft equipment such as cabin vents pass through the firewall, in the event of an engine fire these systems must be isolated to prevent smoke or fire spreading into the cabin

2. Detection SystemsThermoswitch

A thermoswitch is a spot type detection system which uses a number of thermally activated switchesEach switch sensor consists of a bi-metallic thermoswitch which closes at a predetermined temperature completing an electrical circuitIn a single loop system a number of sensors are connected in parallel with each other with the group of switches connected in series with the annunciator, if one sensor switches on the annunciator will illuminateThe double loop consists of multiple detectors connected in parallel between two loops of wire, this allows multiple detectors to be placed in the same area and reduces the chance of a false indication

2. Detection SystemsThermocouple

The thermocouple system is used to measure temperature in relatively high temperature areas of the engine, for example cylinder head temperature or exhaust gas temperatureA thermocouple is a circuit made of two dissimilar metals, when the hot junction is heated a current will be produced When a certain temperature is reached a relay will be tripped and annunciator will be shownIn piston aircraft the metals are either copper-constantan or iron-constantan which are capable of measuring temperature up to 400⁰CTurbine engines use chrome-alumel types which can measure up to 1100⁰C

2. Detection SystemsFenwal Resistance Detector

The Fenwal system is a type of continuous-loop fire detection system It utilizes a metal Inconel tube with a nickel centre conductor. The nickel is insulated from the outer tube by ceramic beads coated with eutectic salts which have a resistance that varies with temperatureWhen a fire or overheat condition occurs the eutectic salt melts resulting in resistance reducing and current flows between the centre conductor and groundAs the system cools the eutectic salts will solidify returning to their original state

2. Detection SystemsKidde Resistance Detector

The Kidde system is a continuous loop system which has two core conductors inside a Inconel tube, the tube is filled with a thermistor material the resistance of which will reduce with an increase in temperature, similar to the eutectic salts in the Fenwal systemThe advantage of this system is that each conductor is on a separate circuit, providing redundancy should either circuit fail

2. Detection SystemsGraviner Fire wire Capacitance Detector

The Graviner system is similar to the Kidde system but consists of a single wire inside a Inconel tube. The insulating material in the tube not only has variable resistance with a change in temperature but also variable capacitanceThe system is less prone to false alarms, by monitoring both resistance and capacitance should a short occur in the sensing element the alarm will not be triggered

2. Detection SystemsSystron Donner System

The Systron Donner system is a pneumatic continuous loop system which utilizes a gas filled tube with a titanium wire running thorough it. The tube is filled with helium, as the temperature increase pressure increases The titanium wire acts as a spot detector, if localised heat is present the titanium will release hydrogen further increasing pressureAn aneroid capsule will be used to set off an electric annunciatorIf the temperature drops below the critical value the titanium will reabsorb the hydrogen

2. Detection SystemsFlame Detectors

Flame detection systems use photoelectric cells to measure visible light or infrared radiation in a confined area

3. Smoke and gas detectionSmoke and Gas Detection

Smoke detection systems are installed in the cockpit, passenger compartment, cargo compartments, toilets, galleys and crew rest areas where considerable smoke will be produced prior to ignition of flammable substancesDue to air circulation in the cabin the detectors must be placed in areas that will provide the earliest possible warning

3. Smoke and gas detectionCarbon Monoxide Detector

The carbon monoxide Detector consist of either a tube or coloured spot of silica gelIn the presence of carbon monoxide the silica gel will darken, the amount of gas determines how dark the silica gel becomesThese detectors are single use, should they be activated they must be replacedThe detectors have a service life so the use by date must be checked prior to use

3. Smoke and gas detectionPhoto Electric Cell Smoke Detector

This system consists of a diffuser, light source and photoelectric cellAs smoke enters the diffuser light from the light source is refracted towards the photo electric cellThe signal from the photoelectric cell is amplified and sent to the cockpit annunciatorA test lamp is installed so system integrity can be checked

4. Fire Extinguishing SystemsCarbon Dioxide Extinguisher

Normally consists of high pressure CO2 bottles located in the nose wheel well or flight deckThe bottles are fitted with operating valves which control discharge of the agent to the engineThe bottles are connected to the engine via stainless steel tubing and are fired remotely from the cockpit

4. Fire Extinguishing SystemsHigh-Rate Discharge

High-rate discharge extinguishers are fitted to most turbine aircraft. They operate by rapidly discharging a Halon gas into the engine nacelleThey consist of a container, detonator, fusible safety plug and high pressure linesThe detonator consists of a frangible disk and squib, when activated by the pilot the squib is fired into the disk breaking it and releasing the gasEach unit provides one extinguishing shot, often bottles cross feed so that number 1 engine fire system can fire its shot into number 2 engineIn order to increase the chance of success in extinguishing the fire, prior to discharge you should:• Prevent al flammable liquids or bleed air from

entering the engine• Disable the ignition system• Close all external doors

4. Fire Extinguishing SystemsPortable Extinguishers

Handheld extinguishers must be approved for the aircraft type and the extinguishing agent must be matched with the type of fire likely to occurThe extinguishing agents are usually either:• Co2• Dry chemical• Bromochlorodifloromethane (BCF)BCF extinguishers are widely banned outside aviation due to environmental damage, they are exempt due to the effectiveness on electrical firesFollowing use contact with BCF should be avoided

Questions?