1. Fire Protection Systems
Chapter 12

2. Aim
State principals of operation of the aircraft fire protection system

3. Objectives State the use of fire zones
Explain the use of fire detection systems
Describe smoke and gas detection systems
Describe fire extinguishing systems

4. 1. Fire Zones
Fire Zones
The aircraft is divided into zones (engine, equipment, passengers, cargo, etc.), each zone is isolated by heat shields or bulkheads
The 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 wall
Some 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

5. 2. Detection Systems Thermoswitch
A thermoswitch is a spot type detection system which uses a number of thermally activated switches
Each switch sensor consists of a bi-metallic thermoswitch which closes at a predetermined temperature completing an electrical circuit
In 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 illuminate
The 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

6. 2. Detection Systems Thermocouple
The thermocouple system is used to measure temperature in relatively high temperature areas of the engine, for example cylinder head temperature or exhaust gas temperature
A 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 shown
In piston aircraft the metals are either copper-constantan or iron-constantan which are capable of measuring temperature up to 400⁰C
Turbine engines use chrome-alumel types which can measure up to 1100⁰C

7. 2. Detection Systems Fenwal 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 temperature
When a fire or overheat condition occurs the eutectic salt melts resulting in resistance reducing and current flows between the centre conductor and ground
As the system cools the eutectic salts will solidify returning to their original state

8. 2. Detection Systems Kidde 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 system
The advantage of this system is that each conductor is on a separate circuit, providing redundancy should either circuit fail

9. 2. Detection Systems Graviner 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 capacitance
The 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

10. 2. Detection Systems Systron 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 pressure
An aneroid capsule will be used to set off an electric annunciator
If the temperature drops below the critical value the titanium will reabsorb the hydrogen

11. 2. Detection Systems Flame Detectors
Flame detection systems use photoelectric cells to measure visible light or infrared radiation in a confined area

12. 3. Smoke 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 substances
Due to air circulation in the cabin the detectors must be placed in areas that will provide the earliest possible warning

13. 3. Smoke and gas detection
Carbon Monoxide Detector
The carbon monoxide Detector consist of either a tube or coloured spot of silica gel
In the presence of carbon monoxide the silica gel will darken, the amount of gas determines how dark the silica gel becomes
These detectors are single use, should they be activated they must be replaced
The detectors have a service life so the use by date must be checked prior to use

14. 3. Smoke and gas detection
Photo Electric Cell Smoke Detector
This system consists of a diffuser, light source and photoelectric cell
As smoke enters the diffuser light from the light source is refracted towards the photo electric cell
The signal from the photoelectric cell is amplified and sent to the cockpit annunciator
A test lamp is installed so system integrity can be checked

15. 4. Fire Extinguishing Systems
Carbon Dioxide Extinguisher
Normally consists of high pressure CO2 bottles located in the nose wheel well or flight deck
The bottles are fitted with operating valves which control discharge of the agent to the engine
The bottles are connected to the engine via stainless steel tubing and are fired remotely from the cockpit

16. 4. Fire Extinguishing Systems
High-Rate Discharge
High-rate discharge extinguishers are fitted to most turbine aircraft. They operate by rapidly discharging a Halon gas into the engine nacelle
They consist of a container, detonator, fusible safety plug and high pressure lines
The 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 gas
Each unit provides one extinguishing shot, often bottles cross feed so that number 1 engine fire system can fire its shot into number 2 engine
In 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

17. 4. Fire Extinguishing Systems
Portable Extinguishers
Handheld extinguishers must be approved for the aircraft type and the extinguishing agent must be matched with the type of fire likely to occur
The 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 fires
Following use contact with BCF should be avoided

18. Questions?