SEPARATOR SHELL
OUTFLOW VALVE
8 FIRE PROTECTION
Fire is the most dangerous threat to the safety of an aircraft and is associated with external areas near the main engines and the APU.
Other external hot spots are landing gear bays, where heat from brake units could affect the surrounding equipment and wiring, when the gears are retracted.
Overheating of the structure, equipment and wiring from very hot air, leaking engine compressor bleed air pipes, must also be catered for.
Fire from internal areas such as the passenger, flight deck and toilet compartments as well as cargo, air-conditioning and electrical/electronic equipment bays require protection too.
Indeed any source on an aircraft that the manufacturer or operator considers a likely hazard will be protected.
Ideally, a fire protection system will include as many as possible of the following features:
Rapid warning of fire/overheat and its accurate location
Must not cause false warnings
Continuous warning for duration of fire/overheat
Confirmation that the fire has been extinguishing
Indication that the fire has re-ignited
A means of testing the system from the flight deck
Detectors that are proof against oil, water, vibration and high temperatures
Detectors that are easily accessible throughout the aircraft
Detectors and extinguishers hot wired electrically or powered from emergency electrical buses
Adequate visual and aural indication on the flight deck and vital areas on the aircraft
Separate warnings for each engine and specific areas as determined by the aircraft manufacturers
Therefore, the Fire (and Overheat) Protection system will normally be split into two main subsystems:
Fire/Overheat Detection and Warning
Fire Extinguishing
8.1 FIRE/OVERHEAT DETECTION AND WARNING
Fire/Overheat detectors can be divided into two main groups:
Unit or Spot Type
Continuous Loop (Firewire) Type
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8.1.1 Unit (Spot) Type
This type is fitted at various strategic points within the fire/overheat zone and takes the form of a thermally activated switch.
They are electrically connected in parallel with each other and in series with the audio/visual warning system. This arrangement allows any switch to operate the warning, even if other switches have failed in the remainder of the system.
Some Unit detectors may have a pair of BI-metallic contacts, that close when heated and open when they are cooled down, to make or break the electrical warning circuit.
However, the majority has a thin casing that surrounds two conventional electrical contacts that are normally set apart from each other. When subjected to heat, the casing expands and pulls the two contacts together, completing the warning circuit in a similar manner to the BI-metallic type.
The main advantage of this so-called „High Speed Resetting Switch‟ (HSRS), is its sensitivity and fast reaction time, to initiate the warning and cancel it once the heat is removed.
Spot Detectors are used mainly to detect high temperature leaks from bleed air ducts and are normally positioned at pipe to pipe connections.
Thermoswitch Type Fire Detection System Figure 1
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Thermoswitch Spot Detector Figure 2
8.1.2 Continuous Loop (Fire Wire) Detectors
This method permits more complete coverage of a fire hazard area than any type of spot-type of temperature detectors. The continuous loop uses the principle of capacitance and resistance to indicate a rise in temperature at any point along the length of the detector loop. The commonest type has a stainless steel or Inconel outer tube, an inner pure nickel wire surrounded by ceramic beads wetted by eutectic salt. The effect of this design is that a rise in temperature causes a sharp fall in electrical resistance, as well as a rise in capacitance.
Once the detection unit senses this effect, anywhere along the wire, it will cause an overheat warning to be generated. This continuous loop system is often referred to as a 'firewire' system. The advantage of a firewire system is that a loop can cover the complete powerplant, (Figure 3) within its cowling so that an overheat or fire will be detected quickly no matter where it starts. The firewire will also re-set the control box to remove the warning when the temperature falls below the limit temperature.
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Fire Wire Layout Figure 3
Firewire elements are attached to the airframe structure with quick release clips approximately 6” apart and 4” from the end fittings. The element is supported in clips with a rubber grommet to prevent rubbing and to help damp out vibrations.
(Figure 4). Care is taken to eliminate strain on the element as excessive bending could result in work hardening of the capillary.
Fire Wire Clips and Connections Figure 4
8.1.3 Dual Loop System
Most aircraft use the dual loop system of indication. Each sensing circuit has dual sensing loops. Each Loop A and Loop B is independent of each other. When the loop selector switch is set to BOTH, both loops must detect a fire condition before the warning system is activated. If only one loop detects a fire, the associated loop fault light will illuminate.
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If the selector is switched to a single loop (A or B) full fire warnings will activate if the selected loop senses a fire condition.
Pressing the loop test button simulates a fire condition on the respective loop.
This is done by earthing the inner electrode of the loop that functionally checks the system and checks the continuity of the loop.
8.1.4 Pressure-Type Sensor
The pressure type detection system uses a continuous loop for the detection element. This loop is made from sealed stainless steel tube that contains an element that absorbs gas when it is cold but releases the gas when it is heated.
This tube is connected to a pressure switch that will close when the pressure reaches a pre-determined level.
The commonest make of this type of system is the Systron-Donner system which uses a centre titanium centre wire and the expansion of both helium and hydrogen gas to give the two-stage warnings.
Whilst the firewire system actuates when any part of the loop reaches the limit temperature, the pressure type system will actuate in two different ways. If a localised fire occurs, the hydrogen gas is released and its pressure closes the pressure switch which will set off the warning system, however, if the temperature over a larger area rises to a lower level than a fire warning the helium expands and closes the pressure switch to activate the system warning.
8.2 FIRE ZONES
On light aircraft, the only protection against fire is a stainless steel or titanium bulkhead (firewall), dividing the engine bay from the cabin and the rest of the aircraft. Larger aircraft have the complete engine cowlings isolated from the airframe/wing assemblies and, in addition, aircraft cowlings can be divided into a number of 'fire zones', each one usually having its own warning and extinguishing system.
The types of zone dictate what type of protection that they receive, for example, light aircraft have piston engines and hence, due to the high flow of air through the bay, have no fire protection and depend on isolating the engine of fuel to put out any fire. The example has four zones around the engine that only two have firewires and extinguishing.
8.2.1 Hot And Cool Zones
Engines are usually split into hot and cool zones (Figure 5). The hot zone comprises the combustion chamber turbines and exhaust areas, the cool zone comprises the intake, compressors and accessory drives.
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Engine Fire Zones Figure 5 8.2.2 Fireproof Bulkheads
These prevent fire from spreading to other areas. Auxiliary power units and tail mounted engines are normally contained within such bulkhead compartments separating them from the rest of the airframe. The engine pylons also contain a firewall to separate the engine from the wing. These are made from titanium or stainless steel and all joints are sealed with fireproof sealants
8.2.3 Engine Fire Prevention
There are a number of techniques used to help prevent a fire occurring around engines. These are, the use of flameproof or flame resistant materials, use of bonding strips to prevent arcing, drainage of spilt fuel/oil and efficient cooling.
All pipes which carry fuel, oil or hydraulic fluids are made fire resistant and all electrical components and connections are made flame proof.
It is essential that a fire staring in any zone is contained within that zone and is not allowed to spread to any other part of the aircraft. The engine cowlings form a natural container but they are usually made from light alloy and would not contain a ground fire for long. In flight however cooling airflow‟s through the cowlings, provide sufficient cooling to render the cowlings fireproof. The fireproof bulkheads and any cowling that has no cooling airflow are usually made from titanium or stainless steel.
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All wool, cotton and synthetic fabrics used in interior trim are treated to render them flame resistant. Tests conducted have shown that whilst the foam used in seat cushions is flammable, if covered with a flame-resistant fabric, there is little danger of fire from accidental contact with a cigarette, for example.
Fire protection for the aircraft interior is usually provided by hand-held extinguishers. Various types are available including, Water, CO2 and Dry Powder. Each type is best used on one kind of fire but may be used on other kinds. It is best to be sure which is safe to use on which type of fire.
8.3 SMOKE DETECTION
A smoke detection system monitors certain areas of the aircraft for the presence of smoke, which is could be indicative of a fire condition. These may include cargo and baggage compartments and the toilets of transport category aircraft.
A smoke detection system is used where the type of fire anticipated is expected to generate a substantial amount of smoke before temperature changes are sufficient to actuate a heat/fire detection system.
8.3.1 Carbon Monoxide Detectors
The presence of Carbon Monoxide (CO), or Nitrous Oxides (N2O), is dangerous to flight crew and passengers alike and may indicate a fire condition as it is a by-product of combustion. Detection of the presence of either or both of these gases could be the earliest warning of a possible dangerous situation.
Carbon Monoxide is very dangerous, firstly due to the minute amount required to cause loss of attention and headaches; (this is approximately 2 parts in 10,000).
It is colourless, odourless, tasteless and a non-irritant. Carbon Monoxide detectors are usually used in cabin and cockpit areas.
The detector is usually a small card with a transparent pocket containing silica gel crystals that have been treated with a chemical, which changes colour to green or black when they are exposed to carbon monoxide.
8.3.2 Photoelectric Smoke Detectors.
Air from the monitored compartment is drawn through the detector chamber and a light beam is shone on it. A photoelectric cell installed in the chamber senses the light that is refracted by the smoke particles. The photocell is installed in a bridge circuit that measures any changes, in the amount of current that it conducts. Figure 6 shows a typical photoelectric smoke detector.
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Photo Electric Smoke Detector Figure 6
When there is no smoke in the chamber air, no light is refracted and the photocell produces a reference current. When smoke is in the chamber air, some of the light is refracted and sensed by the photocell. Its conductivity changes, changing the amount of current. These changes in current are amplified and used to initiate a smoke warning signal.
8.3.3 Ionisation Type Smoke Detector.
A small amount of radioactive material is mounted on the side of the detector chamber. This material bombards the oxygen and nitrogen molecules in the air flowing through the chamber and ionises it to the extent that a reference current can flow across the chamber through the ionised gas to an external circuit.
Air inlet
Light beam
Light source
Photoelectric cell Light reflected from smoke into photocell Air outlet
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Ionisation Type Smoke Detector Figure 7
Smoke flowing through the chamber changes the level of ionisation and decreases the current. When the current reduces to a specific level the external circuit initiates a smoke warning signal. Figure 7 shows a typical ionisation smoke detector.
Flame Detectors
This system uses a photoelectric cell to detect a sharp rise in light, such as that from a flame in a closed bay.
8.4 FIRE EXTINGUISHING
There are a variety of aircraft and ramp extinguishing agents. Their use depends upon several variables such as location, proximity to personnel, environment, possible sources of fire, etc. There are integral extinguishing systems on board the aircraft as well as hand held extinguishers
Radioactive material Ionzing beam
Air inlet
Target
Air outlet
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8.4.1 Extinguishing System
Aircraft that have an integral fire extinguisher system have a system similar to the arrangement shown in Figure 8. There are a number of pressurised bottles with extinguishant inside and each bottle has two explosive cartridges, (squibs), which can be fired from the flight deck. Each bottle can feed either the port or starboard engines through a crossfeed. The extinguishant is fed through a series of pipelines and valves to the outlet nozzles and tubes.
In some aircraft, fixed systems may also be provided for the protection of landing gear wheel bays and baggage compartments. These systems may be independent of each other. They may be fully automatic or require the aircrew to initiate them when a fire is indicated.
Basic Aircraft Extinguishing System Figure 8
On multi-engine aircraft there may be one extinguisher bottle provided for each engine or one bottle may feed 2 engines (Figure 9). There is always usually a facility for cross feeding to another engine should the need arise.
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Dual Container System Figure 9
Two bottles giving either two 'shots', to a single engine or, one 'shot' each to either engine (Figure 10). The bottle condition is indicated either through a pressure gauge on each bottle, or a red/green sectioned gauge showing red when the bottle is empty or its pressure is low as well as a discharge indication on the associated fire control panel I the cockpit.
Typical 2 Shot System