11.8 Fire Protection

Fire and Smoke Detection and Warning Systems

• Fire requires fuel, heat, and oxygen to occur.
• Rapid oxidation with heat and light release is combustion or burning.
• Removing any of the three elements extinguishes the fire.
• Aircraft are designed to be intrinsically safe, but fire remains a critical threat.
• Aircraft fire zones have detection and/or extinguishing equipment and high fire resistance.
• "Fixed" fire protection systems are permanently installed, not portable extinguishers.
• Fire protection systems include detection and extinguishing systems.

Designated Fire Zones on Aircraft

• Engines and Auxiliary Power Unit (APU)
• Forward and Aft Baggage and cargo compartments
• Lavatories
• Electrical/electronic equipment compartments
• Wheel wells (undercarriage bays)
• Bleed air ducts
• Detectors are placed in these zones to monitor for fire or overheat.
• Reciprocating engine aircraft use overheat, rate-of-temperature-rise, flame detectors, or crew observation.
• Large turbine engine aircraft use rate-of-temperature-rise, radiation, smoke, overheat, carbon monoxide, combustible mixture, optical detectors, or crew/passenger observation.

Overheat and Fire Protection System Requirements

• Systems must rapidly detect fire or overheat and indicate the affected area.
• Detectors should not automatically activate extinguishing units in flight.
• They may shut down power or fuel to certain areas.
• Fire warning systems alert the flight crew to fire or overheat that could cause a fire.
• Overheat warning systems are in high-temperature areas that may lead to a fire.
• Overheat warnings trigger an amber master caution light and audible alarm.

Fire Detection Systems

• Common types of detectors include thermal switches, thermocouples, and continuous loop sensing elements.
• Modern aircraft use continuous loop systems for greater coverage.
• Thermal switch systems use heat-sensitive switches that complete circuits at a specific temperature.
• Switches are in parallel with each other but in series with the indicator lights.
• Warning lights may have a push-to-test switch to check the bulb and circuit.
• Thermocouple systems operate based on the rate of temperature change.
• They include a relay box, warning lights, and thermocouples.
• Thermocouples consist of two dissimilar metals joined at a hot junction.
• Pressing the test switch sends current through a heater around the thermocouple to test the system continuity.

Continuous Loop Systems

• Large commercial aircraft use continuous thermal sensing elements.
• Two main types exist: thermistor (Kidde and Fenwal) and pneumatic pressure (Lingberg/Systron-Donner/Meggitt Safety Systems).
• A continuous-loop system provides more fire hazard area coverage.

Fenwal System

• Uses an Inconel tube with a thermally sensitive eutectic salt and a nickel wire conductor.
• Series-connected sensing elements with the same or different temperature settings.
• Overheat reduces resistance, causing current flow between the sheath and conductor.
• The current is sensed by the control unit, which activates alarms.
• The system self-resets to standby when the temperature decreases.
• Filling compounds include eutectic salts in ceramic beads or aluminum oxide in fibrous glass.

Kidde System

• Has two wires in an Inconel tube filled with a thermistor core material.
• One conductor is grounded to the tube; the other is connected to the fire detection control unit.
• Increased temperature decreases electrical resistance to the ground.
• Removed heat condition increases the core material resistance.
• The rate of resistance change identifies a short circuit or a fire.

System Test

• Continuous-loop integrity is tested using a test switch to simulate resistance change.
• Damage to the element can cause system failure or false warnings from poor installation or damage.
• Dual-loop systems increase reliability by requiring both loops to signal a fire (AND logic).
• A selector switch disconnects a failed loop, allowing single-loop operation.

Pneumatic Continuous Loop System

• Works on the principle of gas expansion due to heat.
• The current generation of pneumatic systems is designed so that mechanical damage to the sensor tube cannot result in a false alarm.
• Any severe damage to the unit will provide a "no test" indication, not a false alarm.
• responds to average temperature threshold, and discrete localized temperature increase.
• Average temperatures activate by the expansion of helium gas inside the detector.
• Discrete temperatures release hydrogen gas from the detector core whenever a small tube section hears 1100 c for 5 seconds.
• The pneumatic continuous systems use a stainless-steel tube filled with helium gas and a length of titanium wire running through the center.

Lingberg System, Systron-Donner, and Meggitt Safety Systems

• These pneumatic continuous systems use a stainless-steel tube filled with helium gas and a length of titanium wire running through the centre.
• The titanium wire acts as the gas absorption material because it contains a quantity of hydrogen.
• The tube is sealed at one end and connected to a responder at the other end.
• The responder contains two electrical switches operated by a diaphragm assembly.
• One switch, the integrity switch, is held closed with sufficient gas pressure in the tube.
• Test switch operation illuminates the warning light and sounds the bell, indicating serviceability.
• After a fire, the hydrogen gas is reabsorbed, and the responder contacts break, resetting the alarm.
• Dual-loop configurations increase reliability, with both loops required to sense a fire.
• The system isolates a defective loop and reconfigures for single-loop operation.

Fire Detection System Maintenance

• Sensing elements are in high-activity areas around engines, increasing damage risk.
• Maintenance includes inspecting and servicing damaged sections, containing loose material, correcting connections, and replacing elements.
• Sensing elements must be inspected for cracks, abrasion, metal particles, grommet condition, dents, kinks, and proper routing.
• Interference between a cowl brace and a sensing element can cause rubbing.
• Grommets must be installed correctly on its clamp.

Fire Detection System Troubleshooting

• Intermittent alarms are usually due to intermittent shorts in the wiring.
• False alarms occur without fire or overheat.
• Kinks in the sensing element can cause internal wires to short.
• Test switch failure can be caused by a defective test switch or control unit.

Smoke Detection System

• Aircraft smoke-detection systems sample cabin air for smoke.
• These areas include cargo and baggage compartments, equipment bays, and lavatories.
• Any smoke found in the sample causes a change in electric current flow, triggering a warning.
• Two basic types are photoelectric and ionization.

Refraction Photoelectric System

• An air sample passes through a chamber.
• Light reflected onto the photocell decreases circuit resistance, sending output to the AFOLTS and indicating circuits.

Ionisation Type Smoke Detector

• Has an electrode with a small amount of radioactive material.
• Radioactivity ionizes gases, allowing current to pass across electrodes.
• Smoke reduces ionization and current flow.
• A solid-state sensor activates the warning device in the cockpit when current decreases.

Cargo Smoke Detector System

• Utilizes an optical smoke detector with LEDs, intensity monitor photodiodes, and scatter detector photodiodes.
• Smoke detectors monitor air in cargo compartments and fans bring air from the compartment, water separators remove condensation and heaters increase air temperature.
• Smoke reflects light onto the scatter detector, triggering an alarm.

Lavatory Smoke Detector System

• Aircraft carrying 20+ passengers must have lavatory smoke detectors.
• Smoke indications trigger a warning light in the flight deck and/or at the lavatory and flight attendant station.
• Each lavatory has automatic extinguisher and a ceiling smoke detector.
• The lavatory smoke detector is powered by 28 Vdc from the main DC bus.

Operation of Smoke Detector Unit

• Smoke detection activates a red alarm LED, and a pulsed ground is sent to a timing circuit.
• The warning horn and lavatory call light operate intermittently.
• An energized relay sends a ground signal to the Overhead Electronics Unit (OEU) for:
  • Flashing lavatory master call light.
  • Pop-up window on Cabin System Control Panel (CSCP) and Cabin Area Control Panel (CACP).
  • Lavatory call chime.
• Reset the indications push the lavatory call reset switch smoke detector interrupt switch
• The alarm LED stays on as long as smoke is present.

Fire Extinguishing Systems

• Aircraft designated zones with fixed fire detection and extinguisher systems include engines, APUs, baggage/cargo compartments, and lavatories.
• Fire detection systems must rapidly detect fire or overheat but must not automatically operate extinguishers.
• Extinguishing systems dilute the atmosphere with an inert agent.
• The most common agent is Halon 1301, known for its effectiveness and low toxicity.
• Halon 1301 is non-corrosive, requires little clean-up, and doesn't affect materials and although the manufacture of Halon 1301 is banned, it is used.
• Recyclced Halon 1301 is used the ICAO stated that after 28 November 2024 all new aircraft models applying for a type certificate must replace Halon 1301 with other suitable extinguishant for cargo compartments.

Fire Extinguisher Bottles

• The fire extinguisher bottles hold a liquid halogenated extinguishing agent which is typically pressurised with nitrogen.
• Bottles, typically made of stainless steel, have a temperature/pressure-sensitive safety relief.
• A discharge valve with a cartridge (squib) and frangible disk (diaphragm) is installed.

Discharge Valves

• Standard release types use an explosive-driven slug to burst a disc
• High temperature or hermetically sealed units directly impact a steel diaphragm.
• Conventional metallic gasket seals allow replacement after discharge.
• Visual indication gauges are vibration resistant.
• Combination gauges visually show pressure and send an electrical signal if pressure is lost.
• Two-way check valves prevent backflow in twin-engine aircraft.

Discharge Indicators

• There are two types of indicators commonly used (Thermal and Discharge)
• A red thermal discharge indicator on fire bottle relief, indicating the bottle's content has been dumped overboard.
• A yellow discharge indicator ejected from the fuselage skin, indicating system activation and bottle replacement.

Engine Fire System

• Engine fire bottles are usually electrically operated after manual selection by the flight crew.
• APU fire bottles are similar, with automatic shutdown and activation during ground operation.
• Engine fire extinguishing systems must be "two-shot."
• The extinguishing agent is delivered through a spray nozzle.
• Before discharge, the engine is shut down and isolated from all other supplies.

Operation During Fire Warning

• Fire switches or push buttons are used to activate the system.
• Pressing the fire push button triggers:
  • Engine-driven generator shutoff.
  • Bleed air valve closure.
  • Low-pressure fuel shut-off valve closure.
  • Hydraulic valve closure.
  • Central warning system activation.
  • Firing circuits of the fire bottles are armed.
• Pressing the agent push button heats the squib, rupturing the frangible disc.
• An amber discharge light means the bottle is empty.
• Discharge of one fire extinguisher bottle is generally sufficient.
• Discharge the second bottle if the fire warning is still present after 30 seconds.

Auxiliary Power Unit Fire System

• APUs are self-monitoring and auto shut down in the event of:
  • Fire
  • Oil pressure failure
  • Overspeed
  • Overheat
• A manual control panel is included.
• Automatic APU fire extinguisher discharge may performed on some aircraft.
• APUs have their dedicated fire bottles.

Fire Extinguisher System Maintenance

• Scheduled maintenance includes inspecting and servicing bottles, replacing cartridges and testing discharge tubing.
• Fire extinguisher bottles are periodically checked for pressure.
• Compare new bottle weights with the plate to verify fullness.

Bottle Discharge Cartridge (Squib)

• The service life of the fire extinguisher discharge cartridge (squib) is calculated from the manufacturer’s date stamp, which is located on the face of the cartridge.
• Squib cartridge service life is typically 5-10 years.

Risk of High Intensity Radiated Fields (HIRF) to Fire Bottle Cartridges (Squibs)

• Precautions taken to minimize problems caused by HIRF to fire bottle cartridges includes. -Screening of cables sensitive to radio.
  • HIRF or RADHAZ filters, especially with explosive cartridges.
  • Placing sensitive equipment within a Faraday Cage
  • Ensuring bonding of components is within specified parameters
• When the electrical connectors are fitted, the connectors act as a RADHAZ filter.

Agent (Halon) Bottles

• Replacement of squibs and discharge valves requires care.
• The squib must be correctly assembled in the discharge valve and the valve connected to the bottle.

Pipe/Tubing Systems

• Check pipes and bore for restrictions using dry compression air.
• Free the pipes and stray rings from corrosion, cracks and other damage. Fitting of the support clamps must be returned to their correct position as soon as a pipe has been removed.

Valves

• When installing and maintaining, the valves must be inspected for cleanliness, signs of damage, as well as freedom of movement.
• Care must be taken to ensure their correct location with respect to the required flow of extinguishant.
• Care must be taken to ensure their correct location with respect to the required flow of extinguishant.

Cargo Fire System

• Commercial aircraft must have the following capabilities for each cargo or baggage:
  • Visual Detection system with 60 seconds to the flight crew
  • System capable of detecting structural fire at a safe temperature
  • Crew can check functioning of the fire detector circuit

Cargo Extinguishing System

• The Cargo extinguisher can be activated is smoke detectors detect smoke in the compartment system.
• Some aircraft equipped with two types: dump and metered system.
  • Dump: initial system that releases extinguishing agent.
  • Metered: After a time delay metered bottles discharge at a controlled rate through the filter regulator, and keep it correct concentration of extinguishing agent in the cargo.
  • A pressure switch activate the bottle discharge and send an indication to the Flight Deck.

Flight Deck Indications

• Observed indications can include coming on of master warning lights and fire warring aural, along with cargo fire warning lights, and/or warning messages.
• There are take off functions to prevent the operations during take-off.

Lavatory Fire Extinguisher System

• Each lavatory compartment is fitted with a fire extinguisher bottle to extinguish fires within the waste compartment bin.
• The bottle has two nozzles and contains Halon 1301 (or equivalent).
• When the temperature reaches 170 ˚F (76 ˚C), the solder melts, and the Halon is discharged.
• A temperature indicator on the waste compartment bin shows if there has been a fire.
• If black circles show on the indicators, replace the temperature indicator strip and fire bottle.
• The Fire Bottle cannot be refilled and must be replaced.

System Tests

• Each system must be regularly tested (APU/engine starts, pre/post-flight).
• Tests are performed remotely (switch/indication) or manually (visual inspection).
• Systems that need testing include:
  • Engine fire system loops
  • Engine fire system extinguishant bottles
  • Cargo and Lavatory Smoke detection systems
  • Cargo extinguishant systems
  • Lavatory extinguishant systems

Engine/Cargo Bay Fire Bottle and Loop Tests

• Aircraft may have a dedicated test panel for fire and overheat detection testing.
• These tests cause visual Push Button Annunciators (PBAs), Centralized Warning Panel (CWP) annunciators/messages, and aural warnings to initiate.
• Any failure or fault message must be investigated and repaired before the next flight.
• Examples of tests and indications are:

Aircraft 1 Example

• This air craft contains fire-sensing loops in the pipes, main engine, and four fire bottles.
• A dedicated system panel on the overhead panel.
• 'Master test warn' switch is a 3 position switch,spring loading switch to the centre and is located to the top right panel. -3-way test switch selects individual to be tested, and located to the left of the 'Master Test Warn' switch.

Tests

• 'TEST WARN', will sound fire alarm Selecting the Loop 'A' or Loop 'B 'on the 3 way switch for the engine fire loops will select the individual loops to be tested. The 'Master Test Warn' switch is set to 'Test 'and held. The following indications will constitute a successful test:

• Fire bell will sound.

• L(R) ENG FIRE or JETPIPE OVERHEAT message on the CWP will be posted (RED).

• Aural Warning ‘JETPIPE OVERHEAT’ will sound.

• Hyd and Fuel SOV OPEN Caution message will be posted for both engines.

• Both ‘ENG FIRE BOTTLE 1 and 2 ARMED PUSH TO DISCH’ PBAs will come on. These are located on the glareshield on this aircraft (If pushed due to an actual warning they will discharge the bottle extinguishant to the fire).

• 'FAIL'

• By selecting the 'Master Test Warn' switch to 'FAIL' it will simulate a fault on the selected loops and give the following indication:

• Amber L(R) FIRE FAIL, APU FIRE FAIL and L(R) JET OVHT FAIL messages will be posted on the CWP. ENGINE BOTTLE 1 (2)

• When these switches are selected to 'TEST', the squib circuit is tested and a Green 'L/R ENG SQUIB' message is posted on the CWP.

CARGO SMOKE & BOTTLE TEST 1 (2)

• TEST 1: when the 'Cargo Bottle' test is selected, this will test the smoke detector Number: one
• 'Smoke' aural message is heard
• 'Smoke Cargo' Warning message is posted on the CWP
• 'Cargo squib 1' advisory message posted on the CWP
• 'Normal Cargo smoke Push red warning on Cago fire panel
• 'Normal bottle Armed Push to Disch' on the 'Stand By Cargo Smoke push'

Aircraft 2 Example

• This particular aircraft has three sensing elements on the engine, which are (eng fan, eng core, pylon area) and one for the APU (not shown).

• The associated fire panel has three large RED PBAs for engine and APU fire indications and individual system 'TEST' buttons

Engine Test

• Test Panel Reference:
• When the test button is pressed and held, repetitive chime occurs, and master warning light flashes
• Warning appears on ECAM
• FIRE pba ligjts red
• SQUIB flashes on white when supplies available.
• DISCH light will come on amber.
• FIRE lights come on red.

APU Test

• Pushed and held, the following will be heard and seen,Chime sound MASTER WARNING light flash APU FIRE warning appears on ECAM (CWP).

• If the fire test is performed on ground with only batteries to supply the electrical network, both of the following apply

• SQUIB light and DISCH light come on, without ECAM warnings.

• The automatic shutdown of the APU on ground may occur if the flight crew performs this test for more than 3 seconds."

• Red APU FIRE PBA light partially comes on, as one of the sets of bulbs is not electrically available. During the test the APU will not auto shut down.

External APU Maintenance Panel Test

• External Maintenance Fire Panel initiates a test of the 'Fire Warning', 'Auto Extinguishing' and 'Shutdown' circuits.
• If all the circuits are operating correctly then the PBA will indicate Green ‘OK

Cargo Smoke and Bottle Tests

This test uses one fire bottle to cover each of the FWD and AFT cargo bay and each bay has multiple smoke detectors. A special control panel for this system that:

• Self tests
• ECAM WARNING Messages
• RED Smoke Lights on
• Amber ‘DISCH’ lights on

Lavatory Smoke Detection Tests

• Testing is typically part of the pre-flight checks for ground, and air crew.
• The Lavatory smoke detector is preformed by the Test press button, which triggers a warning , and aural alarm sounds and light activation, along with an amber caution on the EICAS display.
• The interrupt button and system will reset.
• Other Lavatory testing will post a malfunction if system has any fault codes.

Lavatory Fire Extinguishant Testing

• These units have no test facility but will automatically discharge their contents at around a temperature of above 77˚C..
• In this case, once the pressure is low it will indicate to the flight crew by posting a RED ‘LAV FIRE’ on the EICAS

Portable Fire Extinguishers

• There must be at least one handheld, portable fire extinguisher in the flight deck compartment, easily accessible.
• At least one hand-held fire extinguisher must be in the passenger compartment of each aircraft that accommodates more than six and less than 30 passengers.
• Each extinguisher be designed to minimize the chance of toxic gas.

Extinguishers Needed Depending on Passenger Size

• 7 to 30 1
• 31 to 60 2
• 61 to 200 3
• 201 to 300 4
• 301 to 400 5
• 401 to 500 6
• 501 to 600 7
• 601 or more 8

Halogenated Hydrocarbons

• Halogenated hydrocarbons (halons) have been the only type of fire extinguishing agent used on commercial aircraft
• Halon has been banned in some parts, but aviation usage had ben granted.
• Two types of halons used in aviation include:
  • Halon 1301 (CBrF3): discharges mostly as a gas.
  • Halon 1211 (CBrClF2): is mostly used in handheld.

Halon Concerns

• May vigorously react with any burning metal
• Do not use halon-based fire extinguishers of D class fire
• HCFC Blend B
• HFC-227ea
• HFC-236fa

Carbon Dioxide (CO2) Extinguishers

• Is a fire effective agent, also non-combustible, It is used in fire extinguishers that are available on the ramp to extinguish fires on the exterior of the aircraft.
• Dilutes air and reduces Oxygen (O2), use may cause suffocation.

Fire Extinguiser Types

• Class A, B, or C fires can be controlled by dry chemical extinguishing agents.

Water

• Class A water type fires is best controlled with water, best use and method is below ignition temperature

Extinguisher Types

• Portable fire extinguishers are used to extinguish fires in the cabin or flight deck. Halon fire extinguishers are used on electrical and flammable liquid fires
• Water – Class A
• Carbon dioxide (CO2) – Class B or C
• Dry chemicals – Class A, B, or C
• Halons – Class A, B, or C
• Halocarbon clean agents – Class A, B, or C
• Specialised dry powder – Class D
• Carbon dioxide (CO2)
• Dry chemicals
• Specialised dry powder, suitable for use in ground operations

Fire Compartments

• Cargo Compartments depend on:
• Accessibility during Flight.
• The status of fire presence detected
• Extinguishing system presence

Classing is the following:

Class A Compartments They provide for visual detection of smoke. They are accessible in flight.

Example: Small cargo compartments between the flight deck and passenger cabin.

Class B Compartments

• Sufficient access is in flight to enable a crewmember to effectively reach all parts of the compartment with a portable fire extinguisher.
• When the access provisions are being used, no hazardous quantity of smoke, flames, or extinguishing agent will enter any compartment occupied by the flight crew or passengers.
• Each compartment shall be equipped with fire detection and warning signal to the flight office Example: Cargo is located by the passenger, by the rear or front of the plane.

Class C Compartments

• There is a separate smoke or fire detector system to give warning to the flight crew
• There is an approved built-in fire extinguishing system controlled by the flight crew.
• a means is provided to exclude hazardous quantities of smoke, flames, or other noxious gases from entering any compartment occupied by the flight crew or passengers.
• Ventilation are all controlled
• Example: Wide body airplanes can maintain cargo with the floor that has smoke control.

Class D Compartments

• Class D is no longer used: this class has been integrated as class C or E On 18th March 1988

Class E Compartments

• Class E is the highest class which contains; means to shut off all airflow with the use of smoke detection Comprising of cargo with controls, which is easy to work within all conditions