Fire Detection System Questions

• Modern aircraft fire protection includes a fire detection system, as well as a fire extinguishing system.
• Typical zones needing fire detection or extinguishing include: -Engine and Auxiliary Power Unit (APU) -Cargo and Baggage compartments -Transport aircraft lavatories -Electronic bays -Wheel wells -Bleed air ducts
• Aircraft fires are dangerous because of the presence of fuel, ignition sources like electrical devices and hot components, and the potential for catastrophic consequences if not addressed early.

Fire/Overheat Detection Systems

• These systems rapidly detect localized fire or overheat conditions, and indicate the area affected.
• Detectors should not automatically activate extinguishing units during flight, but may shut down electrical power or fuel to specific areas.
• Fire detection systems must: -Be reliable -Immediately indicate a fire or overheat condition, and when it is resolved -Fail into an inoperative state rather than causing erroneous readings. -Operate independently -Function when main power is off using aircraft's 28VDC or hot battery bus.
• Aircraft fire warning systems alert the flight crew to fires or overheat, and detector units are in high-risk locations.
• A fire is indicated on the flight deck by simultaneous visual and aural warnings including a red master warning light, a local red warning light showing the fire's location, and a ringing warning bell.
• Overheat warning systems are in high-temperature areas that could lead to a fire.
• They are commonly used in engine turbine areas, nacelles, wheel wells, and hot pneumatic manifolds depending on aircraft type.
• Overheat conditions trigger an amber Master Caution light and a local caution light.
• Fire protection systems often use multiple detection methods, including thermal switches, thermocouples, and continuous loop sensing elements.
• Older aircraft often use thermal switch or thermocouple systems, while newer ones use continuous loop systems for better coverage.

Thermal Switch Detector

• Thermal switches, also known as thermostat switches or spot detectors, are mounted inside a stainless steel housing which heats up causing contacts to close.
• Thermo switch adjustments involve heating the housing to a specific temperature and adjusting tension until the contacts close and are set by the detector manufacturer.
• All thermo switches are bolted to the aircraft structure, grounding one contact with the other contact of each switch connected in parallel.
• When any switch closes, a current path is made for the coil of relay RL1 which when energized, completes a current path for the bell and red light.
• Pressing the bell cancel button energizes RL2 which breaks the bell's current path and completes its own coil's current path, stopping the bell.
• RL2 remains energized until the fire warning disappears, this system typically operates on the 28VDC battery/power bus.
• A test switch completes a ground connection to RL1 to test the wiring continuity, to check the bell rings, and that the red light illuminates.
• If the bell cancel switch when pressed should stop the bell, the red light remains on and individual switches can be tested using an ohmmeter.
• Using a switch with an incorrect operating temperature can lead to false warnings or a lack of warning.

Thermocouple Fire Detection System

• A typical thermocouple fire detector has its hot junction exposed to the air and the cold junction insulated within the detector body.
• When the hot junction heats up, it causes a current increase which is directly proportional to temperature and when the current hits a preset level, a warning is activated.
• This system is used in engines and aircraft pneumatic systems, and operates on the aircraft's 28VDC system for redundancy.
• Testing of thermal switches involves using a Jet-Cal or similar tester at specified periods to measure the temperature at which the test light illuminates.
• A thermocouple system may use multiple thermocouples in fire zones around an engine and a reference thermocouple in an airspace.
• Under normal conditions the temperature around the reference and active thermocouples is relatively even and no current flows, whereas when a fire occurs, the temperature difference creates a current that activates a warning.
• Closing the test function passes current thru a heater around the thermocouple in the thermal test unit causing the output will cause the system to operate.
• The test is designed to check the circuit continuity, and each is not tested.
• Due to low voltage currents, connections must be clean and tight and when being replaced, new thermocouples must be connected with correct polarity.

Fire Wire System - Continuous Loop

• Continuous-loop detectors give more complete fire hazard coverage than spot-type detectors and are overheat detection systems that complete an electrical circuit at a certain temperature.
• There is no rate-of-heat rise sensitivity in a continuous-loop system and Fire Wire systems are made by Fenwal and Walter Kidde, used for engine fire and bleed air system overheating detection.
• The Fenwal system which uses a single conductor element and the Walter Kidde system uses a dual conductor element.
• Fenwal's single conductor element consists of a thin-walled Inconel tube filled with compound, with a nickel conductor embedded in the center.
• Two forms of filling compound include: -Ceramic beads with eutectic salts, where the melting point of the salts is low and the resistance decreases when heated. -Aluminum oxide in fibrous glass, where resistance lowers when heated.
• The Kidde Fire Wire element has an Inconel tube with two nickel conductors in a thermistor material, where resistance decreases as temperature increases.
• One wire is welded to the case for internal ground, the second becomes a hot lead providing a current signal that is dependent on the ceramic core.
• Damage from poor installs, tight bends, or being hit by objects can cause the system to fail or give false warnings.

Pneumatic Continuous Loop System

• Pneumatic continuous-loop systems expand a contained gas due to heat, used for engine overheat and fire detection in transport aircraft.
• False alarms are a major concern, so sensor tubes are designed to avoid mechanical damage causing false alarms; severe damage will give a "no test" indication.
• Flight and maintenance crews do a system integrity test before each engine start to confirm system serviceability.
• Pneumatic detectors have two functions: -Sensing an average temperature threshold -A localized discrete temperature increase

Average Temperature Sensing

• This function expands a fixed helium volume inside the detector to create a pressure increase proportional to absolute temperature and it will set off the alarm switch between 93°C (200°F) to 371°C (700°F).

Discrete Temperature Sensing

• This uses a hydrogen-filled core in the sensor tube.
• Release of hydrogen occurs when a small section of the tube reaches 1,100° Celsius (2000° Fahrenheit) for 5 seconds.
• This outgassing increases pressure to activate the alarm switch, which is then reversible, so when the sensor cools, the hydrogen returns to the core.
• Lindberg and Systron-Donner are known for their methods of construction and operation.

Lindberg System

• It consists of a stainless steel tube with inert gas and material to absorb a portion of the gas which is sealed at one end and the other end connected to a diaphragm switch unit.
• Temperatures surround the element.
• Rising temps heat the discreet material while it releases a portion of the absorbed gas, increasing the pressure in the tube, it forces electrical contacts to close and activate an alarm on the flight deck.
• Testing the Lindberg system, a test unit sends low voltage AC current.
• The heat releases the gas from discrete material which then activates the warning system.
• When switch is released the sensing element cools with material to reabsorb the gas to reduces pressure and opens the detector element.

Systron-Donner System

• This pneumatic continuous system uses a stainless steel tube filled with helium gas and a titanium wire, with the wire contains hydrogen.
• The tube seals on one end with the other at the responder, containing two electrical switches run by a diaphragm assembly.
• An integrity switch closes given high gas pressure/ If tester is running, lights and bells will ensure the alarm will work.

Systron-Donner - Normal temperatures

• Helium gas pressure can not close alarm switch, when the temp gets to a level where the overall line is way up, overall switch contact closes, activating alarm.
• Fire warning is given when the titanium wire has a discrete heat spot like a fire of air escaping and delivers hydrogen gas that increasing the internal pressure in stainless steel tube, triggering alarm.
• After fire has gone, the titanium wire then reabsorbs through the diaphragm, resetting the alarm.

Continuous Loop Maintenance

• Fire components come in different shapes that are shown on the component.
• Joined parts combined to make a component one piece by plug and sockets.

Element installment

• The component is fitted around the engine/ part clamped with seals.
• Once cable is replaced, the cable should be installed the way it was before

Continuous Loop Compliance

• No sharp ends
• Seals should have clamp at correct radius
• Separate gromets so element will touch anything that is not touching
• Component connects should be clean

Continuous Element inspection

• Component can not be broken, damage to surface or chaffed
• Damage under standard maintenance
• Seals should be replaced

Controller types

• There is a series of component control units with fire devices and work with only capacity and resistance

Controller with only Resistance

• Twin conductor uses thermo stuff.
• Temp is preset, current is sent to controller and biases to switch on indicators devices.

Component with Capacity

• Element runs as cap, keeping positive 1/2 cycle and sending it at half time. when temps goes up the element will energize relay.

Overheat Detection

• System does not see test on circuit
• Circuits are connected to warning light.
• The temperature set is 100C.
• Both detectors connected to warning lamp.

Smoke indicator systems

• Monitors cabing air finding smoke that could lead to bigger problem.
• Cargo/ equipment in airplanes (transport aircraft) can get checked

Smoke

• Used when fire is burning to indicate alarm fast. When monoxide is present it can cause flight.
• Two types - Photo electric and lonization

Refaction Photo Electric

• Air passes in pilot lamp where circuit resistance allows overload in the aircraft system

Refaction Electric cell

• Situated at 90 degrees, Light is aimed at air sample which lights 10% which switches some light towards pilot diode to indicate

Photo Diode

• light is applied with circuit resistance decreases allowing signal to overload, card and indicators.

Fire test light

• LED is there as back up
• Indicates ok test lamp, the pilot indicates light.
• Detect all circuits are open

Comparing light

• Light shows smoke
• Sealed box shows air pass and smoke
• Indicator has the diode test and central alarm

Class types

• One electrode has little amount of waste, and radiation travels to electricity gases

Cargo comparments

• Warn if cargo is burning in compartment. System sends messages to pilot
• Sensors keep area clear
• System takes continuous samples for air samples

Smoke detection operation

• Aboard plane, sent to indicator to display aircraft system
• Bottle used to keep it secure while taking action.

Safety measurements

• 3 points need present ( fire 3 side) to be kept safe. oxygen, fuel, ignition to source.

Fire extinguishes

• Keeps safe from plane aircraft.
• Can take away 2 sides to keep safe

Areas fire safety is kept

• Areas fire protection will engage 1 Engines 2 Auxiliary Power Unit (APU) 3 Cargo Hold 4 Lavatories

High pressure release

• Fixed fire safety will discharge at one, more box.

HR systems

• Made from steel, spherically look which has the same four sizes in place.
• Sizes vary but have two holes for 1- plug.

Halon

• Each has part full to seal it
• Tank test shows how much pressure is being applied,
• Squib fires and pushes on the tank to push seal

Fuel indicator

• Blows a red indicator
• Both spots release and test

Multi engine usage

• Equipped with bonnets show the plane is ready to use.

Brown tape

• Shows the pipeline.
• Indicates the system is running

Extinquishes

• Is halon - used to keep the
• color, smell, gas
• Its fast acting & coolant
• Blankets flame
• No stain
• Causes damage.
• Check msds
• Protects from eyewear

Pilot and water

• Aircraft are covered. All have some rubber for fire protect, they test the rubber for safety
• Used for handheld devices for aircraft fires
• Powder used to test
• Fire safety.

Halon Types

• Chemical name is C8rC1F2.
• Fire safe but high heat temp
• The Australian Commonwealth tests it.
• Fire device with aircraft
• USA has different requirements

Questions about fire detection systems, including thermal switches and thermocouple systems. Covers testing procedures, indications, and potential issues with incorrect components. Addresses the function, operation, and inspection of these systems.