Aircraft Systems Fire Protection Systems Chapter 5 PDF

ME3531 Aircraft Systems Chapter 5: Fire Protection Systems For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Introduction A fire is one of the most dangerous threats to an aircraft, the potentia...

ME3531 Aircraft Systems Chapter 5: Fire Protection Systems For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Introduction A fire is one of the most dangerous threats to an aircraft, the potential fire zones of modern multiengine aircraft are protected by a fixed fire protection system. The term “fixed” describes a permanently installed system in contrast to any type of portable fire extinguishing equipment, such as a hand-held Halon or water fire extinguisher. A complete fire protection system on aircraft, includes a fire detection system and a fire extinguishing system. Typical zones on aircraft that have a fixed fire detection and/or fire extinguisher system are: Engines and auxiliary power unit (APU) Cargo and baggage compartments Lavatories on transport aircraft Electronic bays Wheel wells Bleed air ducts Page  2 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Introduction The complete aircraft fire protection systems of most large turbine-engine aircraft incorporate several of these different detection methods. Rate-of-temperature-rise detectors Overheat detectors Smoke detectors Carbon monoxide detectors Observation by crew or passengers Classes of Fires The following classes of fires that are likely to occur on-board aircraft. Class A—fires involving ordinary combustible materials, such as wood, cloth, paper, rubber, and plastics. Class B—fires involving flammable liquids, petroleum oils, greases, tars, oil-based paints, lacquers, solvents, alcohols, and flammable gases. Class C—fires involving energized electrical equipment in which the use of an extinguishing media that is electrically nonconductive is important. Class D—fires involving combustible metals, such as magnesium, titanium, zirconium, sodium, lithium, and potassium. Page  3 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Requirements for Overheat and Fire Protection Systems Fire protection systems on current-production aircraft do not rely on observation by crew members as a primary method of fire detection. An ideal fire detector system includes as many of the following features as possible: 1. No false warnings under any flight or ground condition. 2. Rapid indication of a fire and accurate location of the fire. 3. Accurate indication that a fire is out. 4. Indication that a fire has re-ignited. 5. Continuous indication for duration of a fire. 6. Means for electrically testing the detector system from the aircraft cockpit. 7. Resists damage from exposure to oil, water, vibration, extreme temperatures, or handling. 8. Light in weight and easily adaptable to any mounting position. 9. Circuitry that operates directly from the aircraft power system without inverters. 10. Minimum electrical current requirements when not indicating a fire. 11. Cockpit light that illuminates, indicating the location of the fire, and with an audible alarm system. 12. A separate detector system for each engine. Page  4 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Fire Detection/Overheat Systems A fire detection system should signal the presence of a fire. Units of the system are installed in locations where there are greater possibilities of a fire. Three detector system types in common use are the thermal switch, thermocouple, and the continuous loop. Thermal Switch System The thermal switches are heat-sensitive units that Thermal switch fire circuit complete electrical circuits at a certain temperature. They are connected in parallel with each other but in series with the indicator lights. If the temperature rises above a set value in any one section of the circuit, the thermal switch closes, completing the light circuit to indicate a fire or Spot detector overheat condition. Page  5 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Thermocouple Systems The thermocouple is constructed of two dissimilar metals, such as chromel and constantan. The point at which these metals are joined and exposed to the heat of a fire is called a hot junction. The thermocouple fire warning system operates on an entirely different principle from the thermal switch system. A thermocouple depends on the rate of temperature rise and does not give a warning when an engine slowly overheats or a short circuit develops. Thermocouple If there is a fire, the hot junction heats more rapidly than the reference junction, A warning light to give a visual fire warning will be provided to the aircrew. Page  6 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Continuous-Loop Systems A continuous-loop detector or sensing system permits more complete coverage of a fire hazard area than any of the spot-type temperature detectors and used on powerplant and wheel well area. Two widely used types of continuous-loop systems are the The thermistor type detectors, such as the Kidde and Sensing elements of a the Fenwal systems continuous-loop system The pneumatic pressure detector, such as the Systron-Donner system Page  7 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Pneumatic Continuous-Loop Systems The pneumatic detector has two sensing functions. It responds to an overall average temperature threshold and to a localized discrete temperature increase caused by impinging flame or hot gasses. Both the average and discrete temperature are factory set and are not field adjustable. Averaging Function The fire/overheat detector serves as a fixed-volume device filled with helium gas. The helium gas pressure inside the detector increases in proportion to the absolute temperature and operates a pressure diaphragm that closes an electrical contact, actuating the alarm circuit. The pressure diaphragm within the responder assembly serves as one side of the electrical alarm contact and is the only moving part in the detector. The alarm switch is preset at an average temperature. Typical temperature ranges for average temperature settings are 200 °F (93 °C) to 850 °F (454 °C) Page  8 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Pneumatic Continuous-Loop Systems Discrete Function The fire/overheat detector’s sensor tube also contains a hydrogen-filled core material. Large quantities of hydrogen gas are released from the detector core whenever a small section of the tube is heated to the preset discrete temperature or higher. The core outgassing increases the pressure inside the detector and actuates the alarm switch. Both the averaging and discrete functions are reversible. When the sensor tube is cooled, the average gas pressure is lowered, and the discrete hydrogen gas returns to the core material. The reduction of internal pressure allows the alarm switch to return to its normal position, opening the electrical alarm circuit. Test circuits, which include a pressure warning switch, will indicate the operational condition of the system. If helium gas pressure is lost, the test circuit warns the flightcrew that the system is not operational. Page  9 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Smoke Detection System A smoke detection system monitors the lavatories and cargo baggage compartments for the presence of smoke, which is indicative of a fire condition. Smoke detection instruments that collect air for sampling are mounted in the compartments in strategic locations. Two common types used are light refraction and ionization. Light Refraction Type The light refraction type of smoke detector contains a photoelectric cell that detects light refracted by smoke particles. Smoke particles refract the light to the photoelectric cell and, when it senses enough change in the amount of light, it creates an electrical current that sets off a warning light. This type of smoke detector is referred to as a photoelectrical device. Photoelectric device Page  10 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Smoke Detection System Ionization Type Ionization type smoke detectors use a small amount of radioactive material to ionize some of the oxygen and nitrogen molecules in the air sample drawn into the detector cell. These ions permit a small current to flow through the detector chamber test circuit. If smoke is present in the air sample being drawn through the detector small particles of the smoke will attach themselves to the oxygen and nitrogen ions, reducing the electrical current flow in the Ionization type smoke detectors test circuit. If the current flow falls below a preset value, the alarm circuit will activate visual and aural cockpit alarms. Page  11 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Smoke Detector System Optical Type The optical smoke detector consists of source light emitting diodes (LEDs), intensity monitor photodiodes, and scatter detector photodiodes. Inside the smoke detection chamber, the fans draw air from the sampling ports through a water separator and a heater unit to the smoke detector which is between a source (LED) and a scatter detector photodiode. Usually, only a small amount of light from the LED gets to the scatter detector. If the air has smoke in it, the smoke particles reflect more light on the scatter detector. This causes an alarm signal. The intensity monitor photodiode makes sure that the source LED is on and keeps the output of the source LED constant. This configuration also finds contamination of the LED and photodiodes. A defective diode, or contamination, causes the detector to change to the other set of diodes. The detector sends a fault message. Optical Type Page  12 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Lavatory Smoke Detectors Smoke indications provide a warning light in the cockpit or provide a warning light or audible warning at the lavatory and at flight attendant stations that would be readily detected by a flight attendant. Each lavatory must have a built-in fire extinguisher that discharges automatically. The smoke detector is located in the ceiling of the lavatory. If there is smoke in the sensing chamber of the smoke detector, the alarm LED (red) comes on. The warning horn and lavatory call light operate intermittently Lavatory smoke detector Page  13 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Flame Detection System Optical sensors, often referred to as flame detectors, are designed to alarm when they detect the presence of prominent, specific radiation emissions from hydrocarbon flames. The two types of optical sensors available are infrared (IR) and ultraviolet (UV) When radiation emitted by the fire crosses the airspace between the fire and the detector, it impinges on the detector front face and window. The window allows a broad spectrum of radiation to pass into the detector where it strikes the sensing device filter. The filter allows only radiation in a tight waveband centered on 4.3 micrometers in the IR band to pass on to the radiation-sensitive surface of the sensing device. The radiation striking the sensing device minutely raises its temperature causing small thermoelectric voltages to be generated. These voltages are fed to an amplifier whose output is connected to various analytical electronic processing circuits. The processing electronics are tailored exactly to the time signature of all known hydrocarbon flame sources and ignores false alarm sources, such as incandescent lights and sunlight Page  14 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Carbon Monoxide Detection System Carbon monoxide is a colorless, odorless gas that is a byproduct of incomplete combustion. Its presence in the breathing air of human beings can be deadly. To ensure crew and passenger safety, carbon monoxide detectors are used in aircraft cabins and cockpits. There are several types of carbon monoxide detectors. Electronic detectors and Chemical color-change types that are common. Normally, the color of the chemical is tan. In the presence of carbon monoxide, the chemical darkens to grey or even black. If contaminated, installing a new indicating element allows a carbon monoxide portable test unit to be returned to service. Page  15 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Extinguishing Agents Halogenated Hydrocarbons Although Halon is an ozone depleting and global warming chemical, Halon has been the fire extinguishing agent of choice in civil aviation because it is extremely effective on a per unit weight basis over a wide range of aircraft environmental conditions. It is a clean agent (no residue), electrically nonconducting, and has relatively low toxicity. Two types of Halons are employed in aviation: Halon 1301a total flooding agent, and Halon 1211 a streaming agent. Inert Cold Gases Carbon dioxide (CO2) is an effective extinguishing agent. It is most often used in fire extinguishers that are available on the ramp to fight fires on the exterior of the aircraft, such as engine or APU fires. CO2 is effective as an extinguishing agent primarily because it dilutes the air and reduces the oxygen content so that combustion is no longer supported. Page  16 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Installed Fire Extinguisher Systems Transport aircraft have fixed fire extinguishing systems installed in: Turbine Engine compartments APU compartments Cargo and baggage compartments Lavatories Cargo and Baggage Compartment Fire Detection and Extinguisher System The cargo compartment smoke detection system gives warnings in the flight deck if there is smoke in a cargo compartment. Each compartment is equipped with a smoke detector. The smoke detectors monitor air in the cargo compartments for smoke. The following indications occur in the cockpit if there is smoke in a cargo compartment: Master warning lights come on. Fire warning aural operates. A cargo fire warning message shows. Cargo fire warning light comes on. Cargo fire detection warning Page  17 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Cargo Compartment Extinguisher System The cargo compartment extinguishing system is activated by the flight crew if the smoke detectors detect smoke in the cargo compartment. Some aircraft are outfitted with two types of fire extinguisher containers. The first system is the dump system that releases the extinguishing agent directly when the cargo fire discharge switch is activated. This action extinguishes the fire. The second system is the metered system. After a time delay, the metered bottles discharge slowly and at a controlled rate through the filter regulator. Halon from the metered bottles Cargo and baggage compartment extinguishing system. replaces the extinguishing agent leakage. This keeps the correct concentration of extinguishing agent in the cargo compartment to keep the fire extinguished for 180 minutes. Page  18 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Cargo Compartment Extinguisher System The extinguishing bottles are outfitted with squibs. The squib is an electrically operated explosive device. It is adjacent to a bottle diaphragm that can break. The diaphragm normally seals the pressurized bottle. When the cargo discharge switch is activated, the squib fires and the explosion breaks the diaphragm. Nitrogen pressure inside the bottle pushes the Halon through the discharge port into the cargo compartment. When the bottle discharges, a pressure switch is activated that sends an indication to the flight Cargo and baggage compartment extinguishing system. deck that a bottle has been discharged. Page  19 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Lavatory Smoke Extinguisher System The lavatory compartment is outfitted with a fire extinguisher bottle to extinguish fires in the waste compartment. The fire extinguisher is a bottle with two nozzles. The bottle contains pressurized Halon 1301 or equivalent fire extinguishing agent. When the temperature in the waste compartment reaches approximately 170 °F, the solder that seals the nozzles melt and the Halon is discharged. Lavatory fire extinguishing bottle. Page  20 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Fire Detection System Maintenance Fire detector sensing elements are located in many high activity areas around aircraft engines. Their location, together with their small size, increases the chance of damage to the sensing elements during maintenance. General maintenance of a fire detection system typically includes the inspection and servicing of damaged sections, containment of loose material that could short detector terminals, correcting connection joints and shielding, and replacement of damaged sensing elements. An inspection and maintenance program for all types of continuous-loop systems should include the following visual checks. Sensing elements of a continuous-loop system should Sensing element defects. be inspected for the following: 1. Dents and kinks in sensing element sections. Limits on the element diameter, acceptable dents and kinks, and degree of smoothness of tubing contour are specified by manufacturers. No attempt should be made to straighten any acceptable dent or kink, since stresses may be set up that could cause tubing failure. Page  21 For Training Purpose Only  Official (Closed), Non-Sensitive Fire Protection Systems Fire Detection System Maintenance Sensing elements of a continuous-loop system should be inspected for the following: Connector joint fitting attached to the structure. 2. Nuts at the end of the sensing elements should be inspected for tightness and safety wire. Loose nuts should be retorqued to the value specified by the manufacturer’s instructions. 3. Sensing element routing and clamping should Rubbing interference. be inspected carefully. Long, unsupported sections may permit excessive vibration that can cause breakage. 4. Grommets should be installed on the sensing element so that both ends are centered on its clamp. The split end of the grommet should Inspection of fire detector loop clamp. face the outside of the nearest bend. Clamps and grommets should fit the element snugly. Page  22 For Training Purpose Only