The document discusses fire protection systems for aircraft. It describes the four classes of fires based on the type of fuel (A-D) and appropriate extinguishing agents for each class. It also outlines various fire detection systems, including thermo-switch, thermocouple, Fenwal, and pneumatic systems. Fire extinguishing agents work by displacing oxygen or chemically combining with oxygen to prevent combustion. Common agents are carbon dioxide, freon, halon 1301, and nitrogen. Fire extinguishing systems can be conventional or high rate discharge, with the latter utilizing compressed gases or liquids under high pressure.

1. Fire Protection Systems Prepared by: Lindsay V. Oczak Fall 2000

2. Nature of Fires Three essentials needed for fire Fuel Combustible gas, liquid or solid Oxygen Heat To bring fuel to its ignition point References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

3. Classification of Fire Four classes of fires Class A Paper, wood, textiles and rubbish Class B Liquids, such as gas, alcohol, oil and grease Class C Electrical Class D Occur in certain metals like magnesium, sodium, potassium, titanium or zirconium References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

4. Principals of Extinguishing Fires Cool the fuel below it’s ignition point Remove the oxygen supply Separate the fuel from the oxygen References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

5. Extinguishing Agents Class A Respond best to water or water type which lower the fuel below it’s ignition point. Class B Respond to carbon dioxide, halogenated hydocarbons (halons), and dry chemicals, all of which displace the oxygen supply making combustion impossible. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

6. Extinguishing Agents Class C Respond best to carbon dioxide which displaces the oxygen. Must have a non-metallic horn to prevent static electricity and transmission of electricity to operator from the fire. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

7. Extinguishing Agents Class C cont. Halons or dry chemicals may be used but the disadvantage it the powder contamination which can lead to further damage of electrical components. Class D Responds best to dry powder which prevents oxidation and and the resulting flame. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

8. Identification of Fire Extinguishers Class A Triangle containing the letter A Green Class B Square containing the letter B Red References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

9. Identification of Fire Extinguishers Class C Circle containing the letter C Blue Class D Five point star containing the letter D Yellow References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

10. Requirements for Fire Protection Systems Fire warning system must provide an immediate warning of fire or overheat by means of a red light and an audible signal in the flight deck. The system must accurately indicate that a fire had been extinguished and indicate if the fire re-ignites. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

11. Requirements for Fire Protection Systems The system must be durable and resistant to damage from all the environmental factors that may exist in the location where it is installed. The system must include an accurate and effective method for testing to assure system integrity. The system must be easily inspected, removed and installed. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

12. Requirements for Fire Protection Systems The system and components must be designed so the possibility of false indications is unlikely. The system must require a minimum of electrical power and must operate from the aircraft electrical system without inverters or other special equipment. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

13. Fire Detection Systems Four systems Thermo-switch Thermocouple Tubular type Fenwal Kidde Pneumatic type (Systron-Donner) References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

14. Thermo-switch System A circuit in which one or more thermal switches are connected to an electrical circuit with a warning horn and an aural alarm to alert the flight crew that an over-heat condition is present. If more than one thermal switch is used they are connected in parallel, so closing of any one switch will provide warning. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

15. Thermo-switch System The thermal switch, sometimes called a spot detector, works by expansion of the outer casing in the unit. When exposed to heat the casing becomes longer, causing the two contacts inside to meet, thus closing the circuit. Closing the circuit activated the warning system on the flight deck. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

16. Thermocouple System Also called a “rate of rise” detection system. A circuit where one or more thermocouples are connected in series to activate an alarm when there is a sufficient temperature increase at the sensor. Thermocouples are made of two dissimilar metals which are twisted together inside an open frame. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

17. Thermocouple System The frame allows air to flow over the wires without exposing the wires to damage. The exposed wires make a hot junction. The cold junction is located under the insulating material in the sensor unit. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

18. Thermocouple System When there is a difference in temperature a current is created. About 4 mA The current created sets off a sensitive relay activating the alarm. If the temperature rise is slow so that the cold junction heats up along with the hot junction then the relay will not be activated. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

19. Fenwal System Continuous loop system Consists of small, lightweight, flexible Inconel tube with a pure nickel conductor wire-center conductors. The space between the the nickel conductor and tubing wall is filled with porous aluminum-oxide, ceramic insulating material. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

20. Fenwal System Any voids or clearances are saturated with a eutectic salt mixture which has a low melting point. The tube is hermetically sealed at both ends with insulating material and threading fittings. When heated sufficiently, current can flow between the center wire and the tube wall because the eutectic salt melts, and the resistance drops rapidly. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

21. Fenwal System The increased current flow provides a signal which is used in the control unit to sound the alarm system. Once the fire is extinguished or the over-heat condition is corrected the eutectic salt increases its resistance and the system will return to a stand-by mode. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

22. Kidde System Continuous loop system Utilizes an Inconel tube with transmistor material embedded with two electrical conductors. One conductor is grounded to the outer shell at the end of the tube. When heated the transmistor material drops in resistance. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

23. Kidde System The change in resistance is sensed by the electronic control circuit monitoring the system and sends a warning signal to illuminate the fire warning light and activate the aural warning device. When the condition is corrected the the system returns to stand-by mode. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

24. Kidde System The sensing element in the Kidde system is unique because it consists of two wire conductors. The wire conductors are on two different circuits and allow for complete system redundancy. The control unit is a transistorized electronic device. Consists of two component board assemblies, a test switch, test jacks, wiring harness and an electrical receptacle, all enclosed in a metal case. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

25. Pneumatic System (Systron-Donner) Continuous-length system The sensing element consists of a stainless steel tube containing two separate gases plus a gas absorption material in the form of wire inside the tube. Normally the tube is filled with helium gas under pressure. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

26. Pneumatic System (Systron-Donner) The titanium center wire, which is the gas absorption material, contains hydrogen gas. The wire is wrapped in a helical fashion with an inert metal tape for stabilization and protection. Gaps between the turns of tape allow for rapid release of the hydrogen gas from the wire when the temperature reaches the required level. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

27. Pneumatic System (Systron-Donner) The sensor acts in accordance with the law of gases If the volume is held constant, its pressure will increase as temperature increases. The helium gas in the tube exerts a pressure which closes the pneumatic switch and operates the warning system. After the situation is corrected the titanium reabsorbs the hydrogen and the system returns to a stand-by mode. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

28. Fire Extinguishing Agents Agents work by two methods Displacing the oxygen Chemically combining with oxygen to prevent combustion Agents commonly used Carbon dioxide (CO 2 ) Freon (chlorinated hydrocarbon) Halon 1301 (monobromotrifluoromethane – CF 3 Br) Nitrogen (N 2 ) References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

29. Fire Extinguishing Agents CO 2 is usually limited to to older, reciprocating engine powered aircraft. Freon is used in modern aircraft. Halon 1301 is used in modern aircraft. N 2 is typically used primarily in current systems as a propellant for the other chemicals. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

30. Fire Extinguishing Agents Liquid Freon and Halon 1301 are stored under pressure in liquid form but when released are in gaseous form. When released the gas can cause frostbite due to extreme low temperatures during evaporation. Dry chemical extinguishers are not used because of the damage caused by toxic and corrosive chemicals. Some gaseous agents may be considered toxic because of the rapid displacement of oxygen when used. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

31. Fire Extinguishing Systems Conventional Used on older reciprocating aircraft Usually utilizes CO 2 High Rate of Discharge System (HRD system) Used widely and more efficient then the conventional system Utilizes Freon or Halon 1301 Used N 2 to propel the extinguishing agent at higher speeds References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

32. Conventional System Uses a perforated ring and distributor nozzle discharge arrangement. When activated the, the CO bottles are opened and the gas flows through the lines to the selected engine. At the engine the gas flows out of the perforated ring and distributor nozzles to smother the fire. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

33. HRD System Utilizes spherical bottles under high pressure actuated by explosive charges to flood the compartment and displace the oxygen. Operation Fire switch closed by crew member in the area where a fire is indicated. Explosive charge at the neck of the bottle is detonated and a cutter is driven through the sealing disk in the neck of the container. This instantly releases the extinguishing agent to the area selected. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614

34. Thermal Discharge Bottles in conventional and HRD systems utilize pressure relief fitting for discharge in high heat or pressure situations. Two methods used to determine thermal discharge Gauge on bottle visible through window on the outside of the aircraft Red blow-out disk A small pressure line is run to an open area like a wheel well and when a thermal discharge occurs pressure in the line will the blow out the disk. In a regular discharge situation either a gauge can be read or a yellow blow-out will be missing. References: Aircraft Powerplants pg: 649-657, Aircraft Maintenance and Repair pg: 602-614