AMT – 111 Fire Protec/on Systems Chapter Fire Protec/on Systems •  Fire is one of the most dangerous threats to an aircra= •  Fire zones An area, or region, of an aircra= designed by the manufacturer to require fire detec/on and/or fire ex/nguishing equipment and a high degree of inherent fire resistance   Loca/on – fixed or non-­‐fixed 
•  Non-­‐fixed are serviced by hand held ex/nguishers •  Examples: Fixed Fire Zones Engines and auxiliary power unit (APU)   Cargo and baggage compartments   Lavatories on transport aircra=   Electronic bays   Wheel wells   Bleed air ducts 
•  Serviced by fire detec/on and ex/nguisher systems Detec/on Systems •  Most common: Overheat detectors   Rate-­‐of-­‐temperature-­‐rise detectors   Flame detectors   Observa/on by crewmembers 
Detec/on Systems •  Addi/onal op/ons: Radia/on sensing detectors   Smoke detectors   Overheat detectors   Carbon monoxide detectors   Combus/ble mixture detectors   Op/cal detectors 
Classes of Fires •  Class A -­‐ fires involving ordinary combus/ble materials, such as wood, cloth, paper, rubber, and plas/cs •  Class B -­‐ fires involving flammable liquids, petroleum oils, greases, tars, oil-­‐based paints, lacquers, solvents, alcohols, and flammable gases Classes of Fires •  Class Class C -­‐ Energized electrical equipment •  Class D -­‐ fires involving combus/ble metals, such as magnesium, /tanium, zirconium, sodium, lithium, and potassium Detec/on & Protec/on Systems •  Requirements: No false warnings under any flight or ground condi/on   Rapid indica/on of a fire and accurate loca/on of the fire   Accurate indica/on that a fire is out   Indica/on that a fire has re-­‐ignited   Con/nuous indica/on for dura/on of a fire   Means for electrically tes/ng the detector system from the aircra= cockpit 
Detec/on & Protec/on Systems •  Requirements: Resists damage from exposure to oil, water, vibra/on, extreme temperatures, or handling   Light in weight and easily adaptable to any moun/ng posi/on   Circuitry that operates directly from the aircra= power system without inverters   Minimum electrical current requirements when not indica/ng a fire   Cockpit light that illuminates, indica/ng the loca/on of the fire, and with an audible alarm system   A separate detector system for each engine 
Fire/Overheat Detec/on Systems •  Types: Thermal switch   Thermocouple   Con/nuous Loop 
Thermal Switch Thermal Switch Systems •  Thermal switches are bimetallic switches •  Thermal switches complete a circuit when they reach a cri/cal temperature •  Mul/ple thermal switches are placed in parallel If any of the thermal switches reaches the cri/cal temperature, the alarm circuit is complete   A failed switch will not take down the whole system 
•  All the thermal switches are in series with the indicator light or circuit Thermocouple Systems Thermocouple Systems •  Thermocouple system measure the rate of temperature rise A fast rise will indicate a fire   Doesn’t use a cri/cal temperature 
•  Thermocouple are polarized Hot junc/on   Reference junc/on (cold temperature) 
•  Thermocouples generate a small amount of current as they heat up 
Current must be amplified •  Thermocouples are connected in series Con/nuous Loop Detec/on Systems •
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Used in powerplant and wheel wells Superior detec/on and coverage Rugged Type: 
Thermistor •  Kidde •  Fenwal 
Pneuma/c •  Lindberg Con/nuous Loop Detec/on Systems Fenwal System Fenwal System •  Single wire system •  Outer tube is conduc/ve •  Tube filled with eutec/c salt Lowers resistance when tube gets hot   Completes circuit between outer tube and internal wire   Once the salt cools, the circuit is opened (reset) again 
•  Can be connected in parallel for redundancy (protect against failures) Kidde System Kidde System •  Similar to Fenwal but the electrical connec/ons are two internal wires (two d’s in Kidde) •  Core is a thermistor core material More linear change to temperature changes than eutec/c salt   Can double as a temperature sensor   Can give separate indica/ons for overheat or fire condi/on 
Pneuma/c Con/nuous-­‐Loop Systems Pneuma/c Con/nuous-­‐Loop Systems •  Lindberg 
Also called Systron-­‐Donner or Meggi^ Safety Systems •  Heat causes the internal gases to expand and trip a pressure sensor Helium gas averages the temperature over the whole sensor   Hydrogen gas is used to detect spot hea/ng   When gases cool, the system resets 
Con/nuous-­‐Loop Systems •  Physical damage to the tubes will damage them 
Dents, kinks, or crushed •  Tubes come pre-­‐bent 
Don’t make your own bends •  Damaged tubes can’t be repaired, they must be replaced •  Class A zone 
Fire Zones Area of heavy airflow past regular arrangements of similarly shaped obstruc/ons. •  Example: Power sec/on of a reciproca/ng engine •  Class B zone 
Area of heavy airflow past aerodynamically clean obstruc/ons •  Example: heat exchanger ducts & exhaust manifold shrouds •  Areas where the inside of the enclosing cowling or other closure is smooth, free of pockets, and adequately drained so leaking flammables cannot puddle •  Class C zone 
Fire Zones Area of low airflow past regular arrangements of similarly shaped obstruc/ons. •  Example: Engine accessory compartment separated from the power sec/on •  Class D zone 
Area of very li^le or no airflow •  Examples: Wing compartments and wheel wells •  Class X zone 
Area of heavy airflow and of unusual construc/on •  Difficult to create uniform distribu/on of the ex/nguishing agent Smoke Detec/on Systems •  Presence of smoke is an indica/on of fire •  Used in lavatories plus cargo and baggage compartments Limited airflow   Substan/al amount of smoke is generated before temperature changes are sufficient to actuate a heat detec/on system 
•  Types (method of detec/on): Light Refrac/on   Ioniza/on 
Smoke Detec/on Systems •  Light Refrac/on Photoelectric cell that detects light refracted by smoke par/cles   Uses a reference light source to “see smoke” 
•  Ioniza/on 
Detects a change in ion density due to smoke in the cabin Carbon Monoxide Detectors •  Colorless, odorless and tasteless gas Byproduct of incomplete combus/on   Lighter than air   The most common type of fatal air poisoning in many countries 
•  Carbon monoxide detectors are used in areas with limited airflow such as cockpits or passenger compartments Carbon Monoxide Detectors •  Chemical Detector Silica-­‐gel crystals that have been dyed with a yellow indicator dye   Turns green when exposed to carbon monoxide 
Carbon Monoxide Detectors Carbon Monoxide Detectors •  Chemical Detector Silica-­‐gel crystals that have been dyed with a yellow indicator dye   Turns green when exposed to carbon monoxide   Lasts 90 days and then must be replaced 
•  Can also electrically detected and measure by: Biometric– A chemical reac/on that changes color like blood and measured electrically   Electrochemical – reacts with sulfuric acid to generate electricity   Sensors have limited life and must be replaced 
Ex/nguishers Ex/nguishers •  Portable ex/nguishers 
Usually Halon or CO2 •  Some/mes dry powder A commercial aircra= requires a least one portable ex/nguisher in the cockpit   Aircra= carrying over 6 people will have a minimum portable ex/nguisher requirements   Non-­‐metallic (plas/c horns) are required for an electrical fire   FAA says dry-­‐powder ex/nguishers should be used on brake fires 
•  Combus/ble metals are used in wheel and landing gears •  Halon Ex/nguisher Agents Halogenated hydrocarbons   Harms the earth’s ozone layer 
•  Produc/on is banned Clean agent (no residue), electrically non-­‐conduc/ng and “rela/vely” low toxicity   Halon 1301 – total flooding agent   Halon 1211 – streaming agent 
•  Halon Ex/nguisher Agents Other Halons too poisonous to use   Should be recycled   Being replaced with halocarbons HCFC Blend B, HFC-­‐227ea, HFC-­‐2 and HCL-­‐125   Used on Class A, B, & C fires 
•  Not Class D – may react with burning metals 
Most common agent used in ex/nguishing systems (non-­‐portable) •  CO2 
Ex/nguisher Agents Carbon dioxide •  Two oxygen atoms 
1 ½ /mes heavier than air •  Drops to the base of the fire and smothers the fire by displacing the oxygen Cools the fire   Mildly toxic   Used on Class A, B, & C fires   Clean 
•  Good for small engine fires •  Water Ex/nguisher Agents Smothers and cools fire   Prevents re-­‐igni/on   Used on Class A fires 
•  Dry Power Messy   Class of fire varies with chemicals used in powder 
•  Check label 
Specialized dry powder can be used on Class D powders Ex/nguishing Systems •  Components: Agent Containers   Discharge Valves   Pressure Indicators   Two-­‐way Check Vales   Thermal Discharge Indicator (Red Disk)   Yellow Disk Discharge Indicator   Fire Switch (ac/va/on switches) 
Ex/nguishing System Agent Container Ex/nguishing Systems •  Agent Containers Stores fire ex/nguishing agent   Usually pressurized with nitrogen   Has an over pressure release 
•  Discharge Valves An explosive charge (squib) is electrically detonated   The explosion breaks a frangible disc that releases the ex/nguishing agent 
•  Pressure Indicators 
Indicates the status of the containers load Ex/nguishing Systems •  Two-­‐Way Check Valves 
Directs agent to the correct loca/on •  Thermal Discharge Indicator (Red Disk) Broken/missing red disk indicates the container has released agent due to overpressure   Container must be serviced 
•  Yellow Disk Discharge Indicator Missing yellow disk indicates the agent has been used   Container must be serviced 
Ex/nguishing Systems •  Fire Switch (ac/va/on switches) 
Releases the agent Ex/nguishing System Maintenance •  Regular inspec/ons are required •  Portable ex/nguishers are checked by weight or pressure check •  Discharge valve squib must be handled with care 
Ground/short the terminals when working with the squib •  Prevents accidental discharge Insure no voltage is present on squib trigger leads when installing squib   Squib and valve may be a matched set and must be replaced as a set 
Ex/nguishing System Maintenance •  Ex/nguishing systems are check by pressure Acceptable pressure range will vary by temperature   Containers out of range must be serviced 
Ex/nguishing Pressure Chart Ex/nguishing Pressure Chart €
Ex/nguisher Pressure Problem •  FAA problem requires interpola/ng a chart •  Perform the following equa/on for both the Minimum and Maximum column to determine the range Temp = SmallTemp +
(L argeTemp − SmallTemp) X (Given Pr es − SmallPr es)
(L argePr es − SmallPr es)
Ex/nguishing System •  Reminder: Aircra= plumbing that contains fire-­‐
ex/nguishing agents is color coded with a stripe of brown tape and a series of diamonds.