Fire Protection System

[Ref Aviation Maintenance Technician Handbook–Airframe, Volume 2 (FAA-H-8083-31A) Chapter 17, AERO QTR_02.11, FAA AC No: 25.857-1, ICAO]

Fire Zone – is an area of an aircraft designed by manufacture to require fire protection and/or fire extinguishing equipment.

5 types of Fire Zone of Powerplant compartments:

  1. Class A – heavy airflow that past regular arrangements of similarly shaped obstructions
    (e.g reciprocating engine’s power section)
  2. Class B – heavy airflow that past aerodynamically clean obstructions
    (e.g turbine engine compartment)
  3. Class C – low airflow
    (e.g engine accessory compartment)
  4. Class D – very little or no airflow
    (e.g wing compartment, wheel wells)
  5. Class X – heavy airflow and unusual construction that making uniform distribution of the extinguishing agent very difficult.

DON’t mixed upPowerplant compartments Classification” with “Cargo compartment Classification

5 types of Cargo Compartment:
[The Federal Aviation Regulations classify cargo compartments into four categories:]

  1. Class A
    • located so close to the station of a crewmember that the crewmember would easily discover the presence of a fire and
    • each part of the compartment is easily accessible in flight.
  2. Class B
    • Class B cargo compartments are limited to the main deck
    • must have a “liner
    • accessible in flight but more remote from the crewmember’s station than a Class A compartment and must, therefore, incorporate a smoke or fire detection system to give warning at the pilot or flight engineer station
    • accessible and allows a crewmember to use hand fire extinguisher reach any part of the compartment to fight a fire
  3. Class F
    • is similar to a Class B  compartment – Class F must be located on the main deck of the airplane
    • is similar to a Class C  compartment – there are means to extinguish or suppress the fire without requiring a person to enter the compartment
  4. Class C
    • it is NOT required to be accessible in flight and must, therefore, have a built-in fire
      extinguishing or suppression system that is controllable from the flight deck.
  5. Class D – Class D cargo compartment has been eliminated from the FAA regulations due to limited effectiveness in providing fire protection;
    Class D compartments were designed to control a fire by severely restricting the supply of available oxygen. Due to several uncontrollable fires in Class D compartments, they were eliminated by Amendment 25-93, effective March 19, 1998.
  6. Class E
    • must have a “liner
    • is the entire cabin of an all-cargo airplane – allowed only on airplanes used strictly for carrying cargo (e.g., lower-deck, inaccessible cargo compartments)
    • air shut-off means : There are means to shut off the ventilating airflow to, or within, the compartment, and the controls for these means are accessible to the flight crew in the crew compartment.

Summary of Cargo Compartment Classification:

  • 5 (or 6) types of cargo compartment – A, B, F, C, E (and D is still existing but there is no longer have new approval)
  • Class B, F, C, and E cargo compartments have smoke detection systems that provide active fire protection.
  • Class F, C have a built-in fire extinguishing or suppression system, that is controllable from the flight deck, to extinguish or suppress the fire without requiring a person to enter the compartment
  • Class B, F must be located on main deck
  • Class E must be for Cargo Only
  • Class B, E must have liner


4 Classesof fires – A through D

  1. Class A– ordinary combustible material(e.g wood, cloth, paper, rubber and plastics)
  2. Class B– flammable liquids and gases(e.g petroleum oils, alcohols)
  3. Class C– energized electrical equipment
  4. Class D– combustible metals(e.g Mg, Ti, Na, Li…)


There are2 common types of fire protection system

  1. fixed – a permanently installed system
  2. portable – hand-hold fire extinguisher


3 common types of Fire/Overheat Detection system

  1. thermal switch
  2. thermocouple
  3. continuous loop


Thermal switch

  • used in older ACFT
  • if temperature > preset value in any section of the circuit, the thermal switch closes, completing the light circuit to indicate a fire or overheat conditio


  • Application: thermocouple is common for the Engine fire detection
  • consists of two dissimillar metal (e.g chromel and constantan)
  • If temperature rises rapidly, voltage produced between “reference junction’ and “hot junction”.
    (the “reference junction” is enclosed in a dead air space and the “hot junction” is open-ended and exposed as a detection point)
  • disadvantage: depend on the rate-of temperature rise – not give a warning if an engine slowly overheats or a short circuit develops

Continuous loop

  • Application: used in transports aircraft – the powerplant (engines) and wheel well
  • There are 2 sub-type of continuous loop detection system
    1. thermistor type detector (e.g Kiddle, Fenwal System)
      (- temperature changed the resistance and in turn current is changed and detected by the control unit)
    2. pneumatic pressure detector (e.g Lingberg system)


Principle of thermistor type detector is that temperature changes the resistance of the sensing elements and in turn current is changed which is detected by the control unit

Principle of Fenwal System

The sensing elements are connected in series to a control unit. If overheat condition occurs at any point along the sensing elements, resistance of “Eutectic salt packing” drops sharply, and current is produced between the inner center conductor and “outer sheath

Principle of Kiddle System

Two wire are imbedded in an inconel tube. One conductor (wires) has a ground connection to the tube, the other conductor connects to the fire detection control unit. If overheat condition occurs, resistance to the ground decreases, control unit can monitor this resistance.



In general, fire/Overheat detection system is dual-loop system and using “AND Logic” – two complete basic fire detection system must output same signal to trigger the fire warning. The benefit is that there is better reliability against false warning.


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