Aviation Investigation Report A98H0003

1.6.12  Aircraft Fire Protection System

  1. - General
  2. - Portable Fire Extinguishers
  3. - Engine/APU/Cargo and Lavatory Fire Extinguisher Bottles  General (STI1-19)

While no zone of an aircraft is immune to in-flight fires, fire protection systems used in transport category aircraft have evolved based on the probability of fire ignition within particular zones of the aircraft. These aircraft are equipped with a variety of built-in detectors and, in some cases, associated suppression systems designed to assist the aircraft crew in identifying and extinguishing an in-flight fire. In accordance with FAA airworthiness certification requirements, the occurrence aircraft was equipped with built-in fire detection and suppression capabilities in the aircraft's designated fire zones (see Figure 2). FAR 25.1181 states that a designated fire zone includes engines, APUs, and any fuel-burning heater or combustion equipment. In addition, specific regions of the aircraft, such as cargo compartments and lavatories, have been identified as "potential fire zones"[41] that require various built-in detection and suppression capabilities.

The fire risk to the remainder of the pressure vessel was such that it did not have, nor was it required to have, built-in detection and suppression equipment. Therefore, the remaining zones of the aircraft were solely dependent on human intervention for both detection and suppression of an in-flight fire. For the purposes of this report, the remaining zones of the aircraft for which built-in detection and suppression are not specified are referred to as "non-specified fire zones."  Portable Fire Extinguishers

The aircraft was equipped with eight portable fire extinguishers, which were held by brackets mounted in designated locations and distributed throughout the aircraft. In the passenger cabin there were five, 2.5-pound (lb) bromochlorodifluoromethane (Halon 1211) fire extinguishers, and two 5-lb monoammonium phosphate (dry chemical) fire extinguishers. The cockpit contained one 2.5-lb Halon 1211 fire extinguisher held by a bracket mounted on the cockpit rear wall (see Figure 17).

Five of the six Halon 1211 extinguishers, and both dry chemical fire extinguishers, were recovered. (STI1-20) It was not possible to determine where these extinguishers had originally been located in the aircraft, primarily because each extinguisher was identical in design, and there were no additional identifying features. Three Halon extinguishers exhibited markings indicating that they were still in their mounting brackets at the time of impact. Two of the three extinguishers still contained a charge of fire extinguishing agent. The pre-impact charge state of the remaining Halon 1211 extinguishers could not be determined, owing to punctures and other damage incurred at the time of impact.

One of the two dry chemical extinguishers showed marks indicating that it was in its mounting bracket at the time of impact. Its charge state at the time of impact could not be determined. The other dry chemical extinguisher was charged at the time of impact, with its locking pin intact; it could not be determined whether this extinguisher was in its mounting bracket at the time of impact.  Engine/APU/Cargo and Lavatory Fire Extinguisher Bottles (STI1-21)

The aircraft was equipped with nine fire extinguishing bottles, containing bromotrifluoromethane (Halon 1301) in the engine, the APU, and cargo areas. Eight of the nine bottles were recovered. Fire handles, which control the activation of the engine fire bottles, were installed on the overhead panel in the cockpit. (See Figure 11.) When the fire handle is pulled and turned, electrically activated explosive cartridges rupture a frangible disc and the extinguishing agent is released from the bottles. The APU bottle is activated automatically when a fire occurs in the APU compartment. The cargo fire bottles are activated by push buttons in the cockpit.

There is no indication that any of these engine/APU/cargo fire extinguishing bottles were discharged by flight crew actions, although some bottles showed signs that they had been discharged by the explosive cartridges, most likely at the time of impact.

A total of four lavatory fire extinguishers were recovered; none could be identified as to its installed location. There was no soot or heat damage on any of the extinguishers. From the recorded information, there was no indication that any of the smoke detectors in the lavatories activated.

[41]    The Transportation Safety Board of Canada (TSB) defines a "potential fire zone" as a region of the aircraft in which the identified risk of fire, by the regulatory authority, mandates an appropriate measure of built-in detection and suppression.

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