ValuJet Flight 592 - Investigation

Investigation

The NTSB investigation eventually determined that the fire that downed Flight 592 began in a cargo compartment below the passenger cabin. The cargo compartment was of a Class D design, in which fire suppression is accomplished by sealing off the hold from outside air. Any fire in such an airtight compartment will in theory quickly exhaust all available oxygen and then burn itself out. As the fire suppression is accomplished without any intervention by the crew, such holds are not equipped with smoke detectors. However, the NTSB determined that just before takeoff, expired chemical oxygen generators were placed in the cargo compartment in five boxes marked COMAT (Company-owned material) by ValuJet's maintenance contractor, SabreTech, in contravention of FAA regulations forbidding the transport of hazardous materials in aircraft cargo holds. Failure to cover the firing pins for the generators with the prescribed plastic caps made an accidental activation much more likely. Rather than covering the firing pins, the SabreTech workers simply duct taped the cords around the cans, or cut them, and used tape to stick the ends down. It is also possible that the cylindrical, tennis ball can-sized generators were loaded on board in the mistaken belief that they were just empty canisters, thus being certified as safe to transport in an aircraft cargo compartment. SabreTech employees indicated on the cargo manifest that the "oxy canisters" were "empty" instead of being expired oxygen generators. ValuJet employees interpreted this to mean that they were empty oxygen canisters, when in fact they were neither simple oxygen canisters, nor empty.

Chemical oxygen generators, when activated, produce oxygen. As a byproduct of the exothermic chemical reaction, they also produce a great quantity of heat. These two together were sufficient not only to start an accidental fire, but also to produce enough oxygen to keep the fire burning. The fire risk was made much worse by the presence of combustible aircraft wheels in the hold. Two main tires and wheels and a nose tire and wheel were also included in the COMAT. NTSB investigators theorized that when the plane experienced a slight jolt while taxiing on the runway, an oxygen generator activated, producing oxygen and heat. Laboratory testing showed that canisters of the same type could heat nearby materials up to 500 °F (260 °C), enough to ignite a smouldering fire. The oxygen from the generators fed the resulting fire in the cargo hold without any need for outside air, defeating the airtight fire suppression design. A pop and jolt heard on the cockpit voice recording and correlated with a brief and dramatic spike in the altimeter reading in the flight data recording were attributed to the sudden cabin pressure change caused by a semi-inflated aircraft wheel in the cargo hold exploding in the fire.

Smoke detectors in the cargo holds can alert the flight crew of a fire long before the problem becomes apparent in the cabin, and a fire suppression system buys valuable time to land the plane safely. In February 1998, the FAA issued revised standards requiring all Class D cargo holds to be converted by early 2001 to Class C or E; these types of holds have additional fire detection and suppression equipment.