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Federal safety officials warned the FAA nine years ago about the dangers of carrying oxygen-generating canisters in the cargo holds of jetliners, but were stymied in later attempts to get fire detectors and extinguishers installed in inaccessible cargo areas.

"It was unacceptable that they didn't listen," Barry Sweedler of the National Transportation Safety Board's office of safety recommendations said of the FAA decision not to require fire detectors. "If you get the warning before it gets too bad, you can get the airplane on the ground that much quicker."The canisters, 8-inch stainless steel bottles that can get hotter than 400 degrees when activated, have come under suspicion in Saturday's crash of a ValuJet DC-9 that killed 110 people.

The canisters have been blamed for at least one fire on an airplane, in 1986. And the Federal Aviation Administration said there are two other known incidents of the devices activating by themselves without causing a fire.

NTSB officials have revealed that 50 to 60 of the canisters were in the ValuJet's front cargo hold when it plunged into the Everglades shortly after takeoff from the Miami airport.

The generators are classified as hazardous materials. On Wednesday, federal officials said ValuJet was not authorized to transport such materials.

The crew of the DC-9 reported smoke in the cockpit and cabin before the plane went down. And investigators said Wednesday that they found soot-covered pieces of the airliner's floor and fire damage on one side of a beam that was clean on the other, indicating the possibility of a fire or explosion in the cargo compartment that runs underneath the passenger section.

The accidental triggering of an oxygen generator in the cargo hold of an empty American Trans Air DC-10 was blamed for a fire that destroyed the plane at Chicago's O'Hare Airport on Aug. 10, 1986.

NTSB investigators believe a mechanic inadvertently triggered one of the stored devices, and it grew hot enough to ignite seats and aircraft oil stored in the cargo compartment. Fire raced through the plane in less than 30 minutes.

The canisters are used in many Boeing jets, the DC-10, the L-1011 and some Airbus models to provide emergency oxygen. Ironically, the ValuJet DC-9 used a conventional stored oxygen system; the plane was simply transporting the canisters.

Jetliners normally have one such canister over each set of seats. The oxygen generators, which resemble small fire extinguishers, are triggered by pulling on the plastic tubing connected to the face masks that drop from the ceiling during sudden depressurization.

Tugging on the cord sets off a small percussion cap inside the tank, initiating a chemical reaction that produce a 15-minute supply of oxygen for four people. The reaction also produces extreme heat.

In a test conducted after the Chicago fire, investigators activated a generator wrapped in a paper lab coat. The tank got hot enough to set the coat on fire. The percussion caps can also activate if the canisters get hotter than 500 degrees.

If an initial fire in the ValuJet cargo hold set off scores of oxygen generators, the sudden burst of oxygen would have fed the fire like gasoline thrown on hot embers.

Following the Chicago incident, the NTSB learned that some air carriers were not aware the oxygen generators were classified as hazardous material that require special precautions when being shipped in airliner cargo holds. Those precautions include labeling the canisters and storing them in durable cardboard boxes.

The FAA promptly renewed such warnings all domestic air airlines. It was not immediately known if ValuJet, a 3-year-old carrier, was aware of the 1988 warnings.

Two years later, the FAA balked at NTSB recommendations for fire extinguishers and fire detectors in cargo holds. Those recommendations were prompted by a Feb. 3, 1988, fire in the cargo hold of an American Airlines DC-9 flying from Dallas to Nashville.

That fire - blamed on a drum of improperly packaged chemicals, not oxygen canisters - sent smoke pouring into the cabin. The plane landed in Nashville, and the 126 people aboard escaped serious injury.

The NTSB's concern grew after a Feb. 1, 1991 fire aboard a USAir DC-9 from Charlotte, N.C., to Greensboro, N.C. Although a flight attendant smelled smoke, the fire was not discovered until the plane landed. It was not caused by a canister.

But the FAA again resisted putting fire detectors and fire extinguishers in cargo holds. FAA officials said the cost, estimated at more than $350 million, outweighed the potential benefits.

The agency instead said fire retardant liners already installed in some cargo holds would provide a measure of safety.

In a reply to then-FAA Administrator James Busey, the NTSB warned the solution was unacceptable.

"The board continues to believe that a fire should not be allowed to persist in any state of intensity in an airplane without the knowledge of the flight crew," then-Chairman James Kolsted said.