Report: Regulatory Response to Fire Incident is Too Limited

Air Safety Week, Dec 20, 2004

Here's paradox for you: a raging blaze next to a pipe for draining water, which can quench a fire, came very close to burning its way out of a cargo hold. Here's corrective action for you: replace water line heater tapes with immersion heaters, and clean out all the detritus, crud, grease, lint and other flammable whatnot that can provide fuel for a fire.

There's more in the way of latent hazards and lurking deficiencies, and they'll be addressed directly.

The case involves an aft belly hold fire on an Air Canada B767 that broke out just a few minutes before landing.

The airplane, with 185 passengers and crew aboard, was on a May 13, 2002, flight from Vancouver to Ontario. The circumstances surrounding the plane's close encounter with deadly fire have been covered previously (see ASW, June 10, 2002, and ASW June 17, 2002). The report released Nov. 22 by the Transportation Safety Board (TSB) of Canada provides additional details. The report's itemization of the risks has universal application, not just to the B767 involved in the event.

To outline the case, the term "shambolic" will be used, British slang for a disorderly or chaotic, which is to say not under control, situation:

Shambolic ONE: The set up

The catalyst for the fire was a repair to the potable water drain line, actually two repairs, until a permanent repair could be made. The drain line is located in the aft belly hold. The electric heater ribbon, which runs along the outside of the line to prevent the water in the drain line from freezing, may have been damaged during the course of the temporary repair work, although this could not be established conclusively as the heater ribbon had been completely destroyed by fire.

Investigators nonetheless believe the combination of Teflon and stainless steel tube, steel clamps, and the wrapping of tape and foam insulation created dissimilar heat sinks which, in turn, led to localized overheating, degradation of the heater tape's insulating matrix, migration of its two heating elements until contact was made with the forward edge of the repair, where the steel water line mated with the Teflon one, with resulting arcing. "The exact location of each and every heater ribbon [51 of them on the incident aircraft] is critical, as an improper installation can result in an overheat condition that can lead to a fire," the TSB report said, adding, "There also appears to be a general sense of complacency in the aviation industry with regard to heater ribbon failures."

Shambolic TWO: Feeding the fire

The circuit breaker (CB) for the heater ribbon did not trip. As the TSB report explained, CBs are designed to protect the wires, not the end-items to which they are connected - which may require their own internal CBs. The heater ribbon did not feature an internal CB, so the arcing continued, eating its way forward some eight inches, coming in contact with and igniting thermal acoustic insulation blanketing, at which point, the TSB report observed dryly, "the fire became self-propagating."

It spread, feeding on soiled insulation blankets, litter and other detritus that had fallen between open gaps on the cargo floor into the bilge. The fire burned holes through a floor beam and breached the belly hold's fire liner, penetrating the space between the belly hold and the outer skin of the fuselage, blistering the paint on the outer skin.

The TSB report described as "significant" the fact that "while the fire occurred in a sealed compartment with a fire extinguishing system, the fire had breached the cargo compartment and entered an inaccessible and unprotected area."

"Had the fire extinguishing system not extinguished the fire quickly, the results could have been catastrophic," the TSB report said. In other words, fate and fortune discriminate this event from the deadly outcome involving Swissair Flight 111, which was downed by fire raging in an inaccessible space.

Shambolic THREE: Continued current

When the aft cargo bay fire extinguishing system was armed, electrical power was cut from the galleys overhead, recirculation fans, and to the aft lavatory, but not from the water heating system. The TSB report said:

"Once the fire was detected and the fire extinguishing system activated, it would be expected that power would be removed from all but the required essential systems as a means of eliminating potential ignition sources. This was not the case in the Boeing 767, nor is it a regulatory requirement.

"The heater ribbons remained powered throughout this entire event, and there was no means of deactivating them from the flight deck. As long as power is available to the heater ribbon, the potential for the heater ribbon to arc exists and presents an ongoing risk."

Shambolic FOUR: Contamination

The incident airplane was outfitted with polyethylene terephthalate (PET) thermal acoustic insulation blankets. They had become soiled and frayed by inservice wear. When new, these blankets passed the Federal Aviation Administration (FAA) Bunsen burner test for flammability (i.e., must satisfy a limited burn rate, need not be self-extinguishing). However, dirt, grease, corrosion-inhibiting compounds, fluid residue, etc., had dirtied the blankets. As such, the blankets were flammable. Other flammable debris was present, in the form of paper, candy wrappers, styrofoam packing peanuts, small polyethylene beads, dust and rubber powder that had collected in the bilge area. The arcing damage might have been limited to a small area had not these burnable materials been in the vicinity.