Safety News

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.

The case involves an aft belly hold fire on an Air Canada Boeing 767 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 report released November 22 by the Transportation Safety Board (TSB) of Canada provides details and the report's itemization of the risks has universal application, not just to the 767 involved in the event.

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."

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.

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."

Following this incident, Air Canada inspected its 55-airplane fleet of 767s. "Numerous instances" of overheated and/or burned heater ribbons were found, and 66 were either deactivated or removed. Over a 17-year period, 1985 to June 2002, Boeing identified 67 cases of heater ribbon failures. Charred insulation was identified in many cases, and two involved structural damage from fire. As early as 1992, Boeing had issued an SB, noting the potential for scorching if heater ribbons of 24 watts per foot capacity were powered while no water was in the line. Boeing's SB called for replacement of the existing heater ribbon with a thermostatically controlled 7-watt-per-foot model.

The incident airplane, acquired from Lan Chile, was one of the affected 767s, yet the TSB noted, "Neither the previous operator nor Air Canada complied with the SB." Throwing a service bulletin at a proven problem did not work in this case.

Two weeks after the fire, Boeing issued an SB calling on 767-200 and -300 operators to inspect heater ribbons, clean up surrounding debris, and add protective tape as necessary. The FAA issued an airworthiness directive June 7 (AD 2002-11-11) ordering compliance with the SB.

Qantas reportedly has taken a step further, replacing the ribbon-style heater tapes on its aircraft with coil-type immersion heaters that are installed inside the water lines, where they are more protected from inadvertent damage. -- By David Evans, Air Safety Week

For information on a second, December 23, 2002, Air Canada B767 heater-tape hold fire while at the gate, Click Here.