"Shambolic" is an appropriate term for the following case involving wiring, circuit breakers and electrical system design. The expression is British slang for a disorderly or chaotic situation, which is to say one not under control.
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 Nov. 22, 2004, by the Transportation Safety Board (TSB) of Canada, provides additional details. The report’s itemization of the risks has universal application. (See www.tsb.gc.ca/en/reports/air/2002/a02o0123/a02o0123.asp).
Shambolic One: The Setup
The catalyst for the fire was two repairs to the potable water drain line, located in the aft belly hold. They were to serve until a permanent repair could be made. 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 temporary repair work. But no one can say for sure, as fire completely destroyed the heater ribbon.
Investigators nonetheless believe the combination of Teflon, stainless steel tube, steel clamps, and the wrapping of tape and foam insulation created dissimilar heat sinks that, in turn, led to localized overheating. The heat degraded the heater tape’s insulating matrix, leading to migration of its two heating elements until contact was made with the forward edge of the repair. There 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. An improper installation can result in an overheat condition that can lead to a fire," the TSB report says, 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 for the B767’s heater ribbon did not trip. As the TSB report explains, circuit breakers are designed to protect the wires, not the end-items to which they are connected–which may require their own internal circuit breakers. The heater ribbon did not feature an internal circuit breaker, so the arcing continued, eating its way forward and coming in contact with and igniting thermal acoustic insulation blanketing.
The fire spread, feeding on soiled insulation blankets, litter and other detritus that had fallen between gaps on the cargo floor into the bilge. The fire burned holes through a floor beam and breached the belly hold’s fire liner. It penetrated the space between the belly hold and the fuselage’s outer skin, blistering the skin paint.
The TSB report describes 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."
Shambolic Three: Continued Current
When the B767’s aft cargo bay fire extinguishing system was armed, electrical power was cut from the galleys and recirculation fans and to the aft lavatory, but not from the water heating system. The TSB report states that 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…this was not the case in the Boeing 767, nor is it a regulatory requirement." It goes on to say, "The heater ribbons remained powered throughout this entire event, and there was no means of deactivating them from the flight deck."
Shambolic Four: Contamination
The incident B767 was outfitted with polyethylene terephthalate thermal acoustic insulation blankets, which had become soiled and frayed by in-service wear. When new, these blankets passed the FAA Bunsen burner test for flammability. However, dirt, grease, corrosion-inhibiting compounds, fluid residue, etc., had dirtied the blankets, making them 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.
Shambolic Five: Flight Recorders
Air Canada downloaded the B767’s flight data recorder (FDR), which was returned to service without the TSB’s knowledge. It was overwritten. The paper printout covering the incident, subsequently obtained by the TSB, did not contain all of the data.
The cockpit voice recorder (CVR) had not been deactivated following the occurrence and had been overwritten. With more than a dozen such FDR/CVR lost data problems known to the TSB, its report says, "Loss of recorded information on serious incidents caused by overwriting [or] premature erasure is a recurring problem." And there is "no requirement" to train crews on the need to preserve the data, the TSB report adds.
Shambolic Six: The Service Bulletin
Following this incident, Air Canada inspected its 55-plane fleet of B767 aircraft. It unearthed "numerous instances" of overheated and/or burned heater ribbons. Sixty-six of the ribbons were either deactivated or removed.
Over a 17-year period, from 1985 to June 2002, Boeing identified 67 cases of heater ribbon failures. It detected charred insulation in many cases, two of which involved structural damage from fire. As early as 1992, Boeing had issued a service bulletin, noting the potential for scorching if heater ribbons of 24 watts per foot capacity were powered when no water was in the line. The service bulletin 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 B767s, yet the TSB notes, "Neither the previous operator nor Air Canada complied with the service bulletin." Throwing a service bulletin at a proven problem did not work in this case.
Two weeks after the fire, Boeing issued a service bulletin calling on B767-200 and -300 operators to inspect heater ribbons, clean up surrounding debris, and add protective tape as necessary. The FAA issued an airworthiness directive (AD) June 7 (AD 2002-11-11) ordering compliance with the Boeing service bulletin. Qantas reportedly has gone 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.
Although the TSB had fulsome praise for Air Canada’s prompt response to its interim recommendations, the agency is less sanguine about FAA’s overall response and that of its equivalent Canadian regulatory authority, Transport Canada. Extracts of the TSB’s statement of "safety concerns" indicate that actions taken as a result of the Air Canada incident represent a limited, if not languid response, and more needs to be done for all aircraft.
"The Board is concerned that the FAA action is limited to Boeing 747 and 767 aircraft…" TSB says. "The FAA believes that heater ribbons do not need to be removed or replaced in closed-in areas because such areas do not accumulate sufficient debris and contamination to pose a risk of a self-sustaining fire."
"Despite all of the action taken to date by the various agencies, the problem of contamination in closed-in areas still exists…dust and lint accumulation on wires has led to self-sustaining fires in closed-in areas, and the potential for such fires still exists."
To which two points need to be added: (1) installing safer immersion heaters ought to be required for airplanes used in extended range twin-engine operations (ETOPS), whose routes can be hours distant from divert airfields; and (2) as a matter of design philosophy, power needs to be cut to everything possible when the fire detection system covering an inaccessible space activates. After all, water in the line is not likely to freeze in a fire, but maintaining current definitely can worsen the conflagration.
David Evans can be reached by e-mail at [email protected].