BRIAN DUNN / MONTREAL
The investigation into a fire in an Air Canada Boeing 767 cargo hold on 13 May has determined that short-circuiting in a damaged water pipe heater ribbon caused the ignition, and a contaminated insulation blanket and debris on the floor helped to propagate the fire.
Canada's Transportation Safety Board (TSB) has become particularly concerned about the danger of "fuselage fires" as a result of interim findings from the long-running inquiry into the 1998 Swissair 111 Boeing MD-11 crash. It has recommended that Transport Canada should issue short- and long-term directives to correct the heater ribbon problem. The TSB says the problem could affect up to 250 Boeing aircraft in Canada.
The fire was extinguished about 50s after a cargo-hold fire-alert sounded on final approach to Toronto's Pearson Airport. The aircraft landed safely although passengers and crew were able to smell smoke in the cabin, says the TSB.
The investigation has determined that an Electrofilm heater ribbon, used to prevent the potable water drain/supply line from freezing, failed and showed signs of overheating and arcing just behind the aft baggage compartment loading door. The heater ribbon, spiral wrapped around the water line, burned through the protective tape used to hold it in place and the Rubatex foam thermal insulation material covering it, igniting the non-metallised PET-covering of the thermal acoustic insulation blanket mounted on the vertical web of the floor beam.
The fire spread to the PET insulation blanket covering the bottom of the pressurised hull and ignited debris on the floor of the aft cargo compartment. The fire spread about 460mm (18in) up the right side wall of the aircraft before it was extinguished by halon. The heat burned holes through the aluminium web of a floor beam and distorted the beam structure.
On 7 June, the US Federal Aviation Administration issued an airworthiness directive (AD) requiring inspections of the particular pipes and replacement of defective heater tapes, but on 15 July, Transport Canada wrote to the FAA expressing concern that the AD did not address other areas of the aircraft that use heater tapes, and that the action did not address the design issue.
Air Canada inspected its fleet of 55 767-200s and 767-300s and replaced heating ribbons in 30 of them. But when investigators examined the replacements on the aircraft where the fire occurred, they found one of the new ribbons had already overheated and burned.
Source: Flight International
