History of the Flight
On 13 May 2002, Air Canada Flight 116, a Boeing 767-300 aircraft (registration C-GHML, serial number 24948), departed Vancouver International Airport, British Columbia, at 1617 UTC on a scheduled instrument flight rules flight to Toronto/Lester B. Pearson International Airport, Ontario. The aircraft carried 8 crew members and 177 passengers. The flight was uneventful until 2132 UTC, approximately 10 miles from the airport on final approach, when the Master Warning Fire/Overheat light illuminated, the fire warning bell sounded, and the Aft Cargo Fire light illuminated. The flight crew reset the master warning and followed the cargo fire procedures from the Quick Reference Handbook, activating the cargo fire extinguishing system. An emergency was declared and emergency vehicles were requested. At 2132:59, the Aft Cargo Fire light returned to normal. The crew informed the flight attendants that the indication had cleared.
The aircraft landed on runway 06L and stopped on the runway at 2138:31. Airport firefighters examined the aircraft externally with visual inspection and forward-looking infrared (FLIR) cameras, detecting no signs of fire. However, the crew reported an acrid smell and slight haze in the passenger cabin. The aircraft taxied to the gate with firefighters following. The captain stopped approximately 40 feet from the gate and requested a firefighter inspection of the aft cargo compartment. Exits remained armed while maintenance opened the compartment. When the doors opened, smoke and fumes flowed out. Firefighters entered with a handheld FLIR camera and located a single heat source behind the aft wall of the cargo compartment.
Fire and Damage
Firefighters did not detect active fire but noted smoke. The flight crew prepared for emergency evacuation, but the situation was secured and passengers deplaned via portable stairs. The aircraft was taken to a hangar for further inspection. Company maintenance personnel discovered substantial soot and fire damage on the cargo bay floor. The fire had breached the cargo compartment liner and spread behind the right sloping sidewall, but was effectively extinguished by the onboard system.
Investigation Findings
The investigation identified the ignition source as a B110 heater ribbon attached longitudinally along a water supply line. The heater ribbon failed just aft of fuselage station 1395, at a recent water line repair. The repair used a stainless steel tube and clamps, creating dissimilar heat sinks (original Teflon water line, stainless steel components, and an air gap). This caused localized overheating of the heater ribbon, exacerbated by multiple layers of 3M 474 tape and Rubatex foam insulation. The overheating degraded the EPDM insulating matrix, allowing the heating elements to arc and ignite surrounding material. The arcing ceased when a conductive path was no longer available. The circuit breaker did not trip because the failure did not produce an over-current condition. The heater ribbons remained powered throughout the event, as there was no means to deactivate them from the flight deck.
The most significant contributor to fire propagation was the presence of flammable materials. The PET covering material on insulation blankets was contaminated and flammable, and was likely the first material to ignite. Debris in the fire area, including paper, candy wrappers, Styrofoam packing peanuts, and polyethylene beads, was consumed. The investigation noted that contamination in aircraft is an ongoing problem, especially in inaccessible areas where debris and fluids can accumulate.
Heater Ribbon Failures
Research indicated a propensity for heater ribbon failures, particularly older vinyl-covered types with lower continuous operating temperatures. Failures often go undetected until major inspections. Most failures cause localized damage, but in one instance a heater ribbon melted a water line leading to an EICAS computer failure. The Flight 116 occurrence demonstrated that a failed heater ribbon could ignite a self-propagating fire in a sealed compartment with a fire extinguishing system. The installation procedures for heater ribbons were generic and ambiguous, leading to confusion. The investigation noted a general complacency in the industry regarding heater ribbon failures, considered non-critical until a serious occurrence.