AO2O0123-Air Safety Recommendations - In-Flight Fire Aboard an Air Canada 767-300

To date, the investigation has determined that an Electrofilm ® brand heater ribbon, used to prevent the potable water drain/supply line from freezing, failed and exhibited signs of overheating and arcing in the vicinity of station 1395. The heater ribbon, which was spiral wrapped around the water line, burned through both the protective tape used to hold the heater ribbon in place and the Rubatex ® foam thermal insulation material wrapped on top of the protective tape, igniting the non-metalized Polyethylene Teraphthalate (PET)-covering (Mylar®)of the thermal acoustic insulation blanket mounted on the vertical web of the floor beam. 

Floor beam damage. Floor beam damage.

The fire then spread to the PET insulation blanket covering on the bottom of the pressurized hull and ignited debris in the non-fully enclosed floor area of the aft cargo compartment. The fire became self-propagating, burning its way forward, inboard and outboard, spreading approximately 46 centimetres (18 inches) up the right side wall of the aircraft before it was extinguished by halon from the fire extinguishing system. Heat from the fire was intense enough to burn holes through the aluminum web of a floor beam and significantly distort the top cap (chord) of the beam structure.

Heater ribbons are used on the Boeing 767 and other aircraft to prevent water lines from freezing. They generally consist of a number of heating elements encased in an insulating material such as vinyl, rubber (Ethylene Propylene Diene Terpolymer, EPDM) or silicone rubber. Typically, a heater ribbon system is thermostatically controlled with power being applied, both on the ground and in the air, when the water temperature in the line approaches freezing. In C-GHML, the aft cargo compartment ribbon heater control thermostat was located 85 inches aft of station 1395, below the bulk cargo floor. The thermostat was set to turn the ribbon heaters on when the ambient air temperature reached 50 F and off at 60 F. Although the exact failure mechanism of the heater ribbon on the occurrence aircraft is still under investigation, examination of other failed heater ribbons suggests that an internal short or arcing event between two of the elements occurred.

Discolouration, consistent with overheating while in service, was observed on some intact heater ribbons on C-GHML and on numerous other aircraft examined by investigators and Air Canada maintenance personnel. Overheating can cause the heater ribbon insulating material to degrade. This degradation may contribute, in some instances, to an internal short or arcing event between two of the heating elements, which can cause the insulating material between the elements to carbonise and/or burn. A reduction of material separating the elements could allow them to migrate towards each other and arc again.
 

Heater Ribbon Installation.
Heater Ribbon Installation

 

TSB has identified safety deficiencies related to the potential for heater ribbon  installations to start a fire, and the potential for contaminated thermal acoustic insulation blankets and debris in the vicinity of heater ribbons to propagate a fire

.Failed Heater Ribbon

Failed Heater Ribbon

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