James T. McKenna/Washington

       British investigators are assessing whether recent maintenance and the use of

       polyimide-insulated wiring contributed to an inflight electrical fire on a

       United Airlines ETOPS 767.

       The U.K.'s Air Accidents Investigation Branch (AAIB) is running the

       investigation into the Jan. 9 incident, which led to the diversion of the

       Zurich-Washington flight to London. The U.S. National Transportation Safety

       Board is participating.

       The problems on United Flight 965 began shortly after takeoff from Zurich for

       a transatlantic flight to Washington's Dulles International Airport, when a

       series of apparently unrelated electrical problems began occurring, the flight

       crew told investigators.

       When the aircraft was west-northwest of Paris, the flight crew was approaching

       the point at which they had to decide whether to continue the extended-range,

       twin-engine overwater operations (ETOPS) flight over the Atlantic. ETOPS rules

       permit twin-engine aircraft to operate on routes 180

       min. or more from the nearest emergency landing airport only if the aircraft's

       systems meet strict conditions. Given the electrical problems, the crew opted

       to abort the flight and divert to London's Heathrow International Airport.

       After an uneventful landing at Heathrow, flight attendants reported smoke in

       the first-class cabin and galley. The captain ordered an evacuation after he

       brought the 767 to a stop on a taxiway. There were a handful of minor injuries

       during the evacuation.

       AAIB investigators examining the aircraft found about 10 circuit breakers open

       in the cockpit. It is not clear whether the breakers opened in flight or on

       the ground.

       They also found a roughly 7-in.-long section in a bundle of more than 100

        wires that was severely burned and melted. The bundle was in the electrical

        and electronics (E&E) bay of the 767-322ER, directly below the first-class

        galley.

       About three dozen wires in the bundle were damaged by heat or fire. The

       twisted strands in one wire were fused into a single strand of copper,

       indicating its exposure to sustained high temperatures. There were copper

       globules in the damaged area, indicating active arcing occurred there.

       All of the damage to the bundle occurred on the inside bend of the bundle's

       curve over the top and down the side of a refrigeration unit. There is no

       evidence that the fire or heat extended up around the circumference of the

       bundle to the top of the bundle.

       The exterior of wires on another bundle about an inch away suffered thermal

       damage, as did foam on the rear wall of the refrigeration unit.

       The unit, an 86-lb. chiller for the first-class galley, had recently undergone

       maintenance. Investigators are trying to determine whether mechanics may have

        nicked insulation on a wire in the damaged bundle in the process of

       maneuvering the chiller out of or into its perch in the E&E bay.

       Most of the wires in the damaged bundle used ETFE insulation, but all of the

       individual damaged wires used Kapton, a type of polyimide-film insulation.

       Polyimide insulation has been known to break down under ``arc tracking'' if it

       has been previously damaged or mishandled.

       In arc tracking, a short circuit arcs the polyimide insulated wire and another

       conductor. This chars the insulator, making it conductive and capable of

       sustaining the arc. Sustained arcs have been shown to propagate along the wire

       through continuous insulation charring, triggering arcing in other polyimide-

       insulated wires in a bundle.

       Photograph: After the United Airlines 767 flight crew diverted to Heathrow,

       investigators found concentrated fire and heat damage in one wire bundle in