Recent Repairs, Wire Arcing
Eyed in 767 ETOPS Diversion
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
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
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
the aircraft's electronics compartment. Bill Hough photo. -AW&ST 2/9/98