By Stephen Thorne / The Canadian Press

Renton, Wa. - An electrical problem was likely the main contributor to the crash of a Swissair jet off Nova Scotia last September, though other factors may have dealt the plane's final blow, says Boeing's safety director.

Canadian investigators, working with industry and other safety agency officials, have determined some wires on the MD-11 aircraft were damaged from the inside out, not the outside in, Ron Hinderberger said in an interview.

"Every indication is that the electrical system is the origin of whatever occurred," he said this week. "Whether it was the reason why it got so bad, we don't know."

Boeing's chief crash investigator shed new light on the Swissair investigation and expressed confidence in the Canadians who are running it. His comments:

- Investigators have found that heavy sooting and other fire-related damage spread well into the plane's first-class section, further than previously thought.

- Evidence suggests the co-pilot was at the controls while pilot's seat was not in a flying position when the plane crashed.

- Boeing offered to inspect wiring in a suspect entertainment system a California contractor installed in part of its fleet last year, but Swissair turned them down and opted to take it out.

"They deactivated it," said Hinderberger. "Subsequent to their deactivation, they decided to remove the wiring that was in the front end of the airplane."

Swiss pilot Urs Zimmermann reported cockpit smoke about 20 minutes before his MD-11 jet went down off Peggy's Cove, N.S., last Sept. 2. All 229 aboard were killed.

Key to the probe has been the question of whether wires in the downed plane arced after fire damaged their insulation.

It now appears, however, that a wire-related problem such as an electrical overload, or cracked and chafed insulation like that found on other MD-11s caused the lethal, lightning-like jumps from wire to wire. Fire is a likely result in either case.

"There are some wires in the forward part of the airplane that have been identified as having been heated from the inside out," Hinderberger said.

Hinderberger, who has had an investigator on the scene since the investigation began, refused to say if the telltale wires were original to the jet or if they were from the inflight entertainment system. Both sets were damaged.

Investigators are confounded by the degree of heat damage and scorching patterns in the ceiling just aft of the plane's cockpit wall, he said.

"Electrical problems don't usually manifest themselves in that much heat. So what occurred that generated as much heat as it did?"

The cockpit's emergency oxygen supply is located in the same general area and could conceiveably have fed such a fire.

But Hinderberger said makers of the three oxygen bottles that supplied the cockpit crew determined they were pressurized - "at least partly full" - when the aircraft hit the water.

"If an oxygen line had severed, my feeling is those bottles would have emptied pretty quickly."

Some experts believe a flashover - an explosive electrical fire - occurred aboard the Swissair jet. Hinderberger said there has been little talk of that among investigators.

There has been speculation that one or both pilots abandoned the cockpit due to intense heat.

He said the copilot's seat was in the flying mode - in and forward - and the stress on his belt indicated he was in his chair when the plane hit the water. Zimmermann's seat was in the "back-and-out" position.

But the position of the captain's track-mounted seat - away from the control panel and off to one side - doesn't necessarily mean Zimmermann bolted, said Hinderberger.

"It could have been for a number of reasons."

The pilot, Swissair's chief instructor, could have been reaching to get a manual, or gotten up to get another manual and then gotten back in his seat to read it while copilot Stefan Loew flew the plane.

The investigation, he said, is "going to be a long road."

Yet Hinderberger says he's confident chief investigator Vic Gerden and his team will find the main causes of the disaster.

"Based on the tenacity of the Transportation Safety Board of Canada, I would say it's very likely," he said, adding the team is analysing parts that most other investigative agencies would have dismissed as irrelevant.