CP file Relatives of victims of Swissair Flight 111 console each other on the rocks of Peggys Cove soon after the 1998 crash.

By Alison Auld / The Canadian Press

Investigators who have pored over the wreckage of Swissair Flight 111 for more than four years aren't expected to produce a definitive cause of the crash, but will likely zero in on critical flaws that set off a devastating chain of events, experts say.

Among them is a controversial inflight entertainment system said to be a part of findings by the Transportation Safety Board, which will release its final report into the 1998 crash on Thursday.

Aviation experts who have followed the lengthy investigation say they'll be surprised if the program is not found to be a main contributor to the massive electrical failure that brought the jetliner down off Nova Scotia, killing all 229 people on board.

"The entertainment system, from an engineering background, was a power-hungry monster and it gobbled up a lot of energy, creating so much heat that they required a rebalancing of the air conditioners," Gerry Einarsson, a former Transport Canada engineer who specializes in avionics, said from Ottawa.

"I can't with any degree of evidence say it caused it, but there's a great deal of reason to suspect it."

Investigators know a fire that raced along wires crippled the jetliner by disabling its electrical system, but they have yet to clearly state its source and likely won't.

Einarsson, who has lobbied Ottawa to improve aviation safety, believes the entertainment unit is key to the fire. He says the system was so hastily installed on the MD-11 that the proper inspections weren't done to ensure it could operate safely in the air.

He and others blame the powerful American Federal Aviation Administration in part for allegedly shirking its duties in certifying the system - something they say the safety board should address in its report.

"I seriously doubt that the Canadians will go as far as I think they should, because of political reasons," says Bernard Loeb, the former head of the U.S. National Transportation Safety Board.

"If I was the Canadians, I'd be a little cautious about suggesting an FAA process has significant holes in it."

Critics allege the agency didn't pay close enough attention to the devices, the installers and the manufacturers even though concerns had been expressed about them.

The system, which allowed passengers to gamble, play video games and watch movies, was found on test flights to raise cabin temperatures and cause hard drives in the seats to fail.

Despite that, Swissair ordered the system to be installed on 21 of its planes - including the jetliner that would plunge into waters off Peggy's Cove just over an hour after leaving New York.

The system came into sharp focus in the days after the crash, when investigators recovered 21 short-circuited electrical wires, including at least seven that came from the system. A wire that shorts can cause a spark or fire that could ignite other materials.

Swissair, once one of the world's most elite carriers and now bankrupt, voluntarily disconnected the system three weeks after the crash as a "precautionary measure."

The TSB would not comment on its forthcoming report.

The report is also expected to mention Kapton wiring, a disputed insulation that has been banned in some U.S. military aircraft because of its propensity to chafe, crack or break down.

The safety board, which has spent more than $60 million on the investigation, recovered pieces of the charred wire near where the fire was thought to have started just behind the pilots in the

ceiling.

The Kapton wire had arced, a phenomenon in which the outer insulation is cracked or chafed and the wire is exposed to another surface. Electrical sparks can escape and set off a chain reaction, burning along the wire almost like a fuse.

Even though the discovery helped narrow the possibilities, the difficulty for investigators was trying to determine which came first.

Ed Block, a wiring expert, is convinced it was the source.

"I have seen this wiring-cancer attack the military, the commercial fleet and the general aviation fleet," said Block, a former U.S. Defence Department employee.

"I am hoping the TSB sends a clear message to the world about this hidden danger."