By STEPHEN THORNE / The Canadian Press

The pilot of doomed Swissair Flight 111 may have sealed his own fate and that of 228 others aboard by resetting a circuit breaker as his troubled jet descended below 10,000 feet, say aviation experts. Pilot Urs Zimmermann may only have been following procedure as he brought his Boeing MD-11 down to an altitude that allowed him to open a window to clear cockpit smoke, say wire-and-cable experts. But what he likely didn't know may have killed him.

The wiring aboard Zimmermann's plane was laden with aromatic polyimide tape insulation, or Kapton, a widely used aviation product which the U.S. military banned 11 years ago because it's prone to breakdown and can promote electrical arcing.

"There's billions of feet of Kapton in the air,"Edward Block said in an interview from Washington, D.C. It has a proven track record until this condition (arcing) occurs but, when it does, you have the absolute worst-case scenario you can imagine." Block said emergency checklists often recommend testing circuits by resetting circuit breakers once enough cockpit smoke has cleared. Detailed air-traffic control transcripts released Tuesday suggest Zimmermann and co-pilot Stephan Loew were following a checklist as they tried to overcome the difficulties they faced.

But procedure is the last thing that should be followed when Kapton-encased wiring

starts smoking, said Block, an industry consultant.

Block was the U.S. Defence Department's wire-and-cable expert when Kapton was banned from American military aircraft in 1987. He is now investigating last year's TWA 800 crash off Long Island, N.Y.

He said many airline pilots are unaware of problems specific to Kapton that could spell disaster if flight crews follow normal procedure once a circuit breaker is tripped by an electrical problem.

"Your tendency would be to reset that circuit breaker to see what happens and, in so doing, you've sealed your fate. It's just the beginning of the end."

The U.S. military had laboratory test results indicating problems with Kapton as early as 1981, particularly aboard the navy's S3 small-transport aircraft.

"They went to the navy research lab and found out that under certain conditions it hydrolizes, meaning it drinks water, and therefore it becomes susceptible to the insulation cracking and peeling," said Block.

"Once you have that degradation in the outer insulation, you have what's called an arc-track scenario set up, where you have a metal-to-metal contact where it reaches out and touches an adjacent wire or fuselage or some ground.

"Then it's actually a trickle thing. It starts to slowly but surely blacken or char the wire insulation. And finally, if it gets hot enough to blow a circuit breaker, it's de-activated."

Investigators have said evidence in the crash of Flight 111 off Peggy's Cove suggests an electrical failure below 10,000 feet, or about 3,000 metres. The plane's signature beacon cut out denying air-traffic controllers critical altitude information.

Communications were also lost, and the flight-data recorder apparently quit after Zimmermann turned away to lose excess altitude and dump heavy fuel.

Early information from the flight-data recorder released Tuesday suggests systems aboard the MD-11 did not all fail at once, said Vic Gerden, chief investigator for the Transportation Safety Board of Canada.

Block said a pilot would typically isolate the source of the smoke then, below 10,000 feet, help vent the air by opening the cockpit window.

"Then you reset the circuit-breaker," said Block. "In so doing, you've now sent a charge of electricity to a charred wire insulation that is now going to actually ignite into what is called a flash-over effect."

In Zurich, Swissair's chief pilot-designate, Rainer Hiltebrand, said Tuesday a total loss of power was virtually unthinkable.

The MD-11 has three normal generators, as well as a back-up generator and an emergency generator, he said. There is also a battery which would power all the plane's essential functions for 15 minutes, Hiltebrand said.

But experts said no amount of back-up could compensate for the kind of problems Kapton can generate. A flash-over, or arcing, refers to electricity escaping from a wire like a bolt of lightning.

Patrick Price, a retired wiring technician who built Boeing's arc-tracking laboratory in Seattle, called aromatic polyimide tape insulation, or Kapton, "the most explosive wire that they can put on an airplane right now.

"Every time you get into an airplane that's flying with Kapton wiring, you are flying with a potential incendiary bomb that's ready to go off at any time if the conditions are just right."

Price lab-tested a thumb-sized bundle of Kapton about a metre long on a Formica board for Boeing in 1985. More than three-quarters of the length of the bundle was totally destroyed, he said. "The only reason the firing stopped was because the electrical energy stopped - either circuit breakers were tripped and-or the wire melted. You could see just exactly where the wire bundle was lying because of the heavy black soot that had been burned into the Formica board."

He said the arcing can jump from one wire bundle to another like a fuse causing "massive destruction."

And Price, a 37-year Boeing veteran, had some advice for pilots: "If you start noticing smoke you should shut the power off and go to emergency battery power immediately.

"But you see, if they weren't aware of how dangerous a Kapton condition can develop and what it can do, they probably wouldn't do that."

Wiring problems aboard MD-11s have been the subject of several U.S. Federal Aviation Administration directives warning of potentially hazardous configurations in the cockpit and in a rear console for flight attendants.

Swissair president Jeffrey Katz has said his company complied with all FAA directives. However, the directives did not say the Kapton insulation should be changed.

Commercial aircraft cannot be rewired anyway, said Block, who echoed others' criticisms that the U.S. regulator has failed to stay on top of wire-and-cable problems in commercial aircraft.

"You're seeing a serious under-appreciation of wire and cable," said Block. "People used to die of heart attacks until they realized arterial sclerosis was the real culprit. I think that kind of mindset is working here.