By Stephen Thorne / The Canadian Press

Ottawa - Insulation used inside the bulkheads of aircraft to keep heat in and noise out spread the fire that brought down Swissair Flight 111, prompting regulators to ban the material in commercial aircraft Wednesday.

Based partly on the recommendations of Canadian investigators, aviation's top regulator is ordering the metalized mylar insulation blankets replaced on 699 U.S.-registered airliners within four years.

The U.S. Federal Aviation Administration, whose standards are followed internationally, said the material fails a proposed new flame-retardency guidelines.

A total of 1,230 aircraft will be affected worldwide. No Canadian-registered aircraft left the factory fitted with the insulation.

"Tests show that metalized mylar falls far below the new test standard," said an FAA statement. "It ignites much more easily than other materials and can spread fire because its properties are much different."

The action comes on the heels of an urgent advisory from the Transportation Safety Board of Canada recommending use of metalized mylar blankets be reduced or eliminated based on wreckage from Swissair Flight 111, which crashed into the ocean off Nova Scotia last September, killing all 229 people on board.

"There are clear indications that a significant source of the combustible materials that sustained the fire was thermal acoustical insulation blanket material," the safety board said in a letter to the FAA written Friday.

"Burnt remnants of this material, quenched by sea water, were found in the wreckage."

Shortcomings in the material's fire resistance and in the FAA testing standards were cited by Swissair crash investigators and others some time ago.

Authorities from the FAA stepped up efforts to develop new testing standards for insulation blankets, and several companies, including at least one in Canada, have been working on replacements.

"Certainly Swissair and the involvement of metalized mylar in that accident has called (its use) into question," Beth Erickson, director of the FAA aircraft certification service, told a teleconference.

"I believe that after the Swissair accident we pushed it up in priority."

The blankets have been identified as a major factor in at least five other aircraft fires. McDonnell Douglas, Boeing-owned makers of the MD-11, told operators they should stop using metalized mylar in 1997.

Airlines apparently initiated a limited response, primarily because of the cost - estimated at between $380,000 US and $880,000 US per aircraft.

"It is the board's view that the operation of aircraft outfitted with thermal acoustical insulation blankets incorporating metalized PET cover material constitutes an unnecessary risk," the safety board said.

The airworthiness directive, when actually enacted after a 45-day response period, will not require the sweeping insulation replacement the FAA had discussed in October.

FAA officials said Wednesday the agency backed off the earlier plan, under which it would have ordered insulation replacements in nearly every U.S. commercial airplane, because recent research showed most existing insulation passes or only narrowly fails the new flame test.

Instead, the more limited order will apply to 699 McDonnell Douglas planes. They include the MD-80, MD-88, MD-90, DC-10 and the MD-11, the type of plane that crashed off Nova Scotia.

There are no MD aircraft flown in Canada. Canadian-operated DC-10s were built before metalized mylar came into use between 1987 and 1994, but some may have been retrofitted with the material, said board spokesman Jim Harris.

Affected airlines include American, Delta, Continental, Trans World, Alaska, Federal Express, Reno Air, Aeromexico and US Airways. Swissair will be subject to a similar order expected from its own authority.

"The work must be accomplished at the earliest maintenance check but no later than four years," said the FAA, noting a "safe and deliberative process" will help prevent wiring damage - always a risk in such retrofits.

The Swiss pilots of the New York-Geneva flight reported smoke in the cockpit and were minutes from Halifax airport when they turned away to dump fuel. They crashed into the Atlantic Ocean near Peggys Cove.

Investigators found evidence of fire just fore and aft of the wall between the cockpit and forward galley. Faulty electrical wiring is believed to have started the fire, though authorities have not confirmed that.

American Miles Gerety, whose 56-year-old brother, Pierce, died on Flight 111, was buoyed by the news out of Ottawa.

He said it should help make future air travel safer.

"That's a huge development," Gerety said. "It means that every airline that's got a plane that's got metalized mylar is going to worry about it."

Gerety said it's a piece of the Swissair-disaster puzzle, but stressed many questions remain unanswered.

"In figuring out what caused the crash they may find many things, like the danger of metalized mylar, that may be capable of downing an airplane but may not . . . have been the cause of this particular crash," he said.

Gerety, a lawyer in Bridgeport, Conn., said "the central question with this crash is why the fire was conveyed forward."

He said the victims' families appreciate the effort of Canadian air-crash experts and realize it could take years before they learn what caused the accident.

"I'm convinced in my heart of hearts that they're going to find (the answer). What is important here is that they get it right. All of the families know it will take a long, long time."

With Micheal Lightstone