Airlines would have to replace or modify old insulation in about 800 Boeing (search) jetliners within the next six years because of concerns that the material becomes less fireproof as it ages, under a rule proposed by the Federal Aviation Administration (search) Friday.

Fire is especially dangerous in aircraft because of the amount and flammability of jet fuel, and the fact that there's no escape until — or unless — the plane lands. While a concern, the FAA does not believe the problem poses an imminent danger.

Several air disasters have involved fires, including Swissair Flight 111 (search), which plunged into the ocean off the Nova Scotia coast in 1998, killing all 229 aboard. Canadian investigators said the fire likely started with a spark and fed undetected on insulation above the cockpit.

As a result of that crash, the FAA in 2000 required airlines to replace their insulation if it was the kind used in the Swissair plane, an MD-11 (search). About 700 Boeing and McDonnell-Douglas aircraft had to swap out the insulation that was installed between the jet's aluminum skin and the cabin.

The deadline for that costly retrofit is June 30. Insulation has been replaced on about 90 percent of the aircraft.

In 2002, a fire in a jetliner on the ground led Boeing to look into the fireproof properties of that aircraft's insulation, which was made of a different substance than the insulation that the FAA had ordered removed.

Boeing discovered that age and contamination had affected the fireproof properties of the film that covers the insulation.

The film was made between 1981 and 1988 and the FAA believes it was installed on 831 U.S.-registered Boeing 700-series jetliners.

The FAA is still considering how the film may be modified to meet the requirement.