Wire Type On MD-11 Known To Trigger Fires -- Kapton Used In Hundreds Of Planes
The Swissair MD-11 that crashed off Nova Scotia last week contained miles of wire sheathed in insulation called Kapton, which experts say can cause fire aboard aircraft.
"It burns extremely hot, and it burns other flammable materials in the vicinity, including aluminum," said Frank Campbell, a research chemist who studied Kapton at the Naval Research Lab in Washington.
Investigators are focusing on the possibility that electrical problems brought down Flight 111, killing all 229 people on board, after the pilot declared an emergency and said there was smoke in the cockpit.
John Maxwell, director of Air Investigations for the Transportation Safety Board of Canada, said, "We already strongly suspect an electrical problem behind the smoke and fire."
Investigators are also looking at airworthiness directives issued by the Federal Aviation Administration on the MD-11, paying particular attention to orders that involve wiring. Among them is a June order to inspect wiring behind the flight deck that can chafe and "result in a fire in the wire bundles and smoke in the cockpit."
Experts have found the material is prone to a phenomenon called arc tracking.
If the insulation is damaged by rubbing against another surface, sparks can jump from the damaged wire to nearby metal. The insulation becomes carbonized and conducts electricity, which can spread to adjacent wires in the bundle. What can result is an explosive "flashover," with exposed wires spewing molten metal.
Kapton is used in hundreds of aircraft now flying, including the MD-11 and some older Boeing 737s and 767s. But the wire has caused problems on civilian aircraft and is blamed for a fatal crash of a Navy plane:
-- In 1990, a Navy E-2C Hawkeye surveillance plane crashed as it returned to base in Puerto Rico. Arcing from a failed Kapton wiring bundle created a blowtorch effect that likely penetrated a fuel line. All seven crew members died.
-- Also in 1990, a USAir MD-80 was forced to return to Buffalo, N.Y., when the cockpit filled with smoke from overheated wire insulation.
-- In 1991, a Delta Lockheed L-1011 was forced to make an emergency landing after flames erupted from behind the cabin wall.
-- And industry sources said TWA had found 22 instances of severe arcing on its fleet of L-1011 jetliners between 1972 and 1981.
Five years ago, the U.S. Air Force helped test a new type of insulation, called hybrid wire, designed to solve those problems. The new wiring was selected for newer Air Force aircraft.
Boeing, which acquired McDonnell Douglas last year, said MD-11s were made with Kapton until 1995, when it was phased out in favor of the newer hybrid insulation. But Boeing spokeswoman Susan Bradley said the change was not made for safety reasons: "It was introduced for economic reasons because it's lighter and less expensive."
Kapton can be a problem because under certain conditions "it goes from an insulator to a conductor," said George Slenski, a materials engineer at Wright Patterson Air Force Base in Dayton, Ohio.
But Slenski said that the flashovers are not spontaneous events. The wire insulation must be damaged, wires must be tightly bundled and the current must be high enough to carbonize the Kapton. Many aircraft wires carry a current too low to cause a problem. "It is a rare event," he said. "The probability of all these things happening, fortunately, is fairly low."
--------------------- Swissair developments ---------------------
-- The U.S. Navy salvage ship USS Grapple, which helped with the recovery of TWA Flight 800 wreckage in 1996, arrived in Nova Scotia with an experimental laser camera and new sonar that can detect objects as small as a seashell.
-- Canadian divers were trying to retrieve the cockpit-voice recorder. A signal has been detected, but bad weather had delayed diving operations.
-- Investigators have found heat damage on fragments of the MD-11's cockpit.
-- Swissair and Boeing were served with a $50 million lawsuit this morning by former boxing champion Jake LaMotta, whose son was aboard the flight.
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