Burning odor reported less than 24 before United cockpit fire the forced emergency landing

WASHINGTON (AP) — A burning or heating odor was reported in the cockpit of a United Airlines plane less than 24 hours before an in-flight fire erupted, forcing an emergency landing, a federal safety panel said Tuesday.

National Transportation Safety Board investigators are looking into whether long-known problems with the cockpit window heating systems in Boeing 757s played a role in a fire that forced United Flight 27 to make an emergency landing at Dulles International Airport near Washington on Sunday night.

"We understand that there was an odor in the airplane, but no smoke," said Keith Holloway, a spokesman for the safety board. "We are still evaluating the maintenance actions that were taken as a result of that event."

United officials didn't immediately reply for a request for comment.

The plane was en route from New York to Los Angeles with 112 people aboard when the fire broke out. Among the passengers were several celebrities, including actress and fashion designer Ashley Olsen.

Passengers said there was consider smoke in the front of the cabin and they could see flames in the cockpit when the captain opened the door to request a flight attendant retrieve a fire extinguisher. Photos taken by passengers show the cockpit window was badly cracked.

While any fire is a concern, a small fire can usually be put out. Of greater concern is that heat generated by a fire could cause a window to completely shatter in-flight, allowing wind to gust in at great force.

Boeing Co. spokeswoman Sandra Angers said on Monday that Boeing knows of 29 incidents over eight years involving the window heating system on its 747s, 757s, 767s, and 777s.

Boeing issued safety bulletins to airlines between 2004 and 2007 requiring them to check for a loose screw holding the wires connected to the heating system. And planes made since 2005 have used a different wiring system that Boeing believes has solved the problem, Angers said.

The NTSB recommended to the FAA in 2007 that airlines be required to replace the windshield heat terminal blocks on all Boeing 747s, 757s, 767s and 777s. The FAA proposed a rule in March 2008 that gave airlines a choice of regularly inspecting the heat terminal blocks or replacing them. However, the rule isn't expected to be made final until August.