DENVER, NOV. 19 -- Investigators probing the wreckage of the Continental Airlines jet that crashed Sunday in a snowstorm here focused on the jet's engines today, looking for evidence that could indicate the plane had ice on its wings.
Engine sounds heard on the cockpit voice recorder were described by a source close to the investigation as being similar to an engine compressor stall, which could be caused by a disruption of air flow over the wings. In a compressor stall, the engines continue running, but not smoothly, and the engine blades could be damaged if the stall continues.
The sounds were described as sharp thumps, similar to the sound of a car backfiring, and they occurred after the jet lifted off the runway, but before the plane began to tip to the side, officials said.
The plane crashed a few seconds after taking off from Denver's Stapleton International Airport and broke up between parallel runways. Twenty-eight of the 82 on board, including the captain, copilot and a senior flight attendant, were killed.
Investigators from the National Transportation Safety Board also are looking for signs of soot on the interior of the engines, which could indicate there was a compressor stall. If there was ice on the wings, the air flowing over them would be distorted, interfering with the plane's ability to lift and requiring greater speed to get off the ground, officials said.
In a technical sense, a plane could become airborne with ice on its wings if it were moving fast enough, but Federal Aviation Administration regulations prohibit takeoffs if ice, snow or frost are adhering to the aircraft's surface. Evidence of an engine compressor stall is symptomatic of various problems, including the presence of ice on the wings.
National Transportation Safety Board chairman Jim Burnett, who accompanied the 17-member investigation team here, said a passenger described abnormal engine sounds to investigators. "A loud poof and a couple of loud noises," Burnett said, adding that the sounds were heard after the plane began to tilt into the ground.
Earlier in the day, Richard Hillman, vice president of flight operations for Continental, said he did not think ice accumulation on the wings was a major factor that contributed to the crash.
Asked at a briefing today to respond to a statement from an unnamed source in the Airline Pilots Association's Washington headquarters that the accident sounded like "a classic case of ice," Hillman replied: "I have pushed aside the icing. Based on my personal opinion, it is not a classic icing."
The pilots association has asked to be made a party to the Flight 1713 crash investigation, but the safety board has denied the request because the association has not represented Continental pilots since the union's Continental unit was broken in a strike that began in 1983. Representatives from the aircraft manufacturer, the FAA, the airline and employe unions participate in safety board investigations of air accidents.
The jet, which was headed to Boise, Idaho, had its ice removed at Continental's de-icing pad. Airline officials estimated the pad is about a two-minute taxi from the end of the runway.
The jet stood in the snowstorm for 23 minutes after it was de-iced before it received clearance from air traffic controllers to take off, officials said.
Three Continental flights that took off just before Flight 1713 waited only 15 to 16 1/2 minutes from the time they requested clearance from the de-icing pad to the time they were given clearance to take off, officials said.
Because of confusion in radio communications between the crew and air traffic controllers, Flight 1713 got out of sequence. A Continental jet that passed through the de-icing pad after Flight 1713 was allowed to take off before it, Burnett said. But, he added, if Flight 1713 had not been out of sequence, it would have saved only two minutes and 15 seconds.
As for the rest of the delay, Burnett said, the jet "could not be given clearance due to arrival of aircraft."
Investigators are trying to reconstruct the air traffic control operations just prior to the crash. In the 13 minutes prior to the time the plane was cleared to take off, there were four departures and five arrivals. The arriving jets were landing on Runway 35 Right, which is 1,900 feet away from a parallel runway from which jets were taking off.
Investigators are trying to determine if the plane was affected by turbulence from a Delta jet that landed just before the Continental jet began to take off. The flight data recorder of the Delta jet, a Boeing 767, is being reviewed to determine the time and location it touched down on the runway.