The cockpit voice recorder on Continental Airlines flight 1713 picked up sounds of an engine surge four times as the DC9 struggled to take off in a blinding snowstorm from Denver's Stapleton Airport Nov. 15, a transcript showed yesterday.
The sounds -- which alone do not pinpoint the cause of the crash -- add to the evidence that ice may have accumulated on the wings before the pilots attempted to take off for a mid-afternoon flight to Boise, Idaho. Investigators have estimated that the plane rose only 40 feet from the runway before it tipped to one side, then the other, flipped onto its back and broke into three pieces. Of the 82 people on board, 28 were killed, including both pilots and a senior flight attendant.
Almost from the start, the investigation has focused on the possibility of ice on the wings. Investigators have determined that about three inches of snow fell during the afternoon of the crash. They also have confirmed that the plane sat in the snowstorm for 23 minutes after it left Continental's new high-tech de-icing pad, until air traffic controllers cleared it to take off.
The plane, an older model DC9-10, was a small version of the widely used jet and lacked slats -- devices on the front edge of the wings that assist in lift. In three earlier incidents, the same model failed to take off in icy winter conditions. And in 1986, manufacturer Douglas Aircraft Corp. warned owners to watch the plane closely in icy conditions. Slats were added to later models of the DC9.
On the cockpit voice recorder, the sounds of an engine surge and a loud bang came in rapid succession in the final five seconds before the sound of impact. After the first engine surge, one of the pilots muttered an expletive.
Then came the next surge, the bang, and two final surges.
The surge, which would indicate an engine compressor stall, can be caused by a disruption of air flow over the wings. In a compressor stall, the engines continue operating, but not smoothly. Frequently, compressor stalls are accompanied by a bang, which sounds like a backfiring car engine.
Ted Lopatkiewicz, a transporation board spokesman, said the board has not determined if the engine compressor stalls occurred as a result of the plane losing its aerodynamics.
The tape also confirms that copilot Lee Bruecher was at the controls when the crash occurred. Bruecher, 26, was a former commuter airline pilot hired by Continental last July. He was making his second trip as a DC9 copilot.
Up to the moment of the crash, the tape reveals a textbook flight. Bruecher and Capt. Frank Zvonek ticked off the items on the preflight checklists.
"I've got the brakes on. You've got the airplane," Zvonek said to the copilot after the plane was in position.
Nine seconds later, the plane was cleared for takeoff.
"There's a hundred knots, looking for one-thirty-nine," Zvonek said, as the plane rolled down the runway. He voiced the final calls as the jet reached takeoff speed. "Positive rate," he said, meaning the jet was climbing normally.
Three seconds later came the sound of the first engine surge.
The 32-minute tape does not contain any conversation about the snowstorm -- although it was snowing so hard that the controllers could not see the jet.