WASHINGTON - The mechanical problem aboard Alaska Airlines Flight 261 apparently festered for most of its two-hour, 43-minute journey, but the pilots may have thought the problem was more of an irritant than a threat and they postponed a decision to attempt an emergency landing, new data released Tuesday indicates.
In the most comprehensive account yet of the ill-fated flight Jan. 31, National Transportation Safety Board Chairman James Hall said the pilots turned off the autopilot soon after departure from from Puerto Vallarta, Mexico. They then flew manually for an hour and 53 minutes.
A veteran pilot said the decision to fly manually was unusual and suggests the crew ran into a problem but decided it was manageable.
"This is significant," said Lou Aaronson, a retired pilot who flew jets of the same type for Continental.
"One does not normally disengage the autopilot at that point. In hindsight, you can draw the conclusion that this is where the problem began. There might have been a structural failure that was getting worse and worse as the flight continued, something that had worn out and was beginning to give way."
In radio conversations with the ground, the pilots indicated their problem was with the plane's horizontal stabilizer, the winglike portion of the tail that controls the up-and-down pitch of the aircraft's nose. Aaronson said the pilots probably chose to fly manually because that would make it easier to control the pitch.
However, the problem worsened, and after almost two hours, the cockpit crew requested clearance for an emergency landing in Los Angeles. They never made it.
The twin-engine MD-83 jetliner crashed into the sea near Port Hueneme, Calif., killing all 88 people aboard.
Whether an earlier attempt to land the plane might have prevented a crash is a matter of speculation. Experts point out that the strain of reconfiguring the MD-83 for a landing, even early in the fight, might have exacerbated the problem.
Tuesday, Hall provided sobering details of the plane's final plunge, which lasted just more than a minute, from 17,900 feet.
In the initial moments of the dive, occupants were thrust toward the cabin roof with forces three times the pull of gravity. The aircraft then rolled to the left and turned upside down as it fell.
The plane's cockpit voice recorder picked up a loud noise just before the dive began. Radar suggests a piece of the plane broke away at about the time of the noise, falling into the sea about four miles from the main debris field. Navy ships have begun a search for this piece.
Hall said the Navy has recovered two fragments of wreckage from the main debris field - both of them sections of the stabilizer. Experts have suggested the stabilizer may have broken apart slowly in flight.
Experts have suggested mechanical failures and human decisions both played a role, but the NTSB said it could be months before any official conclusions are drawn.
In the wake of the crash, Boeing on Monday issued an advisory to pilots of similar aircraft, recommending an emergency landing if a standard checklist fails to resolve a stabilizer problem.
Hall reported the pilots had turned on the autopilot as the plane was climbing through 7,500 feet but turned it off 13 minutes later as the aircraft climbed through 29,000 feet.
"They might disconnect the autopilot thinking they can manually keep the plane in trim," Aaronson said. The crew could use other control surfaces to counteract a problem.
As the cockpit voice recorder tape begins, some 31 minutes before the crash, the pilots were discussing their problem with the stabilizer.
About 20 minutes later, with the plane still at 31,000 feet, the pilots turned on the autopilot again.
Then - virtually simultaneously - the autopilot was turned off again and the stabilizer moved into the full nose-down position, Hall said.
The plane began to dive at a rate of 7,000 feet per minute, more than three times the normal descent rate from cruising altitude. On the voice recorder, the crew could be heard trying to arrest the dive. The plane's speed brakes - flaps on the wings - were deployed, and after about a minute Flight 261 leveled off at 24,300 feet.
For the next nine minutes or so, the plane apparently was under control. The stabilizer was jammed in the nose-down position, but the crew was able to compensate by pulling back hard on the yoke, tipping up the plane's elevator, which consists of hinged panels on the trailing edge of the stabilizer. The crew announced their intention to make an emergency landing in LA.
"Then things began to happen very quickly," Hall said.
The pilots had extended the flaps and slats - hinged panels in the back and front of the wings - in their continuing efforts to gain the upper hand. That slowed the airplane and may have added to the nose-down pressure.
Suddenly, at about the same time the loud noise was heard, the plane pitched down, ultimately reaching a nose-down angle of 70 degrees.
"At this point, they had no pitch control at all," Aaronson said. "They are turning left, right, pulling, pushing, trying every combination to find something that will give them control. But it's too late - at this point, there is nothing they can do."
Rolling slowly left until it was inverted, the plane slammed into the Pacific.