Ramon Lopez/WASHINGTON DC
An inflight mechanical failure of the aircraft's pitch trim control system may have caused the 31 January 2000 Alaska Airlines McDonnell Douglas MD-83 crash, according to information presented at a public hearing on the accident. The cockpit voice recorder (CVR) has yielded clues not revealed before.
On the initial day of the four-day hearing, the US National Transportation Safety Board released the CVR transcript covering the last 30 minutes of the flight. Together with flight data recorder information, it points to an in-flight failure of the screwjack mechanism that controls the angle of incidence of the horizontal stabiliser and limits its range of motion.
Much of the hearing concerned the design and maintenance of the screwjack assembly. The Federal Aviation Administration does not believe that a re-design is necessary since the event may be isolated. Meanwhile, Safety Board investigators are testing whether incompatible greases had been mixed on the screwjack.
No conclusions were to be offered during the public hearing. A date for the final report has not been forecast.
The hearing revealed that the aircraft went into a dive at 26,000ft (7,900m). At 24,000ft the crew regained control, and the aircraft stabilised and slowed. It was then cleared to descend to 17,000ft, to prepare for a landing at Los Angeles airport.
At an airspeed of 270kt (500km/h), however, the flaps started to deploy. One second later, an extremely loud noise was heard on the CVR. Alaska pilots who have analysed the CVR transcript believe that noise "to be the result of a catastrophic failure of the aircraft's screwjack".
The next several seconds of FDR data show a maximum nose down pitch rate of close to 25í/s, and the vertical acceleration reaching -3g within 3s The aircraft reached 80í nose down pitch attitude within 6s and rolled inverted prior to impact.
The screwjack mechanism was recovered, but it was fouled by metal shavings and fractured. The FAA subsequently ordered inspection of over 1,000 DC-9s, MD-80/90s and Boeing 717s, with airlines replacing screwjacks on 18 aircraft.
Source: Flight International
