The news that ground investigators had discovered the jackscrew from the Ethiopian Airlines 737 MAX that crashed Sunday evoked strong memories and comparisons to the 2000 crash of Alaska Airlines Flight 261.
But there are key differences between the role of the jackscrew in the two fatal crashes.
In the crash of Flight 261 off the Southern California coast in January 2000, which killed all 88 people on board, the National Transportation Safety Board (NTSB) found that the jackscrew on the MD-83 failed because it had not been adequately lubricated, causing the part to fail and send the plane into a dive. The mechanism moves the plane’s horizontal tail, a winglike structure on the tail that helps the plane climb and descend.
The board also found that the Federal Aviation Administration had allowed Alaska to engage in risky maintenance practices. The plane plunged into the Pacific Ocean, en route from Puerto Vallarta, Mexico, to San Francisco and Seattle.
In the Ethiopian crash, which killed all 157 people on board, there's no suggestion that the jackscrew on the Boeing plane failed.
The significance for the crash is that when the jackscrew was found on the ground among the wreckage, its position indicated that the jet's movable horizontal tail, also known as the stabilizer, was deflected in an unusual position.
This information was provided by John Cox, chief executive of Safety Operating Systems and formerly the top safety official for the Air Line Pilots Association, who was privately briefed on this piece of evidence Thursday by someone familiar with the investigation findings.
The deflection of the tail, combined with the high speed the jet was traveling at according to satellite-based data, would have made it uncontrollable, Cox said.
Cox cautioned that there are several systems that in the course of normal flying could malfunction and deflect the tail.
But because the preliminary investigation into the October crash of a Lion Air 737 MAX pointed to a new flight-control system on the MAX called MCAS (Maneuvering Characteristics Augmentation System), that system is now a prime suspect as potentially the cause of both tragedies.
In this case, the jackscrew provides one more clue.