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A failsafe jackscrew mechanism designed to improve the safety of space launch operations might have prevented the fatal January 2000 crash of an Alaska Airlines (ALK) twinjet.
Engineers who developed the new jackscrew design to support Space Shuttle operations say it can readily be applied to commercial aircraft and, indeed, can be retrofitted to hundreds of existing aircraft to provide an assured means of trim control.
On the Alaska MD-83, pitch trim is controlled by the horizontal stabilizer; it moves nose-up and nose-down by means of an electric motor driving a jackscrew assembly. This device is comprised of an electric motor which drives an acme screw through an acme nut. In this distinctive T-tail arrangement, the aluminum-bronze alloy acme nut is affixed to the tailfin. The motorized steel jackscrew, attached to the horizontal stabilizer, moves up and down through the acme nut. Safe operation of the mechanism depends on proper lubrication and periodic checking of the "end play" between the threads of the screw and nut.
In its ongoing investigation into the Alaska crash, National Transportation Safety Board (NTSB) investigators found improper mixing of lubricants, which may have accelerated wear on the acme nut. Additionally, during the airplane's last heavy maintenance check in 1997, the wrong tool was used to check the "end play," which was right at the "no go" margin. Without a spare jackscrew and acme nut in stock - the two are mated, serialized parts - the end play was checked again, found to be within limits, and the airplane was put back in service. The airplane flew for more than two years until this improperly-maintained assembly broke in flight, and the airplane plummeted into the Pacific Ocean waters off Los Angeles, killing all 88 passengers and crew aboard.
When the wreckage was pulled from the water, the motorized jackscrew and acme nut were found bereft of lubrication, and the threads in the acme nut were almost completely worn away by the abrasive action of the jackscrew (see ASW, Jan. 1, 2001). In truth, the steel jackscrew acted like a machine tool on the...