Failure-Prone Jackscrews, Again

From the Australian Transport Safety Bureau (ATSB), released May 6:

"On 13 March 2003, while on final approach during a training flight, the crew of a Lear 45 ... reported feeling a severe vibration through their respective control columns, followed by a rapid 10 to 15 degree nose-down pitch change. Attempts to manually trim the nose of the aircraft up failed, with both crew members required to pull back on the control columns in order to regain control of the aircraft.

"A subsequent inspection of the aircraft ... revealed that the aircraft's horizontal stabilizer could be moved by hand, vertically, approximately four inches at the leading edge ... the actuator was found to have free movement of its shaft in and out of the actuator body. Further detailed examination indicated that the actuator's primary 'Acme screw' had failed. With this screw failed, the horizontal stabilizer load should have been retained through a secondary rod, however, the threaded retaining nut had unscrewed from the rod."

As a result of this incident, the U.S. Federal Aviation Administration (FAA) issued an emergency airworthiness directive (AD), which was followed by the Australian Civil Aviation Safety Authority (CASA). The ATSB report notes that the actuator manufacturer completed a new design and has since had it certified for fitting on all affected Lear 45 aircraft. Recall that the failure of a similar jackscrew led to the crash of Alaska Airlines Flight 261 (see ASW, Jan. 20, 2003). The accident aircraft was an MD-83, and the accident was blamed on a shortage of grease. The problem persists, however, as evidenced by AD 2005-07-04 issued by the FAA on March 30 for the Airbus A330 and A340 fleets. The AD calls for repetitive inspections of the ball screw assembly for the trimmable horizontal stabilizer, corrective action if necessary, and repetitive greasing of the ball nut.