The US Federal Aviation Administration wants operators of a wide variety of Boeing 737s to perform repetitive inspections, lubrications, repairs and overhauls to the horizontal stabilizer trim actuators on the aircraft to prevent possible loss-of-control accidents.
The proposed airworthiness directive (AD) comes as a result of a Boeing design review and safety analysis of the trim units on all its aircraft following the January 2000 loss of an Alaska Airlines MD-83 and all 88 passengers and crew due to an improperly lubricated jackscrew.
During Boeing’s review, one operator of a 757 reported “extensive corrosion” of the primary load path ball bearings in the ballscrew assembly of the trim mechanism, according to the FAA.
The agency says the trim mechanisms in the Boeing aircraft, although different in design to the failed MD-83 system, perform “similar functions and have the same airplane-level effect following failure.”
“This condition, if not corrected could result in drive mechanism of the horizontal stabilizer trim actuator and subsequent wear and failure of the secondary load path, which cold lead to loss of control of the horizontal stabilizer and consequent loss of control of the aircraft,” says the FAA.
Given that the ballscrew assembly on the 757 is similar to the system on the 737, the FAA contends that “all of these models may be subject to the same unsafe condition” and says it is considering a similar rulemaking for the 757 trim system.
Source: FlightGlobal.com
