GUY NORRIS / LOS ANGELES
Boeing expected to issue stabiliser service bulletin
Operators of more than 1,000 Boeing 757-200/300s have begun checking stabiliser ballscrew actuator assemblies for grease contamination which could in extreme cases cause pitch-trim runaway.
Boeing, which was expected to release an alert service bulletin (SB) mandating the inspections late last week, declines to comment. The alert follows the discovery of an unsuspected failure condition in the dual-redundant system. The discovery follows increased operator scrutiny of pitch trim control systems after the 31 January 2000 crash of an Alaska Airlines Boeing MD-83.
The issue revolves around the braking system for the assembly which prevents it from backdriving and causing a trim runaway. The stabiliser ballscrew actuator is powered by two hydraulic motors with two hydraulically released brakes. Two stabiliser trim control modules are electrically controlled to supply hydraulic power to the brakes and motors. A secondary mechanical brake is located on the ballscrew and it is this unit which is susceptible to contamination by grease from a nearby support bearing holding the assembly.
Although sources stress that the availability of the hydraulic system makes the chances of a runaway "virtually nil", the problem reduces the system to single redundancy level. Initial checks appear to have uncovered a considerable number of affected brakes, and Boeing's SB is expected to call for the assembly to be serviced at 30,000h rather than "on-condition".
Boeing has told operators of 3,284 of its 737s, 747s and 757s to inspect for potentially faulty centre wing fuel tank pumps, but to continue operating provided the pumps are not allowed to run in nearly-empty tanks. This does not alter operating instructions issued by the US Federal Aviation Administration in a 30 August emergency AD, but it dramatically increases the number of pumps believed to be at risk (Flight International, 10-16 September). Boeing 767s and 777s are unaffected.Source: Flight International
