A REVIEW OF THE design of Boeing 737 flight control has uncovered no flaws, which could have caused the unexplained crashes of two aircraft, says the US Federal Aviation Administration. The critical design review of the flight control system, was prompted by the 1991 United Airlines crash at Colorado Springs and the 1994 USAir crash at Pittsburgh.
Unexplained rudder deflection has been implicated in both accidents. Although the review team found no design faults linked to the crashes, it has made 27 recommendations to improve the control systems of the 737 and other aircraft. "No safety issue has been found that requires immediate corrective action," the FAA says.
The recommendations cover enhanced alternative means of controlling the aircraft; enhanced flight-crew training for response to failures and flight-path upset; improved design, protection and maintenance of flight-control components; and improved surveillance of design, manufacture and repair of replacement parts.
Of the 16 recommendations specific to the 737, several relate to the yaw damper and standby rudder-actuator, for which the team identified failures which could result in a rudder hard-over - an event which Boeing says it "...has never known happen".
The review team examined all possible control-system failures, regardless of their likelihood, and the alternative means of control available in the event of a failure. Follow-on activity will determine the probabilities of the failure modes identified, the FAA says.
"We considered those [failures] with even the slightest possibility, where continued safe flight was doubtful," says Tom McSweeney, FAA director of aircraft-certification services.
The team examined all design, maintenance and operational aspects of the 737's lateral and directional control systems and assumed the worst-case flight-crew reaction to the failures identified. Boeing's 737 flight simulator, was to evaluate the effects of flight control failures, "many of which have never occurred on the 737", McSweeney says.
The review team wants adequate alternative means of directional control to be "clearly demonstrated" throughout the operational envelope. Boeing says that simulation showed that the 737's lateral controls are sufficient to counteract a rudder hard-over in flight conditions similar to those of the Pittsburgh accident, "...if the crew gets on to it real quick."
The report recommends further analysis of yaw-damper failure modes and urges action to reduce the "unsatisfactory rate" of failures. Boeing says that it has "a lot of data" on the unit and plans "a couple of special flight-tests soon". It emphasises that it "...can and will reduce yaw-damper failures".
The team recommends action to eliminate galling of the standby rudder actuator, which can cause it to lock up, with the result that the yaw damper exceeds its 3° rudder-deflection limit. Boeing is aware of "about 12" instances of lock-up, all of which were detected on the ground during control checks. No galling was present on the Pittsburgh 737.
Source: Flight International
