Ramon Lopez/WASHINGTON DC
The US National Transportation Safety Board (NTSB) has recommended that the US Federal Aviation Administration require all Boeing 737s to have a "reliably redundant" rudder system, following its investigation of the September 1994 fatal crash of a US Airways Boeing 737-300 near Pittsburgh, Pennsylvania.
The US Airways crash occurred because of an uncommanded rudder hard-over that was "most likely" caused by a jam of the main rudder power control unit (PCU) servo valve secondary slide, according to the NTSB.
The NTSB says the same rudder failure was the probable cause of the March 1991 crash of a United Airlines 737-200 near Colorado Springs and the June 1996 yaw/roll incident involving an Eastwind Airlines 737-200 on approach to Richmond, Virginia.
FAA and Boeing officials say changes ordered since 1997, including a PCU redesign, yaw damper modifications, the addition of a rudder limiter and improved flightcrew training, have eliminated the problem.
The NTSB, however, says the dual, concentric servo valve on 737s is not reliably redundant and that the system design enhancements ordered by the FAA do not go far enough.
Tom McSweeny, the FAA's associate administrator for regulation and certification, says he needs to talk to the NTSB to establish what it means by the term "reliably redundant".
The NTSB also recommends the creation of a government and industry engineering test group to continue research on the rudder safety issue.
Charles Higgins, Boeing's vice-president for airplane safety and airworthiness, says: "What we've done to date takes care of all the problems we know about.
"We welcome an independent look at the rudder system. If they make recommendations that improve it, we will be happy to consider them."
Source: Flight International
