FAA to mandate Boeing 737 rudder redesign

Following a year-long investigation into the flight control system of the Boeing 737, prompted by two fatal accidents where a failure of the aircraft's rudder system was thought to be responsible, FAA announced that within a year it will "propose mandating" a complete redesign of the aircraft's rudder system.

FAA's announcement coincided with the release the final report of the Engineering Test and Evaluation Board (ETEB), the blue-ribbon panel charged with investigating the 737. Download the full text of the ETEB executive summary, including the Findings and Recommendations and Introduction, from Aerospace Online's Download Library.

In the meantime, in addition to changes already made to portions of the rudder system - specifically a redesigned rudder power control unit, Boeing says it is taking the following steps:

• Crew procedures: current cockpit procedures for dealing with jammed or restricted rudder operations have been clarified and simplified;
  
• Maintenance procedures: rudder system maintenance procedures on 737 Initial and Classic airplanes (-100 through -500 models) will be modified to match procedures developed for Next-Generation 737s.
  
• Rudder system: NTSB has recommended that the 737 rudder Power Control Unit (PCU) be made "reliably redundant." Boeing says it has developed a concept that ensures "the reliability and fundamental safety that is built into the airplane."

"The changes we are going to propose will make an aircraft with an excellent safety record even safer," said FAA Administrator Jane Garvey. Boeing echoed this sentiment in its own statement: "These enhancements should be taken in context," said Carolyn Corvi, VP and GM of Boeing's 737 program. "The 737 family has been, and continues to be, among the safest of all jetliners; in fact its safety record is twice as good as the average for the world's commercial jet fleet."

New 737 models with a newly designed rudder system could be available by mid-2003, said Boeing, with retrofits beginning later that year. The company says it believes the retrofit can be accomplished during regular maintenance intervals.

For its part, however, the National Transportation Safety Board (NTSB) remains skeptical. "The major finding … is that the Boeing 737 rudder control system has numerous potential failure modes that represent an unacceptable risk to the travelling public," said NTSB chairman James Hall following the FAA's announcement. The ETEB report, says Hall, identified "dozens of single failures and jams and latent failures in the 737 rudder system, in addition to the single point of failure we identified in our accident report, that can result in the loss of control of the airplane."

"Although the failure mechanism that we believed led to the crashes of United Airlines flight 585 in 1991 and USAir flight 427 in 1994, and the near loss-of-control of Eastwind Airlines flight 517 in 1996, appears to have been eliminated through a redesigned rudder power control unit, the results of the ETEB echo our findings that failure modes still exist in the Boeing 737 rudder system."

Hall said NTSB was "very concerned" that some of the ETEB recommendations would not be adopted - particularly a requirement for an independent switch to stop the hydraulic flow to the rudder and a rudder position indicator in 737 cockpits, but was "pleased that both the FAA and Boeing Aircraft Company agree that there is a need for a redesign to the rudder actuator system."

As of the end of July 2000, Boeing had delivered 3,740 B-737 aircraft to 300 operators in more than 100 countries. The company says a 737 takes off or lands every 5.5 seconds and approximately 1,000 are in the air at any given time.

Edited by David Robb
Managing Editor, AerospaceOnline.com