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| The various fixes from ten+ years ago appear to have stopped the "uncommanded rudder" yaw upset events suffered by the Boeing 737-100/500 during the 1980's and 1990's. The ADs were sequential, not concurrent, therefore someone has surely worked backward to discern which one or more was/were effective. The 737-NG's didn't suffer the problem - big clue? It certainly wasn't the dual-concentric hydraulic valve replacement as the USA NTSB suspected. MetroJet 2710 and at least 15 other events with redesigned valves proved that. MetroJet 2710 had NTSB scratching their heads at the USAir427 Sunshine back in the spring of '99. We passengers are relieved that the fixes worked, however, let us have a discussion of which were the good ones for educational purposes. Doug | |
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Yes, the history of the rudder problem is confusing and the "fix" is hard to pinpoint. I did find a web site that covered the story well and the most significant info in there seemed to be:
and....13 Sep 2000 - The FAA reaches an agreement with Boeing to redesign the rudder. Once the directive is issued, the company will have about five years to make the changes in planes currently flying. The new design will be required inall newly made 737s. Because the redesign could take years to implement, the FAAsaid it will also announce new training procedures for pilots to use in the event of rudder problems.
http://www.b737.org.uk/rudder.htm12 Nov 2002 - FAA issues AD 2002-20-07 R1 (Revision issued a month afteroriginal AD)
The actions specified in this AD are intended to prevent an uncommanded rudder hardover event and consequent loss of control of the airplane due to inherent failure modes,including single-jam modes, and certain latent failures or jams combined with a second failure or jam.
Install a new rudder control system that includes new components such as an aft torque tube, hydraulic actuators, and associated control rods, and additional wiring throughout the airplane to support failure annunciation of the rudder control system in the flight deck. The system also must incorporate two separate inputs, each with an override mechanism, to two separate servo valves on the main rudder power control unit (PCU); and an input to the standby PCU that also will include an override mechanism. -
B737 Yaw Upset History
Bob, a check of the various data bases reveals that the yaw upsets subsided in late 2000, therefore the effective fix predates the FAA required ADs of 2002. Someone surely has cross checked the earlier ADs against the event episodes. I suspect that AD/B737/123, which replaced the analog yaw damper coupler with the digital coupler, was the one that did the trick but have no proof. The NG 737's had the digital version as built. It was basically the B757 coupler.
The old Forum on airdisaster.com had several posts from me with support for a theory involving a failure mode of the old analog yaw damper coupler. The same failure mode was discovered during the FAA's Engineering Test and Evaluation Board (ETEB) year-long effort back in 1999-2000. They called the 11-degree rudder a "yaw damper hard over." It contradicted the Boeing position that the yaw damper could only add, at most, 3 degrees of rudder. The material was not seen by many and resided on ad.com for over a year.
I then sent the link to three or four of the attorneys involved with litigation against Parker-Hannifan, the maker of the rudder Power Control Unit. About three weeks later, the ad.com host announced that the popular Forum was being taken down. A link to this forum was suggested.
Cause-effect? Hummm.
The NTSB's USAir 427 report (back in one of the last appendices) gave the date of AD/B737/123 to change the analog yaw damper coupler with the digital version as three years starting in August 1997. Honeywell had a lot of them to build, so they probably didn't all get replaced until late 2000 - that's when the yaw upsets stopped.
Maybe so, maybe not.Comment
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