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Boeing devising limiter for 737 rudder moves
November 3, 1996
New safety feature for jets would reverse company's stance
By Byron Acohido
The move to retrofit the existing 737 fleet with the devices, known as limiters, would be a stunning reversal of Boeing's longstanding position that the jet's rudder-control system is safe and doesn't need changing.
A company spokeswoman, Susan Bradley, yesterday acknowledged that Boeing has held discussions with the Federal Aviation Administration about the possibility of retrofitting the fleet with limiters, but denied the implementation is imminent.
"We haven't made any final decisions," Bradley said. "We're not marching to a decision date. We'll make an announcement when it makes sense and we have enough data to make an announcement."
Another person, who has been involved in the discussions but who asked not to be identified, said a decision to implement the retrofit has been made and will be announced soon.
In a separate development, on Friday Boeing acknowledged discovering a 737 rudder-control problem that could imperil flights, and issued an advisory to airlines flying 737s to test all planes for a potentially jammed valve part. The FAA quickly issued an emergency order mandating the tests within the next 10 days.
Airlines will have to repeat the emergency inspection every 250 flight hours about every 30 days until a solution for the valve jamming is found.
Pilots have reported troubling, inadvertent movements of the 737 rudders since the plane entered service in the late 1960s, but the plane's rudder-control system didn't come under intense scrutiny until a 1991 crash in Colorado that investigators suspected was caused by an extreme uncommanded rudder movement, which engineers call a hardover.
If such an extreme rudder movement happens at low altitude and slow speed, it can snap the plane into a dive that a pilot may not be able to recover from.
United Airlines Flight 585 was at about 1,000 feet approaching landing in Colorado Springs when it suddenly flipped into a dive and crashed in March 1991, killing 25 people. A USAir flight was descending on approach to Pittsburgh when it crashed nose-first into a ravine in September 1994, killing 132.
Though it didn't solve the Colorado Springs crash, the National Transportation Safety Board nonetheless called for two improvements in the 737's rudder system. The FAA subsequently ordered airlines to implement one of the changes designed to make it harder for a specific kind of hardover to occur.
But pilots continued reporting scores of flights disrupted in a manner that suggested rudder malfunction. Then came the Pittsburgh crash in 1994.
The NTSB's investigation of the Pittsburgh crash is now in its 26th month with no official finding about what caused the disaster.
Test turns up unexpected result
Boeing's recent discovery of a new way the rudder could fail grew out of an unusual set of circumstances. In February, frustrated that the Pittsburgh investigation was going nowhere, Safety Board chairman James Hall convened a panel of independent experts to review all investigative work from both the Colorado Springs and Pittsburgh probes.
In particular, Hall asked the panel to re-examine whether dirty hydraulic fluid could have jammed the main rudder power-control unit, or PCU, an assembly of valves, levers and linkages that work together to control rudder movements.
Concern centered on the dual servo valve, a device at the heart of the PCU about the size of a soft-drink can. The servo valve contains two slides, which work together in a delicate sequence, involving movements smaller than the width of a dime, to direct pressurized hydraulic fluid to move the rudder.
The panel devised an experiment to assess whether hot, dirty fluid directed into a very cold servo could cause the slides to jam, misdirect pressure and trigger a hardover. Using the USAir jet's servo valve, the panel ran the test in late August at an independent lab in Valencia, Calif., and triggered a jam.
Boeing asked to repeat the experiment at its lab in Seattle. On Oct. 11 with the panel and NTSB investigators present, Boeing repeated the test and the USAir jet's servo again jammed. Boeing engineers then decided to analyze the computer data generated by the test to assess what was going on inside the servo.
The data showed Boeing engineers one of the servo's two slides could be jammed without pilots or mechanics noticing. If a pilot depressed a rudder pedal with authority with that particular slide jammed, the servo could misdirect fluid and cause an uncommanded rudder hardover.
The new problem had nothing to do with the temperature of the hydraulic fluid or the servo valve, said Jean McGrew, former chief 737 engineer.
Boeing did not notify the independent panel or NTSB investigators of its discovery. Bradley said that was because of Boeing's policy "not to go forward raising issues and concerns until we have good data. We want to have compete data. We want to understand the issue thoroughly and then take it forward. It's not productive to speculate."
On Oct. 23, again without notifying the NTSB, Boeing began running a different set of laboratory experiments, which confirmed that if the servo's outer slide jammed for whatever reason, any rapid command to move the rudder would result in a hardover.
Why Boeing didn't notify NTSB
Last Tuesday, Boeing conducted a ground test on a Boeing 737 at Boeing Field. The jet's servo was rigged so that the outer slide was jammed. When Boeing chief test pilot Jim McRoberts stomped on the pedal, the pedal sprang back and the rudder swung hard over to the left.
"The McRoberts test was verification of what we were seeing," said Boeing's Bradley.
Last Thursday, the NTSB convened a meeting in Pittsburgh to review findings from its committees responsible for various aspects of the USAir crash.
Boeing representatives at the meeting made no mention of the new rudder-control jam that had been discovered in its tests here.
The Safety Board's "party" system of investigating crashes, which brings together manufacturers, airlines and pilots representatives for technical support, has long been governed by an understanding that all investigative information is shared.
Bradley said Boeing didn't notify the NTSB investigators of what they were discovering because it felt the NTSB's oversight ended after safety-board investigators and panel members left Seattle following the Oct. 11 test of the hot hydraulic fluid and the cold servo valve.
Boeing says the data derived from that test revealed a problem totally unrelated to the Pittsburgh crash investigation.
"Independently looking at the data, we came across something that made us want to look further. So we did, and when we found it (the new problem) we went to the FAA per the process," Bradley said.
On Thursday night Boeing officials contacted Tom McSweeny, FAA director of aircraft certification, and advised him of the findings. Boeing showed him its advisory calling for the inspection of all 2,700-plus 737s around the world for indications of rudder servo-valve jamming.
McSweeny's staff made Boeing's bulletin an emergency directive, making the inspection mandatory for all U.S.-registered 737s, and providing impetus for agencies in Europe and Asia to do likewise.
Friday afternoon, as Boeing was announcing plans for the fleetwide inspection, the Safety Board faxed a statement from Hall to The Times which clearly linked Boeing's discovery to the ongoing Pittsburgh crash investigation.
"Today's action by Boeing to issue a 737 alert bulletin is a direct result of information gained during NTSB's exhaustive tests to find the cause of the USAir Flight 427 accident two years ago in Pittsburgh," the statement said.
"While the NTSB supports Boeing's efforts, we are doing a full
analysis of this data to find out how it relates to the Pittsburgh
and Colorado Springs accidents," Hall's statement continued. "I am
encouraged that this brings us closer to finding the answers to
these twin tragedies."
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