B737 RUDDER FAILURES

IASA Safety wrote:

"The design of this servo valve does not use "O" rings. Instead it relies on very close tolerances to limit hydraulic leakage. The total movement of the primary or secondary valve from center to its extreme travel is about 0.0045 of an inch (about the width of a dime). The  clearance between the primary valve and the secondary or the secondary  to the valve housing is less than a human hair. Close tolerances   required that consideration be given to the effects of a foreign object obstructing movement of the valves.
Chip shear force is a measure of the ability of a valve to shear a foreign object. That is, to actuate normally in spite of the presence of foreign material. The chip shear of the primary valve of the B737 main rudder PCU is significantly less than that of other similar aircraft. This chip shear capability is about 40 pounds on the B737 while the DC-9 and MD-80 are a minimum of 100 pounds. As a result it  may be easier to jam the B737 PCU. The secondary valve of the B737 has a somewhat higher chip shear than the primary valve.

The redundant features of the servo valve are only effective if both valves are free to move. If one valve does not move freely, then a subsequent single failure or jam can cause uncommanded movement of the  rudder. B737 pilots have no way to detect a jam of either a primary or secondary valve."
Comment:
> I simply cannot believe that Boeing designed such a critical valve
> without O rings. The tolerances are as fine as a human hair -but in
> hydraulic systems you'd need a five micron filter to ensure that
> contamination wouldn't eventually lead to the sort of unreproducible
> valve sticks and hard-overs that have plagued the B737 fleet forever.
> Because of the forces available in hydraulic actuation you CAN
> miniaturise components and have very limited valve travel. Methinks
> that they (the designers) do this just to prove that they can (and
> save weight) without any consideration of "in-service" perils of hyd fluid
> impurities and contaminants. It's similar to miniaturising a fine
> Swiss Watch, gifting it to a wharf laborer (and later wondering why it
> stopped working).
> With sensible robust design, that allows for the in-service working
> environment, you just don't get these asinine failures. If you want to
> miniaturise and design to incredibly fine tolerances you've got to
> embrace the possibility of failure and give the pilots easily
> accessible overrides and manual reversion. Otherwise, it's designer-wrought homicide.
> These revelations mark the beginning of the real decline in Boeing's  fortunes.
>

>IASA ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

John King wrote:

Thanks John:

I think ALPA would agree with your analogy of the 737 PCU; "It's similar to miniaturising a fine Swiss Watch, gifting it to a wharf laborer (and later wondering why it stopped working)."

Did you also notice under the section d " FAA Critical Design Review Team", "A visit to Honeywell/Sperry by a CDR team representative resulted in two observations:";

See Item 2. "There are a number of failure modes that could cause the Yaw Damper to command a rudder deflection to the Yaw Damper authority:
a. Electrical shorts or grounds,

b. Open feedback circuit, and......"

Regards,

John
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Subject: Re: USAIR Flight 427 B737's Rudder - Hardovers and HardtoSwallows
Date: Sat, 13 Mar 1999 20:26:01 +0800
From: Safety IASA <safety@iasa.com.au>
To: John King <jking1@mediaone.net>

Yes John I saw that too. The possibility of electrical glitches in all-electric jets is endemic. It's almost a given. Just about any aircraft system is electrically powered, monitored, switched or controlled and vulnerable to ticking faults, shorts, power outages, surges, spikes and a lack of electrical system redundancy.

But the thing that gets my dander up about the B737 rudder saga is that it's taken five years to get to this report, this admission of fundamental technical deficiency - yet if you look at the bottom story on the following site, that idiot McSweeney is now saying essentially that nothing should be done because the hazards of implementing a fix outweigh the number of incidents of rudder hard-overs. That's tantamount to saying that you're better off not having that critical life-saving heart operation because the surgeon might foul it up and you could catch an infection and die.  Basically one of their prime fixes is for the crews to fly faster (i.e. closer to the crossover speed) so that they can accelerate (if they're lucky and not too near the ground) to a speed where rudder authority isn't paramount. That's similar to not driving too fast because your brakes don't work. With McSweeney near the top of that organization they don't have to worry about their credibility showing.
"The Federal Aviation
Administration said on Thursday that a major redesign of the
Boeing 737 rudder was unnecessary and could pose risks of its own."


BoeingLimitsLiability.html  

Subject: FAA's McSweeny statements contradict FAA own Team Findings
Date: Sat, 13 Mar 1999 07:53:41 -0500
From: John King <jking1@mediaone.net>
To: IASA Safety <safety@iasa.com.au>, David Evans <devans@phillips.com>,
Edward Block <EdwBlock@aol.com>, lyn romano <rosebush2@hotmail.com>,
Patrick Price <PAPCECST@aol.com>, Timothy Clark <timothyclark@compuserve.com>,
max lenz <hau-if@datacomm.ch>, res gehriger <res.gehriger@sfdrs.srg-ssr.ch>,
Tim van Beveren <tvb1@prodigy.net>, Ader <ader@compuserve.com>,
Bob Rowland <rwroland@aol.com>, "BabsF342@aol.com" <BabsF342@aol.com>,
Kay Pennington <KPennington@prodigy.net>


To All:

FAA's Tom McSweeny has omitted in a public statement the FAA's own
team findings that electrical malfunctions may have caused these
unexplained 737 rudder hardovers. (This is not the first time Mr.
McSweeny's statements have contradicted FAA or industry data.)

This present contradiction can be seen by reading a reference to that
FAA team finding within a document by the Airline Pilots Association
(ALPA) and by reading a McSweeney's most recent FAA statement by
McSweeny on March 11.

Two articles are linked here regarding this ongoing 737 rudder
controversey. The first article is ALPA's submission to the NTSB
regarding U.S. Air flight 427.

View this at http://airlinesafety.com/articles/ALPA2.HTM

Scroll to - section d; "FAA Critical Design Review Team" (CDR)

Scroll to - "A visit to Honeywell/Sperry by a CDR team representative
resulted in two observations:"

Scroll to - " 2. There are a number of failure modes that could cause
the Yaw Damper to command a rudder deflection to the Yaw Damper
authority:

Note the team findings of;

"a. Electrical shorts of grounds,"

"b. Open feedback circuit, and ..........."

The second article, By Tim Dobbyn (Reuters) was posted in Washington,
March 11 at 11:28 PM ET. This article title was; "FAA says Boeing 737 rudder redesign unwise".

Statements, by McSweeny, omit these FAA team findings of any electrical factors that could explain these hardovers.

See this article at

http://www.infobeat.com/stories/cgi/story.cgi?id=2558773429-160

The text of this article and McSweeny's statements also follow.

"The Federal Aviation Administration said on Thursday that a major redesign of the Boeing 737 rudder was unnecessary and could pose risks of its own. The National Transportation Safety Board (NTSB) is considering recommending design changes at a meeting scheduled for later this month on the probable cause of a 1994 USAirways crash near Pittsburgh.
FAA Associate Administrator Tom McSweeny said his agency had already addressed the three most likely scenarios for that crash with modifications to the 737 rudder and pilot training.
"We've looked at the data. We think there are three scenarios that could have caused this accident and we've dealt
with all three of them," McSweeny said. Major rudder changes would force a whole new system into an existing airframe at repair stations around the world.
"I think the risk of unintended consequences is real," McSweeny told reporters.
There are more than 3,000 Boeing Co. 737s in service around the world and about 1,300 of them are registered in the United States. The NTSB can only make recommendations to the FAA, whose rules are usually adopted by other aviation regulators around the world.

USAir Flight 427 was about to land at Pittsburgh International Airport on Sept. 8, 1994, when it rolled sharply and dived into the ground, killing all 132 people on board. Computer simulations of the accident show the plane's rudder, the hinged rear portion of the vertical tail fin, went hard to the left just prior to the crash.
What caused that rudder movement is still the subject of debate. The 737 rudder is also suspected in a 1991 crash of United Airlines Flight 585 near Colorado Springs, Colorado, that killed all 25 people on board.
An Eastwind Airlines 737 in 1996 experienced an in-flight upset but recovered safely. On Feb. 23, a USAir Metrojet experienced an unintended rudder movement while at cruising altitude. The flight diverted safely to Baltimore-Washington International Airport.
McSweeny is not convinced the two accidents and two incidents have a common cause, but he believes the FAA's initiatives since the Pittsburgh crash have sharply reduced the likelihood of two types of unintended rudder movement or pilot error.
A new design of hydraulic power control unit, which all U.S. operators are required to fit by Aug. 4 this year, eliminated the possibility of a mechanical reversal of the rudder controls found in post-accident research, McSweeny said.
A new digital yaw damper system and hydraulic pressure reducer, to be retrofitted to all older 737s by July 2000, will further limit the potential for a major in-flight upset. Yaw is a side-to-side wiggle along the length of the plane and is
suppressed with small rudder inputs, mainly for passenger comfort.
The FAA has encouraged programs to train flight crews how to recover from unusual flight attitudes, but it has not yet made the sessions mandatory.
The NTSB is due to meet on March 23 in Springfield, Virginia, to discuss the USAir Flight 427 crash."

END TEXT-----------
Please circulate to any other interested paties not listed here.

Thanks,

John D. King

othing is as easy as it looks

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Corollary: If there is a worse time for something to go wrong, it will happen then