In Days of Yaw

Still just another number

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Soft Connections and concentric valves

Your info on the two A300-600 yaw dampers servo valve arrangement is interestingly reminiscent of that 737 rudder's concentric dual pistoned valve….the one that afforded the only notional level of rudder redundancy for that aircraft (until it suddenly didn't on a number of occasions, and as a result is now scheduled to be replaced).

Try and picture an A300’s yaw dampers #1 and #2 at odds with each other and acting via the same concentrically pistoned valve actuator. Sounds mightily suspiciously potential to me.

The relevant part of the Byron Acohido Pulitzer-Prize-winning articles on the 737 rudder PCU is to be found here and another here.

You might agree that (in parts) its applicability to AA587 is a bit eerily familiar. The role that dirty hyd fluid played in those various 737 incidents is not well known. Perhaps that’s a reason why a “soft connection” can suddenly become a hard (same-same hardware mechanical) connection...... and provide non-design feedback from one yaw-damper to the other (or whatever other FCS feedback mechanism then becomes available in the rudder or rudder limiter circuit).

posted 3rd October 2002 23:07     Click Here to See the Profile for Belgique   Click here to Send Belgique a Private Message     Edit/Delete Message  

Just to Clarify

I was asked by a few people to elucidate whatever theory might have been behind the vague comment above (that was in response to FDXMech’s info). My reply may be of interest.

Think in terms of two computers driving a singular servo-valve actuator (only one of the three A300 rudder actuators is connected to the yaw-dampers and that actuator can drive the rudder at 39deg/sec). One yaw damper drives the actuator mechanically and the other soft-drives it by virtue of something akin to a shock-absorber strut or oleo (that is concentric to that same singular primary actuator valve) (i.e. soft =with a 40% flex), Now develop a theory whereby the "soft-drive" suddenly becomes a hard connection because its travel seizes under the angular displacement of a rudder at near to full throw (or, due to stiction, is acting very slowly). The other system's yaw-damper computer sees a bad result (and takes over - as it's designed to do). The soft-drive connection suddenly frees itself and its computer wants to argue about who has precedence. The rudder actuator returns full-stroke and the soft-drive again seizes. etc etc

Get the idea? In an ideal world the "failed" yaw damp system would trip and hand over. My guess is that it doesn't do that in conditions of partial failure when the actuator goes well outside NORMAL travel (such as in a wake event). i.e. it has probably been allowed some latitude (by design) because yaw-damping can be a rather dynamic event. Now also think about an actuator that spends 99.99% of its existence near motionless over a very limited travel zone. It's going to have a particular wear characteristic, yes? But suddenly there's a wake event and the yaw-damper sends it into the Twilight Zone of Travel for that particular actuator. It's now well outside its "comfort zone" of wear and starts twitching, sticking or seizing (or even suffers an internal hyd leak) - perhaps due to encountering corrosion in the untraveled zone.

This might also explain the anecdotal (and incidental) incidence of mild to severe tail-wagging on the A300.

It was a similar mechanism (with the rudder power control unit) that led to the 737 rudder hard-overs. However, before reading further, you should read the 747 SP yaw-damper event narrative at this URL first.
The dismissive comment in that incident report is very telling when you think of AA587. It says:
"System redundancy had operated as required to limit the effect of the upper yaw damper anomaly"

(Well, that's very true for the 747’s split rudder system! The A300 however doesn't have a split rudder AND its yaw dampers operate through a singular dual-action valve). It ain't got no real redundancy at all. And just keep remembering that AA587 had a yaw-damper fail its checkout on pre-start at JFK - and required a maint reset. Do you think it might have been trying to tell them something? Commercial pressures notwithstanding, a failure of a system in my opinion requires more than giving it a quick kick in the guts reset and telling it to work (or else). A pessimist might think: "Jeez, that's failed its checkout and that's a warning that everything might not be mechanically right. We've now convinced it to reset but......well what might’ve caused it to fail initially?"

What else might crank up a yaw damper disagreement? Well most (all?) of these URL links below addresses situations where air data computers, fluctuating airspeed and uncommanded yawing all played a part. In AA587 it was flying in amongst the pressure-spike sharp discontinuities of a wake encounter. At West Palm Beach it was the pressure variations and static port asymmetry that you’d encounter in an inadvertent stall - when the copilot then (tellingly) tried to recover using full rudder.

A. May 97 West Palm Beach.” As the plane's nose pitched up 12 degrees and the bank angle exceeded 50 degrees, the first officer applied full left rudder to correct the roll, NTSB determined.”

B. 25 Nov 2001 - Singapore Airlines A340-300 incident [departing Singapore for Dhaka] (problems with airspeed indicators, overspeed warnings and large rudder movements without pilot input). Post-flight inspection revealed problems with the pitot-static connections to the Air Data Computers (CADC).

C. 17 May 99 - American Airlines A300 experienced uncommanded yaw (crew being unable to control the rudder with their pedals). Problem was attributed to the autopilot.

D. 26 Jul 00 - Kansas City (FEDEX A310-203). N409FE returned shortly after take-off due to overspeed warning actuation followed by rudder system 1 and 2 fault lights illuminating. Maint replaced #2 Air Data Computer and a/c checked out serviceable.

E. 05 Nov 99 - Miami (American A300B4-605R) Operator reported erratic rudder movements during a manual approach, caused by a "double fault" with the autopilot yaw actuator. After replacement of the yaw actuator the system functioned normally. NTSB MIA97LA161

F. An American A300-600 crew departing Lima, Peru, reported "fish tailing" soon after the plane took off, an NTSB report said. The aircraft, N7055A returned to Lima and made an uneventful landing.

G. 26 Jul 2000 Fedex A310 (N409FE) – overspeed warning followed by rudder system 1 and 2 fault, returned to Kansas City. Replacement of air data computer #2 fixed fault. CE20001AC071

H. The FEDEX A300 that broke a rudder actuator (the yaw-damp connected one) in the hangar whilst undergoing maint checks.

So IMHO, the tail didn't fall off - it got blown off. That's where the "kid's swing" analogy comes in. A little rudder repetitively at just the wrong time can build the restorative dampening up to quite a yawing gyration. I used to do it to great extremes at the Jet FTS on my graduation parade aero shows. The final pass was inverted yawing of up to 30 degrees nose left and right. You might have seen it done at air displays. That simply requires in-phase rudder-kicks to build the amplitude.

Once the yawing was underway, it only needed the rudder to be significantly deflected whilst the fin was highly loaded and its ultimate load could be exceeded.

I'd be surprised if something similar to this theory doesn't rate a mention by the NTSB at their public docket show later this month.

As the printed NTSB report stated, the number two pitch trim and the yaw damper would not engage. The corresponding CB was cycled and all was back on line and cleared by line maint..
The cause may have never really been fixed. The yaw damper as well as the pitch trims both serve the FAC, at least on the 310, and there may have been the problem.
In my 20+ years experience in computer technology, I have never been confident in recycling circuit breakers to fix a problem. At best, this process usually masks an intermittent problem that almost always returns. I do not understand why the airline industry, especially in today's electronics dependant aircraft, continue to endorse this process as valid for fixing problems.


Reliable Redundancy


Quite right. And the "cleared" problem need not have been electrical. It might have been, in the case of AA587, mechanical and related to the 3rd servo-actuator (the one that serves up the output for both yaw dampers). Yaw damper actuators spend 99.999% of their time moving through a very limited travel and so probably pick up quite distinctive wear patterns. Perhaps it's only at "those other times" that travel outside their normal comfort zone detects a bit of stiction (or dirty hyd fluid or corrosion/erosion) and causes the FAC to hiccup. Those other times? Pre-start BITE checks and the dynamism of wake encounters or CAT.

Fascinating design arrangement that one. A soft-drive (#2YD) and a hard-drive (#1YD) act through a singular servo-actuator valve. Hardly fail-safe or fail-operational if a momentary intermittency in one causes the system NOT to trip, but to cycle between the two yaw damper outputs. I wonder if that might induce rudder oscillations?

During the NTSB's Public Docket (just finished) they very vaguely referred to a failure mode of those three rudder hyd actuators as "force-fighting" and reassured everyone that because of that FEDEX hangar failure of an actuator they'd implemented a 1300 hour synchronization check. Now that's about as reassuring as Firestone introducing a 10,000 mile safety check on their quality radial tyres. It doesn't make that failure mode evaporate. Until force-fighting was mentioned, I thought that that possibility might just have been a figment of my imagination.

Also interesting that the Airbus chappie gave, as a reason for having three rudder actuators connected to a single rudder panel, the logic that "how else would you provide for each of the 3 hyd systems to be available and cover a dual failure (in the uncontained engine failure case)". But no-one seemed to think it relevant to query whether it constituted "reasonable redundancy". That term was introduced by Dr Loeb in the NTSB Public Docket on the 737 rudder hard-overs. Eventually Boeing has had to introduce a 737 fix. I personally find much similarity between the 737 rudder valve's shenanigans and the misconduct of the A300-600's rudder over the many incidents culminating in AA587. The similarity of the 737's actuator and the dual acting A300 yaw damper servo actuator is very striking. I'm suggesting that it may be the quintessential Achilles' Heel.

Below (in a few links) is the story of two 747 uncommanded yaw incidents. Just reflect upon a similar malfunction involving NOT the life-saving split panel rudder arrangement in the 747 incidents, but the single panel scenario - with a number of FAC driven authorities fighting for rudder command and control (and perhaps passing the ball between them). The dismissive comment in the 747SP incident report is very telling when you think of AA587. It says:
"System redundancy had operated as required to limit the effect of the upper yaw damper anomaly" Here they are referring to the split rudder on that a/c.

A few links
one     747-400 uncommanded full rudder

two   747 full rudder (Anchorage Emergency)

three   QANTAS 747SP rudder incident

four (the final two posts on that page are above)

Some Relevant Links

  Flt AA903 - The West Palm Beach Incident
 Panel 408VU and A300 Uncommanded Yaw Events   Some A300 Autopilot Discrepancies
  The Rudder Trim Switch & Uncommanded Yaw   The Rudder Trim Knob AD
  The CROW Hammer Wake Vortex Effect   Singapore and Peru Events Also