NTSB and Rudders
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PickyPerkins
The actual shape and intensity of the trailing vortices at a dirty configured slow approach speed is very very different to what is shed at low altitude/250kts clean. And high altitude/high TAS is quite different again. The main variables are weight, speed and wing-loading (but assuming one g) - however obviously configuration's a player also. A 747 wake vortex radius at half a mile astern (plus lateral drift) would be around 50 metres across - and at 5 miles that would be around 150m to 200m. To further define that, here we're talking about the radius beyond which the rotational velocity has dropped off greater than 50% of the current maximum. The mounted photos above show the air (as well defined by the smoke) being entrained (dragged into the outer helix by the strongly rotating core, much as a whirlpool sucks in the outer periphery) - but with the very vital core less noticeable as the black bit in the centre. i.e. You have to realise exactly what it is that you're looking at here. Here it's still very fresh and hardly diffused at all. With that large A300 vertical fin, it would not be difficult to imagine how it could take an instant thwartwise hit on the fin and rudder at up to 60 kts (but not the full 100kts if it was on much the same track - due to the fact that it's still a helical rotation). It would have been the next (second) hit, as the A300 went through the 747's port wing wake (with its rudder highly deflected and loading up the fin) that would have broken the right attachment lug(s).
FDXMech
I don't disagree with WINO. He seems to have nailed it. Be very interesting to see how the FAA/DGAC allows Airbus to sneak out of some very expensive mods or add-ons. I've referred the query to a couple of guys who may or may not get back on this thread (or maybe to me). One of them designed the MD-11 FCS and is currently working on the F-35.
The actual shape and intensity of the trailing vortices at a dirty configured slow approach speed is very very different to what is shed at low altitude/250kts clean. And high altitude/high TAS is quite different again. The main variables are weight, speed and wing-loading (but assuming one g) - however obviously configuration's a player also. A 747 wake vortex radius at half a mile astern (plus lateral drift) would be around 50 metres across - and at 5 miles that would be around 150m to 200m. To further define that, here we're talking about the radius beyond which the rotational velocity has dropped off greater than 50% of the current maximum. The mounted photos above show the air (as well defined by the smoke) being entrained (dragged into the outer helix by the strongly rotating core, much as a whirlpool sucks in the outer periphery) - but with the very vital core less noticeable as the black bit in the centre. i.e. You have to realise exactly what it is that you're looking at here. Here it's still very fresh and hardly diffused at all. With that large A300 vertical fin, it would not be difficult to imagine how it could take an instant thwartwise hit on the fin and rudder at up to 60 kts (but not the full 100kts if it was on much the same track - due to the fact that it's still a helical rotation). It would have been the next (second) hit, as the A300 went through the 747's port wing wake (with its rudder highly deflected and loading up the fin) that would have broken the right attachment lug(s).
FDXMech
I don't disagree with WINO. He seems to have nailed it. Be very interesting to see how the FAA/DGAC allows Airbus to sneak out of some very expensive mods or add-ons. I've referred the query to a couple of guys who may or may not get back on this thread (or maybe to me). One of them designed the MD-11 FCS and is currently working on the F-35.
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Further info from Rainman at this link
EXTRACT ONLY
This 13 Nov 01 incident below may be relevant (happened quite remarkably the day after the AA587 crash).
The question that arises with the A300-600 is how this might have played out in its two yaw-damper system - but with both yaw damper actuators acting upon the A300's single rudder (and accepting follow-through inputs that wouldn't have been the case with the 747's upper and lower rudder setup).
It seems ridiculously clear (to me, at least) that dual physical rudder surfaces are the best way to go._ There was simply no way the DC-10 nor the MD-11 could meet the stringent 10^-9 first catastrophic failure rate requirement for autoland with only one rudder. We clearly needed two.
When FedEx began bringing in the A300-600s, and as I learned about their autoland with only a single rudder, I ALWAYS wondered how they showed they met the numbers....and why the US gives this a bilateral certification for autoland with that single-point rudder failure mechanism?
Might this actually lead to a feedback mechanism and mutual excitation_within the two Y/D systems?
I believe the answer to that is categorically yes, with the empirical evidence available to back it up._ The FedEx hangar event was a manifestation of the subject of a prior AD for oscillations induced thru out-of-phase pressure pulses from two different hydraulic systems that feed the yaw damp actuator._ This has always been part of the smoking gun from my vantage point of this accident._ Here in the hydraulics is the precise mode you are talking about Belgique: Two systems allowed to operate in an out-of-phase condition, which sets-up and possibly excites a critical oscillation._ Bad news.
cont'd at link above
EXTRACT ONLY
This 13 Nov 01 incident below may be relevant (happened quite remarkably the day after the AA587 crash).
FACTUAL INFORMATION
A Boeing 747-SP38 aircraft was maintaining Flight Level (FL) 430 with autopilot `A' engaged, when the aircraft yawed abruptly to the right and rolled to a bank angle of approximately 20 degrees. The autopilot was disengaged and the aircraft stabilised in a straight and level attitude. The uncommanded yaw occurred again. The flight crew broadcast a PAN (radio code indicating uncertainty or alert, not yet the level of a Mayday) and received a descent authorisation to FL380.
The upper rudder position indicator showed a rudder displacement of 5-degrees right and the lower rudder indicator showed zero degrees deflection. The flight crew began activating and de-activating the upper and lower yaw damper switches attempting to isolate the problem. During those actions, the aircraft commenced to `Dutch roll' (lateral oscillations with both rolling and yawing components). The crew then successfully isolated the problem to the upper damper and turned the upper damper switch off. With the aircraft at FL380, normal operations ensued. Autopilot `B' was then engaged and the flight proceeded without further incident.
Investigation by company maintenance personnel confirmed an anomaly of the upper yaw damper computer. The unit was replaced and the system tested. Normal operations ensued.
Analysis of Flight Data Recorder information revealed that during the event the upper rudder displaced 4.7 degrees. The data also indicated that the maximum roll encountered was 13 degrees to the right.
System redundancy had operated as required to limit the effect of the upper yaw damper anomaly.
http://www.atsb.gov.au/aviation/occu...l.cfm?ID=381__
A Boeing 747-SP38 aircraft was maintaining Flight Level (FL) 430 with autopilot `A' engaged, when the aircraft yawed abruptly to the right and rolled to a bank angle of approximately 20 degrees. The autopilot was disengaged and the aircraft stabilised in a straight and level attitude. The uncommanded yaw occurred again. The flight crew broadcast a PAN (radio code indicating uncertainty or alert, not yet the level of a Mayday) and received a descent authorisation to FL380.
The upper rudder position indicator showed a rudder displacement of 5-degrees right and the lower rudder indicator showed zero degrees deflection. The flight crew began activating and de-activating the upper and lower yaw damper switches attempting to isolate the problem. During those actions, the aircraft commenced to `Dutch roll' (lateral oscillations with both rolling and yawing components). The crew then successfully isolated the problem to the upper damper and turned the upper damper switch off. With the aircraft at FL380, normal operations ensued. Autopilot `B' was then engaged and the flight proceeded without further incident.
Investigation by company maintenance personnel confirmed an anomaly of the upper yaw damper computer. The unit was replaced and the system tested. Normal operations ensued.
Analysis of Flight Data Recorder information revealed that during the event the upper rudder displaced 4.7 degrees. The data also indicated that the maximum roll encountered was 13 degrees to the right.
System redundancy had operated as required to limit the effect of the upper yaw damper anomaly.
http://www.atsb.gov.au/aviation/occu...l.cfm?ID=381__
The question that arises with the A300-600 is how this might have played out in its two yaw-damper system - but with both yaw damper actuators acting upon the A300's single rudder (and accepting follow-through inputs that wouldn't have been the case with the 747's upper and lower rudder setup).
It seems ridiculously clear (to me, at least) that dual physical rudder surfaces are the best way to go._ There was simply no way the DC-10 nor the MD-11 could meet the stringent 10^-9 first catastrophic failure rate requirement for autoland with only one rudder. We clearly needed two.
When FedEx began bringing in the A300-600s, and as I learned about their autoland with only a single rudder, I ALWAYS wondered how they showed they met the numbers....and why the US gives this a bilateral certification for autoland with that single-point rudder failure mechanism?
Might this actually lead to a feedback mechanism and mutual excitation_within the two Y/D systems?
I believe the answer to that is categorically yes, with the empirical evidence available to back it up._ The FedEx hangar event was a manifestation of the subject of a prior AD for oscillations induced thru out-of-phase pressure pulses from two different hydraulic systems that feed the yaw damp actuator._ This has always been part of the smoking gun from my vantage point of this accident._ Here in the hydraulics is the precise mode you are talking about Belgique: Two systems allowed to operate in an out-of-phase condition, which sets-up and possibly excites a critical oscillation._ Bad news.
cont'd at link above
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Smoke and mirrors
To: Belgique
No aircraft flying has ever met the certification requirements for catastrophic failure rate of 10-9. However, every aircraft flying under those failure rate requirements has demonstrated on paper that they are able to not only meet those requirements but they could exceed the requirements showing a catastrophic system failure rate of 10-12 all the way to 10-17. The manipulation of numbers and the magic of Boolean Algebra do it all.
Another point to consider is that the specified number of 10-9 is for system failure that could cause death or loss of the aircraft.
If the certification authorities specified that the airframe manufacturer had to demonstrate the catastrophic loss of an aircraft using the magic of Boolean Algebra it could be proved that the aircraft had a catastrophic failure rate not of 10-9 but somewhere around 10-6 or 10-7 which is totally another story. That is why it is not required.
There must be a dead rat around here because I can smell it.
There was simply no way the DC-10 nor the MD-11 could meet the stringent 10^-9 first catastrophic failure rate requirement for autoland with only one rudder. We clearly needed two.
Another point to consider is that the specified number of 10-9 is for system failure that could cause death or loss of the aircraft.
If the certification authorities specified that the airframe manufacturer had to demonstrate the catastrophic loss of an aircraft using the magic of Boolean Algebra it could be proved that the aircraft had a catastrophic failure rate not of 10-9 but somewhere around 10-6 or 10-7 which is totally another story. That is why it is not required.
There must be a dead rat around here because I can smell it.
Last edited by Lu Zuckerman; 2nd Sep 2002 at 22:53.
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======= Start of quote =======
cwatters posted 31st August 2002 What diameter are these rotating vorticies?
======= End of quote =======
On another PPRuNe thread, [Flight Deck Forums > Questions > What exactly causes vapour trails?], 152captain has posted a link to a spectacular photo of
vapour trails.
What is interesting is that the vapor trails from each pair of engines on the 747-200 can be seen spiralling around each other giving some indication of the size of the vortices. Also, the intense core of the port vortice can be seen two or three plane lengths back in the trail from the port outer engine. Another feature is the light condensation mist between the two vortices, presumably caused by the lower pressure over the wing. Great photo.
Browsing around the web, I find that the University of California at Berkeley [UCB] claims [and has patented] a way to dissipate wake turbulence faster than normal.
They say that their wind tunnel tests at UCB show that a single vortex is stable. It just slows down with time and finally fades out. A pair of vortices [as in the case of an aircraft] eventually tend to destabilize each other and fade out more quickly than a single vortex. The proposal is to add a vortex-producing flap near each wingtip which would create one additional vortex near each wingtip. The pairs of votices at each wingtip then mutually destabilize each other. The flap adds some drag and could be retracted in cruise. They tried two slightly different designs. One dissipated trails two to three times faster than normal while another design using larger flaps did so four to eight times faster.
There is an excellent video movie on the UCB web page demonstrating this mutual destabilization in a water tank. The web page says you need a broardband connection to view. However, it worked for me with a 56k dial-up connection, but took over an hour to download the 12MB!
I am not in any way connected with UCB.
cwatters posted 31st August 2002 What diameter are these rotating vorticies?
======= End of quote =======
On another PPRuNe thread, [Flight Deck Forums > Questions > What exactly causes vapour trails?], 152captain has posted a link to a spectacular photo of
vapour trails.
What is interesting is that the vapor trails from each pair of engines on the 747-200 can be seen spiralling around each other giving some indication of the size of the vortices. Also, the intense core of the port vortice can be seen two or three plane lengths back in the trail from the port outer engine. Another feature is the light condensation mist between the two vortices, presumably caused by the lower pressure over the wing. Great photo.
Browsing around the web, I find that the University of California at Berkeley [UCB] claims [and has patented] a way to dissipate wake turbulence faster than normal.
They say that their wind tunnel tests at UCB show that a single vortex is stable. It just slows down with time and finally fades out. A pair of vortices [as in the case of an aircraft] eventually tend to destabilize each other and fade out more quickly than a single vortex. The proposal is to add a vortex-producing flap near each wingtip which would create one additional vortex near each wingtip. The pairs of votices at each wingtip then mutually destabilize each other. The flap adds some drag and could be retracted in cruise. They tried two slightly different designs. One dissipated trails two to three times faster than normal while another design using larger flaps did so four to eight times faster.
There is an excellent video movie on the UCB web page demonstrating this mutual destabilization in a water tank. The web page says you need a broardband connection to view. However, it worked for me with a 56k dial-up connection, but took over an hour to download the 12MB!
I am not in any way connected with UCB.
Last edited by PickyPerkins; 22nd Sep 2002 at 13:51.
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I don't have it to hand so I can't cite the reference, but there was an interesting article in the AeroSoc Journal probably 6-12 months ago which addressed the issues of tip vortex and diameter.
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Belgique:
>>>The question that arises with the A300-600 is how this might have played out in its two yaw-damper system - but with both yaw damper actuators acting upon the A300's single rudder (and accepting follow-through inputs that wouldn't have been the case with the 747's upper and lower rudder setup)................................................Might this actually lead to a feedback mechanism and mutual excitation_within the two Y/D systems?<<<
I don't necessarily think so.
Here's why. The A300-600 yaw damp actuator, a single unit with two hydraulic actuators, each independantly controlled and monitored via its respective FAC (flight augmentation computer) have what Airbus calls a, "hydraulic transparency device".
This "transparency device" allows up to a 40% split in position between #1 and #2 actuators. Above the 40% threshold, the systems would be opposing one another causing both yaw damp paddles to drop. Of course before this threshhold is reached, a failure would (should) be detected in one of the channels allowing only one channel to drop offline.
The yaw damp assembly has two channels, #1 controlled by FAC 1 and #2 channel controlled by FAC 2.
Both actuators are mechanically bused together so both output levers operate in unison. This would at face value validate the possibility of mutual excitation.
But in actuality, only the #1 actuator has a direct mechanical connection to the yaw damp output lever, #2 actuator is actually a hydraulically driven cylinder with a concentric inner piston. The concentric inner piston has the capability of relative movement inside the hydraulically driven actuator and is the component within #2 channel that is mechanically connected to the output levers (and thus #1 channel), this making a "soft" connection between channel 1 & 2.
Airbus refers to channel 1 as the, "driver side", and #2 as "driven side".
>>>The question that arises with the A300-600 is how this might have played out in its two yaw-damper system - but with both yaw damper actuators acting upon the A300's single rudder (and accepting follow-through inputs that wouldn't have been the case with the 747's upper and lower rudder setup)................................................Might this actually lead to a feedback mechanism and mutual excitation_within the two Y/D systems?<<<
I don't necessarily think so.
Here's why. The A300-600 yaw damp actuator, a single unit with two hydraulic actuators, each independantly controlled and monitored via its respective FAC (flight augmentation computer) have what Airbus calls a, "hydraulic transparency device".
This "transparency device" allows up to a 40% split in position between #1 and #2 actuators. Above the 40% threshold, the systems would be opposing one another causing both yaw damp paddles to drop. Of course before this threshhold is reached, a failure would (should) be detected in one of the channels allowing only one channel to drop offline.
The yaw damp assembly has two channels, #1 controlled by FAC 1 and #2 channel controlled by FAC 2.
Both actuators are mechanically bused together so both output levers operate in unison. This would at face value validate the possibility of mutual excitation.
But in actuality, only the #1 actuator has a direct mechanical connection to the yaw damp output lever, #2 actuator is actually a hydraulically driven cylinder with a concentric inner piston. The concentric inner piston has the capability of relative movement inside the hydraulically driven actuator and is the component within #2 channel that is mechanically connected to the output levers (and thus #1 channel), this making a "soft" connection between channel 1 & 2.
Airbus refers to channel 1 as the, "driver side", and #2 as "driven side".
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Soft Connections and concentric valves
FDXMech
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).
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).
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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 untravelled 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.
http://www.atsb.gov.au/aviation/occu...ail.cfm?ID=381
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. http://www.iasa-intl.com/store/chaos.html#westpalmbeach 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).
http://www.iasa.com.au/folders/Safet...587update.html
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.
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 untravelled 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.
http://www.atsb.gov.au/aviation/occu...ail.cfm?ID=381
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. http://www.iasa-intl.com/store/chaos.html#westpalmbeach 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).
http://www.iasa.com.au/folders/Safet...587update.html
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.
