NTSB and Rudders

[knows]

I can't beleive no one has thanked Mr Farley for his easy to understand and valuable informed input!

[Roadtrip]

The NTSB recommendations are a bunch of horse s**t. I used to have respect for the NTSB, but after listening to the woman who heads it, I fear that she's cut from the same cloth as Administrator Garvey. If that crap about not agressively using rudder is the best they can come up with, then they're better off keeping their mouth shut until they find some that they can PROVE.

I've had a hardover rudder on a KC-135 so bad it bounced my head off the side window. Damage? NONE.

I've also flown MANY times minimum interval takeoffs (MITO)during the cold war. 12 secs behind multiple aircraft in wake turbulence you only read about, and you know what?? It's not a big deal. Easily handled by both pilot and aircraft.

[747FOCAL]

ignoramusextremus-Quit talking like you got short mans syndrome. Let them speculate about anything they want. Who really cares. I wouldn't want to fly with anybody who thinks they know everything. <img src="rolleyes.gif" border="0">

[wes_wall]

Well, one thing is always for sure, the lawyers can always determine the cause, well before anyone else.

American and Airbus Sued for NY Crash

"The case was filed by Margarita Del Carmen Montan, the widow of Jose Angel Rosa, who said her husband was forced to endure severe mental anguish and fear of impending death before he was killed in the crash.

The suit alleged that structural, electrical and mechanical systems failures caused or contributed to the crash. "

Comforting isn't it.

[Belgique]

My initial post on page two of this thread gave rise to a number of queries, so it may not have been clear enough. Here's another attempt. Apologies for the length.

A Refined Theory

The NTSB is laying the groundwork for a belief that perhaps the pilots could have encountered the initial wake turbulence and then

laid into it with a corrective boot (for whatever reason) but thus creating a rudder reversal situation (beyond the provisions of

FAR25) that overloaded the fin. Their theory infers that, perhaps when they hit the second lot of wake, they were in a significant

sideslip with a large rudder deflection. And supposedly they didn't do this just once, but went for a heavy pedal-throw five

"recorded" times during and just after that second wake encounter, with as little as 0.3 seconds between.

Well I'll tell you now that you won't have any more than 0.5% of the qualified pilot population ever believing that one. Multi-engine

pilots just don't go for big boots of rudder at 240 knots (1.9Vs) when they encounter wake turbulence (and, as you point out, that

happens quite frequently). Pilots might be more prone to utilise rudder in order to stop a further wing drop at low (approach type)

speeds (1.3Vs). That remains a recommended course of action simply because trying to pick up a "dropped" wing with aileron/spoiler

when near the wing's stalling angle of attack only serves to exacerbate the situation and can lead to autorotation (incipient spin).

So there has to be another answer - and that's probably why the US pilots flying the A300 have the wind up. They know that this facile

NTSB explanation just doesn't wash. Neither does it accord with the A300's reputation as a tail-wagger, nor with the recorded

instances of uncommanded yaw. You should not lightly dismiss either of those factors.

There are system failure explanations that might encompass the cause and I've mentioned my latest theory (the effect of yaw on the

static pressure sensed by the Air Data Computer and predicating allowable rudder deflections - see below). Another avenue of

exploration might be this one:

<a href="http://www.casa.gov.au/avreg/aircraft/ad/adfiles/over/ab3/ab3-088.pdf" target="_blank">http://www.casa.gov.au/avreg/aircraft/ad/adfiles/over/ab3/ab3-088.pdf</a>

AD/AB3/88 Amdt 1

. .Rudder Servo Control Desynchronisation 5/98

The desynchronisation check is to be repeated every 1300 flight hours.

Background: This AD is raised to detect and prevent rudder servo-control desynchronisation which

could lead to structural fatigue and adverse aircraft handling qualities. The AD also

mandates structural inspections to detect the onset of fatigue damage resulting from

servo control de-synchronisation. This amendment recognises changes to Service

Bulletin revision status and incorporates a new compliance clause.

The original issue of this airworthiness directive became effective on 2 January 1997.

Unfortunately I don't have the Service Bulletin nor the inside knowledge of the system, so I cannot judge whether it's a valid

connection or not. I'm not sure whether the rudder input actuator rod found on the FEDEX bird had any significance here - but you

cannot discard that either.

. .The NTSB and FAA have disclosed that a previously FAA-accepted NTSB recommendation on the DFDR was not acted upon and so only the

desensitized data was going onto that thoroughly discredited Loral DFDR (in other words, the same as what the pilots were seeing - and

eliminating all the intermediate data-points). When you consider that aspect, and reflect that as little as a quarter of what went on

down back actually went onto the DFDR, I have no trouble seeing those latter tail-end events as "a rattle". And I certainly then

discard any notion that it was the pilots peddling that fast....because they simply couldn't have done so. But the FCS driven system?

Yes, that can move the rudders at a rapid 39 deg/sec.

So, is it a simple 737-type rudder case where the valve is going to full throw and then sticking? No. Something is causing the A300

rudder to fluctuate back and forth - but not rhythmically (an important point). Well we know that there have been prior instances of

the A300 rudder generating uncommanded yaw, some just annoying - but others generating concern, incident reports and flight aborts. So

why didn't they previously cause a fatal fracture of the fin? Easy, it's all about the forces generated on the fin - by both the

rudder's motion and the side-slip angle at the time. For AA587, already in tail-wag mode when they hit the second wake, the overload

was likely caused by the angular velocity of that wake as it hit a deflected rudder's pre-loaded fin. It is most obviously the

combination of those two aerodynamic loads that (for AA587) proved lethal. That is well covered in

<a href="http://www.iasa-intl.com/folders/Safety_Issues/others/aa587/ruddersnapfinoff.html#dornheim" target="_blank">AW&S T's Dornheim article</a>. But

it's not the whole story - it lacks an acceptable cause for the waggle.

OKAY, what could cause the rudder to waggle, (but not rhythmically) and, in the case of AA587, break the fin? Well the answer possibly

lies in the systemic relationship between the ADC and the FCS. The ADC senses pitot and static pressures and provides the speed input

so that the rudder ratio limiter knows how much of the (physically available 30 deg of) full-throw is permitted at any one time. If

you manage to confuse the ADC so that its input to the rudder limiter (owned and operated by the FCS) is a little haphazard, then

every rudder deflection will be inappropriate. Because it is (and proves to be) an inappropriate response, a further corrective rudder

deflection is called for. It too will be incorrect if the speed derived from the ADC isn't near to the actual speed of the aircraft.

The yawing evolution ends up as an over/under-correction followed by yet another over/under-correction and the end result is dependent

upon some other factors. Firstly, if it's not a significantly inappropriate rudder deflection, the massive dampening effect of that

large fin will provide a countervailing restorative moment and the uncommanded yaw will subside. If it is a largish error, then the

inertia of that fuselage swing also becomes a factor, as does the timing of the FCS inputs and any time-lag in the ADC's sensing of

the aircraft's speed.

So why would the ADC tell fibs (to the FCS and rudder limiter) about the aircraft's speed? It might not be the same reason every time

and I covered that in my initial hypothesis

<a href="http://www.pprune.org/ubb/NonCGI/ultimatebb.php?ubb=get_topic&f=1&t=017746&p=2" target="_blank">here</a> ("just another Theory"- "Help me out

here"). But in AA587's case I'd suggest that the initial wake turbulence encounter was enough to set the ball rolling. Normally, in a

yaw, the fact that the port and starboard static ports are Y-connected is enough to iron out (and average) any discrepancies caused

locally at each port (by the venturi effect of yaw angle, air inflow and fuselage curvature). However the design engineers only ever

consider balanced flight and a few different configurations when they decide upon static port positioning and then map the PEC

(position error correction) errors to be fed into the ADC. In yawed flight I'm suggesting that quite large errors can creep in and be

magnified by both the ADC and FCS sampling rate (i.e. how often these gizmos ask for the speed info). As I pointed out in the earlier

article, about a 9mb error (= an 8% error on an ISA day) can cause the airspeed to zero out. I'm not suggesting that that happened,

just pointing out the small magnitude of P.E. error required to cause a largish speed error and an inappropriate rudder deflection.

Remember the 1996 Birgenair 757 that crashed due to static port tape being used to stop water getting into the static system during an

aircraft wash (not being removed)? Well it's a little-known fact that heavy rain and a falling barometer can allow a static port to

suck in water. It's usually not significant because it normally pools in water traps in the system - and gets drained eventually. In a

heated system it's unlikely to freeze and cause the gross errors mentioned in my earlier article. However just consider what effect an

amount of trapped water might have in the static system - particularly upon the timeliness of ADC sensing, and particularly in a yaw?

The water may well flow in an adverse direction, due to the forces of the yaw and inertia, and induce a false static pressure (which

is then picked up by the ADC etc etc). Do you see where I'm going here?

So if you accept my hypothesis, you will see that a clean/dry static pressure system can induce an error into the FCS - and so can a

wet one, but probably a fiercer reaction in the latter case - because of the adverse flow of trapped water during any yawing. Due to

the unpredictable nature of the water-flow and the ADC sensing, rudder ratio changes and resulting erroneous FCS responses, any yaw

meanderings would not be rhythmic, but they could build up and become divergent and gross. Equally likely, the rudder could become

out-of-phase with the aircraft's physical yawing and tend to assist the damping effect of the fin. Luck of the draw I would say - but

in AA587's case I'd suggest that the second wake turbulence encounter came at just the wrong time in the cycle. It can probably

therefore be seen as a once off type accident. But if I am right, then there are things that can be done to resolve any such

discrepancies in the Airbus system's logic and susceptibility to this type of error.

In answer to the question (at the end).

Pitot Pressure (as sensed by a pitot tube facing into the airflow) is a Total Pressure made up of dynamic and static pressure: T =

D + S1. . . .So obviously we need to subtract that S1 (the static pressure – which should be the ambient pressure at that height), because what we

want to see on our Air Speed Indicator is D (dynamic pressure or the pressure due to our forward speed). The formula now becomes D = T

– S1 (a conventional airspeed indicator does this within its internal plumbing but in a glass ship the Air Data Computer obliges).. . . .But where do we source that S1? Well it comes from the static ports (let’s call it S2)…. which live on the port and starboard sides of

the airplane’s aft fuselage (normally two small holes each side / above and below, which are heated against icing but which are open

to the elements inflight and quite often left open and unplugged on the ground). If you park an airplane in the open and there’s heavy

rain and it’s flowing down over those holes then capillary action can cause water to be sucked in, sometimes in fairly large

quantities. I can recall an inflight emergency where I lost all pressure instruments after climbing through freezing level. They

figured out later that, parked in the rain, water had been sucked up the hollow centres of the downward-facing rubber bungs (off which

the water was dripping). When it later froze in the static system, of course I lost the altimeter (it froze at that height), the VSI

went to zero and the Air Speed indicator just wound back to zero (from the 220knot climb speed). It was calculated later that that

will happen (from that IAS) over a climbing height change of 2800feet (about = 9mbs or 8% of the Sea-level ISA pressure). The school

solution is to depressurize and break the glass on the VSI and accept that there will be a fairly gross altitude error (due to using

cockpit static). But that gets your ASI back in the picture (although greatly errored, trends will be good).. . . .Now in an ideal world S2 will always be equal to S1. But we don’t live in an ideal world so there will always be some discrepancies:. . . .a. When we lower flap and/or change AoA, it tends to speed up the airflow over the wing and down past the static ports - so the

venturi effect drops the pressure there and a correction is required (to the speed that we fly). . .D = T – S2 becomes an errored speed indication because it’s now a greater quantity than D = T – S1.. . . .b. If we yaw the aircraft, the fact that the port and starboard static ports are interconnected via Y junctions should stop

the error being of any great magnitude (a higher pressure on one side being in theory cancelled out by a lower pressure on the other).

But if we go into a rapidly yawing mode through a significant angle, then all bets are off and the pitot-static system will be

struggling to provide valid data to those flight control systems with a high sampling rate and requiring accurate pick-offs of static

pressure (and consequently any derived speed will be fluctuating). Note here that I said pitot-static systems – because the pitot is

also yawing away and so it’s also subject to some error). Navigation systems and pressure altimeters aren’t as vulnerable to sensed

errors (resulting in momentary speed discrepancies) as would be the flight-control systems (yaw damper and rudder limiter inputs say). . . . .c. And if we have water trapped in that system and start yawing, then obviously we have to examine the effect of the inertia

of that water on the sensed static pressure (S2). Over a period of (say) five seconds, we might measure (but never see) S2 ± 5mbs

variation. Depending upon the sampling rate of the ADC it may not even record this on the slow-sampling DFDR – but it may provide that

instantaneously erroneous data to the rudder limiter and yaw damper…. With eventual AA587 uncommanded gross yaw type results. . . . .d. Additionally, if we fly into a wake turbulence encounter, we might accept that a wake vortex has within it some wide

variations of pressure. These can vary from “almost a vacuum” to much greater than ambient pressure at that height. In addition the

angular velocities of the encountered (greatly disturbed) rotating wake vortex airflows may mean that there are components of the wake

at an angle to the static port (on one or BOTH sides of the airplane) – and those ports can take in that air pressure much as a pitot

tube does (leading to even greater errors and flux). . . . .So it is starting to look like a real pakapoo ticket is it not? The business of getting a true S2 just became tantamount to impossible

when we entered that first wake and began the tail-wag. When we entered it the second time, it was with a thoroughly confused system

that was doing the wild thing with the rudder and, as luck would have it, the second encounter just happened to be at the worst

possible angle and at the worst possible time. The rudder was already imposing a high load upon the fin and suddenly the additional

load snapped an attachment lug and started the fin “working”. As the fin started its lateral dance of detachment, the rudder was

excitated into a frenzy of fin-swaying corrections, leading to the death-rattle heard on the CVR.. . . .Addressing what you’ve asked below:. .“The static pressure drops to zero” isn’t really the case at all. What happens when water freezes in the static system is that the

ambient pressure at that height is trapped and displayed (and that’s why the altimeter freezes at that height, even though the

aircraft continues to climb). 2800ft later, the altimeter reads the same height - but the subtraction of that greater S2 (trapped

pressure) causes the Air-speed indicator to wind back to zero. Over that 2800ft of climb the ambient pressure drops off about 9mbs

(roughly 3mbs Hg/1000ft). So if there was to be a momentary 9mb drop in the S2 (for any reason including the adverse flow of trapped

static system water) at a speed of 220kts, the sensed speed would be nil, zero, zip. Now obviously that’s never the case because it’s

a very dynamic situation we’re talking about here – but it does give you an idea of the magnitude of the pressure errors as sensed at

the static ports – which could drastically influence the airspeeds being fed into the flight-control system. The speeds sensed could

be at any one point of time either above or below the actual aircraft speed – thus causing the continually inappropriate FCS-dictated

rudder responses (in reply to the gyro-detected yawing moments). And there’s the rub.

Question: I don't quite understand one thing in your "Just another theory". It deals with pitot tubes and static ports,

which is to say dynamic and static pressure. If static pressure drops to zero, why would the speed reading decrease? If the speed is

dynamic pressure minus static pressure, then lower static pressure would suggest a greater speed to me than a lesser one. At greater

perceived speed, the rudder would be more (not less) limited in its arc of travel.

<a href="http://www.iasa-intl.com/folders/Safety_Issues/others/aa587/ruddersnapfinoff.html" target="_blank">a link to the theory in toto</a>

[ojay]

Certainly exploring the pprune envelope on this topic.An absolutely superb analysis from a very distinguished ex-test pilot followed by the usual embittered rantings. My ha'porth on the subject of rudders and jet transports is never touch them except for engine failures and crosswind landings.Over and out.

[ojay]

And by the way as a postscript to my previous post I certainly do not believe that experienced AA pilots would be applying large inputs of rudder in normal flight ops/rgds.

[411A]

....don't be soooo sure, ojay, many of these guys were not around when "dutch roll" was a problem...

[RatherBeFlying]

Further to Belgique's post, if yaw is sensed with a vane (or perhaps differential pressure between the static ports), entering and exiting different flow regions within a wake vortex will likely feed fluctuating and misleading data to the yaw sensing system, resulting in confused yaw damper outputs.

The resulting aircraft response could compound the confusion.

If yaw in the A300 is sensed from ring gyros or other INS, then this theory goes out the window.

Obviously yaw damper and rudder programs need to integrate correctly sensed yaw as well as airspeed to limit rudder movements within the fin strength.

Once Airbus, NTSB and BEA have determined and made the necessary revisions, they should fly an updated A300 through the same flight profile to show that the problem truly has been fixed.

[Belgique]

RatherBeFlying

Think you've misunderstood. Not talking about yaw being sensed by a vane - that's normally a gyro function. The yaw that was sensed was probably quite correct - it was the corrective rudder inputs that may have been continually inappropriate. Why?

What I'm looking at is the airspeed inputs to the yaw dampers and rudder limiters being in error. The reason why they can be in error is fairly clearly explained in that post - but to

summarize:. .Imagine that there's water trapped within that static system and you enter the first wake encounter induced yaw cycle. Water has inertia and will be affected by the yawing.

Flowing adversely (and even out-of-phase) within the static system, the instantaneous pick-offs by the ADC are going to lead to inappropriate rudder input corrections - simply because the sensed airspeed will be wrong (and thus the rudder ratio limiter is mis-set). Because the airspeed derived from a false static pressure can be quite grossly in error (example given) you will have a series of non-rhythmic corrective yaw inputs that will each prove to be "unfinished business". A minor yaw disturbance will result in a diminishing yaw cycle - because of the overpowering damping influence of the vertical fin (which is why the fin is there in the first place). However a larger initial yaw might result in a divergent (or at least self-sustaining) oscillation.

I'm suggesting that this is what has happened with AA587. Unfortunately that cycle was interrupted at just the wrong time (and angle) by the second wake encounter - and that's what broke an attachment lug. After that first lug broke, the fin was soon rocking laterally and that was the death-rattle heard on the CVR (as the rudder was driven into a frenzy trying to compensate for the fin's lateral rocking as it danced its Detachment Dance).

The bent/broken FEDEX rudder input actuator rod. .The event they had in their hangar was related to the hydraulic synchronization issue (hydraulic pressure pulses from different sources getting out of phase-sync). The mechanic who initially heard the "banging" did say it was oscillatory in nature, although there was no one watching the rudder at the time. Remember that only one actuator is moved by the

yaw damper. If it was this one that’s bent? Well then it’s just about case solved. I would then submit that there might have possibly been a battle between a rudder limiter that had run amuck (courtesy of the ADC’s flawed pressure inputs) and a yaw damper that was properly doing its job, as it saw it.

As these threads are very perishable, a page of informed surmise will be maintained <a href="http://www.iasa-intl.com/folders/Safety_Issues/others/aa587/ruddersnapfinoff.html" target="_blank">here</a> (with links of interest at the bottom).. .Feel free to contribute.

[RatherBeFlying]

Belgique,

If yaw on the A300 is sensed by gyro, that would eliminate my last theory/guess, but I would like to get confirmation from somebody with specific A300 knowledge before dropping that line of inquiry.

As to the possibility of incorrect airspeeds to the rudder limiter because of static system disturbances, I am so far disinclined that that was material in this case because the recorded rudder movements were within or close to the limits for the speed. However it may be possible that flight data recorder filtering censored beyond limit rudder movements or that the rudder sensing unit may have refused to signal more than allowed by the limiter.

Somebody's gonna have to fly a properly instrumented A300 through the accident flight profile, preferably under remote control -- and it might take more than one A300 to understand what really happened and verify the fix.

[spinproof]

...sorry to be dragging this out, but are we talking about those two pedal thingeys that when you step on the one on the right the plane turns right, and when you step on the one on the left the plane turns left???. .OK then..we're not suppose to do that anymore? <img src="confused.gif" border="0"> <img src="confused.gif" border="0"> <img src="confused.gif" border="0">

[RatherBeFlying]

On further thought, it looks like I've been garbling my nomenclature; so for the record:

yaw rotation about the normal/vertical axis,

sideslip angle airflow makes with the longitudinal axis.

So the yaw damper and/or rudder program(s) yaw the a/c to zero out sideslip.

And sideslip pretty much has to be sensed by an airflow sensor, be it a vane or a differential pressure between static ports or other opposite side ports in the fuselage or fin (A sensor using a classic ball would surprise me, but I'm not an instrumentation engineer).

As different flow regions are entered and exited within a vortex, the sideslip sensor could be presenting highly variable and possibly misleading sideslip conditions to the rudder program which can result in the yaw damper amplifying the condition rather than dampening it. And possibly it could get caught in FCSIO, the computer equivalent of PIO.

Now if your a/c suddenly yaws heavily, you may likely put in full opposite rudder, but don't forget the NTSB has just published that so doing may break it off <img src="frown.gif" border="0">

You may prefer to turn off the yaw damper and any other automatic rudder control immediately upon entering a wake vortex.

[Few Cloudy]

That could be dangerous advice RBF.

[RatherBeFlying]

Few Cloudy -- My suggestion to turn off the yaw damper and automatic rudder control upon entering a wake vortex [in an A300] is based on my evolving suspicion that the A300 FCS in its confusion overreacted and subsequently overstressed the fin and rudder. I suspect that this accident would not have happened if the rudder only moved in response to pilot inputs.

Certainly, if turning off the A300 yaw damper and automatic rudder control inside a wake vortex would create other hazards, I would very much like to understand how that would happen.

[wsherif1]

@#$%&, Forget the Rudder!!!

The pilots never had a chance to induce any effective recovery control inputs! The clockwise rotating vortices off the left wing tip of the B747 struck the vertical tail surfaces, fin and the rudder, BROADSIDE, at a force of 200 + mph, which induced an instantaneous left yaw. This abrupt yaw motion initiated a left Dutch roll. An eye witness statement, "The right wing was perpendicular to the ground"!

The instantaneous left yaw severed both engines from the structure and accelerated them directionally, the right engine was tossed to the left of the aircraft's track and the left engine to the right of the track.

The instantaneous left yaw presented the right side of the fin and the rudder to the slipstream plus the force of the vortices. These abrupt opposite direction bending forces exceeded the ultimate load factor of the surfaces and the rudder was torn off the structure while the fin was sheared off just above the lug connections.

The rudder movements were a result of the forces of the B747's vortices striking the rudder. These lateral forces began at a value of 0.1G through 0.3 and 0.4G to a final value of 0.8G as the aircraft encountered the fringes of the vortex and then closed on the core.

At one point the Captain exclaimed "wake turbulence". Pilots sure as hell know when we have encountered wake turbulence!
To intimate that he was mistaken is a disservice to the crew and the profession.

There is some question about an early T/O release of AA 587 behind the B747 and also the routing directly in trail of the preceding traffic.

In any case there is no way this accident was in any way connected with pilot error!

[Belgique]

The Importance of that "Old One/Two" Lethal Combination

Both wsherif1 and RBF have good points.
wsherif1 however seems to be missing the fact that the A300 received two wake “hits” - and the real damage was likely to have been done by the second - and for sound reasons.

Unfortunately at the time of that second “hit” the FCs was still responding to the yaw upset caused by the first. It’s like that old pugilistic “one-two” punch set. The first fist (the “right”) puts him off balance and the second (“left uppercut”) follows through at just the correct moment, catching the punch-drunk fighter dazed, off-balance and off-guard. We know that the A300 has a factual and anecdotal history of tail-wagging. However unless that involves external forces and reaches extreme amplitudes or gets nastily out-of-phase, the outcome is simply “noteworthy”, although discomfiting. In AA587’s case the characteristics of the FCS corrective rudder displacement) and the natural dampening stability of that large vertical fin meant that the amount, rate (and direction) of side-slip at the instant of that second hit was wholly adverse - and enough to break the fin off. No need to involve any pilot inputs here, in fact because of the tight timings it’s unwise to do so. But that’s not to say that attempted pilot intervention could NOT have happened. If I was sitting in the seat as PF and felt that the FCS had lost the bubble, I’d be attempting to stop the yaw nonsense (for pax comfort). That doesn’t mean that I’d be laying on a bootful. To the contrary, I’d be tromping on both pedals equally and simultaneously and trying to dampen out oscillations related to any excess pedal travel - in the hope that natural damping would stop the yawing. That’s just about instinctive for any pilot.

If the FCS has a built-in software (or CADC induced) flaw that causes any restorative rudder inputs to overtravel inappropriately, then that would explain both its reputation for tail-wagging and the AA587 outcome. Why so for AA587? Well think of it as a wake-induced yaw that went “a Bridge too Far” and entered dangerous, normally untravelled, territory. Let’s say that, after the first wake encounter, at that IAS a 5 degree rudder displacement would have been appropriate to assist the fin’s natural yaw dampening. But for some (programmed) reason the FCS fed in 10 degrees and at a higher rate than might have been ideal. The end result is an overshooting instability and an even greater amount of slip or skid out the other side - at which point the second wake “hit” occurred. For FCS failure modes see “servo control desynchronisation” and “yaw inertia” in my earlier posts on this thread (pg3).

If it had been another type aircraft (with a less rigid metal fin and rudder incorporating some flexing "give") the dynamics of the side-stresses could have been soaked up by the structure - and possibly without any permanent deformations. But it was an A300 and its vertical fin attachment is via a set of six composite attachment lugs dissimilarly mated to metal brackets on the fuselage hull. If it had been a metal-to-metal attachment and the metal upper attachment lug-set had been firmly part of a fin-embedded metal structure, then the shear of an excessive side-force may not have caused the failures in tension seen just above those composite lugs (it's called "load-sharing"). So it’s back to the composite engineers’ admissions that the composite matrix lay-up is necessarily set so as to give maximum strength in certain force directions - unlike metal. The first wake “hit” engaged an essentially yaw-static airplane. But when the second wake-hit occurred, at just the wrong moment, it engaged with some opposing FCS-induced yaw-dynamics. That combination was dynamic enough to exceed the fin’s ability to withstand lateral stresses and an attachment failure occurred. In fact it probably exceeded ultimate design load and any inherent weakness due to manufacture, repair or prior incident damage would have been minimal factors. In other words any A300 might have suffered a similar failure in the circumstances - and that’s the big worry. The one disquieting aspect of structural composites (as against metal) is that they need a continued 100% integrity. In other words, once you have a composite failure affecting load-bearing integrity, it’s likely to become progressive. The other anxieties about whether or not a visual inspection is sufficient to detect significant delamination or disbonding and the effects of resin aging and moisture ingress? They just add to the disquiet.

My spies tell me that all concerned in the investigation are now leaning heavily towards pilot intervention as being a satisfactory explanation for AA587. Even those who don’t believe that explanation remain convinced that it was an unholy stroke of bad luck that caused all those factors to come together and fail AA587’s fin. Be that as it may, I myself will be thinking twice about traveling the structural composite routes. Catastrophic failure is not supposed to occur, no matter how fast a pilot may (or may not) have been pedaling his wares.

[apfds]

Some of the comments made worry me somewhat;

1. On modern swept wing aircraft it is taught to use aileron to pick up a wing,rudder should not be used. Ailerons are effective into the stall.

2.It is not full deflection of rudder that causes the damage ,it is sudden control reversal.

3. I think that 411 has hit the nail on the head, training is probably at fault and a whole generation of pilots are suffering under the misapprehension that rudder travel limiters will stop you overstressing it.

[Wino]

Now wsherif1...
Since you have access to the APA website you should KNOW that the .8 G loading was after the tail departed and the airplane was going through the airsideways shedding large parts. The other G loadings that you list are not enough to even require a write up under the new AD against the tail.

If you had been following along you would also know that there is a problem with the rudder load limiter on the A300.

Just in recurrent 2 weeks ago and doing a rudder demonstration at 250kts I knocked the sim off its jacks even knowing what was going to happen I couldn't prevent it. The only way you can use the rudder at all in the A300 once it gets going fast is to push with the opposit foot at the same time. The breakout force is higher than the force required to reach the stop. It is guaranteed to cause a POI.



Belgique...

The A300-605R uses spoilers for Roll. There is no out board aileron on the A300-605 though there is on the A300b4. And no matter what you think, you cannon RAISE a wing with spoilers. Ask the B52 pilot that flew into the ground during the airshow.

The fact that it is impossible to apply coorinated rudder at high speeds on the A300-605 is the problem. The B4 had a ratio type limiter. the 600 series has a very poorly done blocking type system.

Cheers
Wino

[411A]

Wino

Are you saying that the A300-605 has a deficient rudder control/limit design? I would have thought after all these years that designers could get it right from the get-go.