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  • #16
    Evan, if for any unexpected reason the airplane suddenly, briskly and intensely largely yaws to the left, what do you think should be the pilot's reaction?

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

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    • #17
      Originally posted by Evan View Post
      Again, Gabriel, a full defection in one direction is not a structural problem.
      Well, if that full rudder deflection in one direction happens after the plane is already in a sideslip and in the direction needed to correct the sideslip, that is beyond the FARs requirement.

      So is the pilot supposed to use the rudder only to induce sideslip, not to correct it?

      --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
      --- Defend what you say with arguments, not by imposing your credentials ---

      Comment


      • #18
        Originally posted by Evan View Post
        Again, Gabriel, a full defection in one direction is not a structural problem.
        Well, if that full rudder deflection in one direction happens after the plane is already in a sideslip and in the direction needed to correct the sideslip, that is beyond the FARs structural requirements.

        So is the pilot supposed to use the rudder only to induce sideslip, not to correct it?

        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
        --- Defend what you say with arguments, not by imposing your credentials ---

        Comment


        • #19
          Originally posted by Gabriel View Post
          Well, if that full rudder deflection in one direction happens after the plane is already in a sideslip and in the direction needed to correct the sideslip, that is beyond the FARs structural requirements.

          So is the pilot supposed to use the rudder only to induce sideslip, not to correct it?
          Seriously Gabriel, are you trolling me now too? Of course the rudder is available to correct sideslip AS I SAID ABOVE. And it is completely safe to do so UNLESS it is used for RAPID RUDDER REVERSALS to extreme loads.

          Why am I the only one who actually does any research around here? Have you read the report?

          As you know, there are limit loads and then there are ultimate loads. The ultimate loads for certifications requirements are 1.5x the limit loads. When the rudder lug failed on AA587, the load was OVER 2X the limit load. Do you realize this? What Boeing rudder can confidently withstand >2X the limit load?!!
          (a) Maneuvering . At speeds from V MC [minimum control speed] to V A [design maneuvering speed], the following maneuvers must be considered. In computing the tail loads, [54] the yawing velocity may be assumed to be zero: (1) With the airplane in unaccelerated flight at zero yaw, it is assumed that the rudder control is suddenly displaced to the maximum deflection, as limited by the control stops, or by a 300 lb. rudder pedal force, whichever is less. (2) With the rudder deflected as specified in subparagraph (1) of this paragraph, it is assumed that the airplane yaws to the resulting sideslip angle. (3) With the airplane yawed to the static sideslip angle corresponding to the rudder deflection specified in subparagraph (1) of this paragraph, it is assumed that the rudder is returned to neutral.

          (b) Lateral gusts . The airplane is assumed to encounter derived gusts normal to the plane of symmetry while in unaccelerated flight. The derived gusts and airplane speeds corresponding to conditions B’ through J’ (in § 25.333(c)) (as determined by §§ 25.341 and 25.345 (a)(2) or 25.345 (c)(2)) must be investigated. The shape of the gust must be as specified in § 25.341. In the absence of a rational investigation of the airplane’s response to a gust, the gust loading on the vertical tail surface must be computed [according to a specific equation].

          Section 25.351(a) does not require a return of the rudder from the overswing sideslip angle to neutral or a full rudder movement in one direction followed by a movement in the opposite direction.

          The other point I keep making but is not penetrating the brain-barrier here is about the use of rudder. In almost any upset situation, unless at an AoA where ailerons become ineffective, rudder input is not required on large aircraft. The Airplane Upset Recovery Training Aid created though a joint consortium in 1996, including Airbus, Boeing, McD, American, United, Delta, the FAA and the pilot's unions added the following language after the AA587 investigation was complete:
          It is important to guard against control reversals. There is no situation that will require rapid full-scale control deflections from one side to the other. The rudders on modern jet transport airplanes are sized to counter the yawing moment associated with an engine failure at very low takeoff speeds and to ensure yaw control throughout the flight envelope, using up to maximum pedal input. This very powerful rudder is also capable of generating large sideslips. An inappropriate rudder input can produce a large sideslip angle, which will generate a large rolling moment that requires significant lateral control input to stop the airplane from rolling. The rudder should not normally be used to induce roll through sideslip because the transient sideslip can induce very rapid roll rates with significant time delay. The combination of rapid roll rates and time delay can startle the pilot, which in turn can cause the pilot to overreact in the opposite direction. The overreaction can induce abrupt yawing moments and violent out of phase roll rates, which can lead to successive cyclic rudder deflections, known as rudder reversals. Large aggressive control reversals can lead to loads that can exceed structural design limits.

          That is NOT specific to Airbus. But don't listen to me. Read the report: http://www.ntsb.gov/investigations/A...ts/AAR0404.pdf

          Comment


          • #20
            [QUOTE=Evan;636670]Seriously Gabriel, are you trolling me now too? Of course the rudder is available to correct sideslip AS I SAID ABOVE.[quote]
            I swear I am not trolling. If you said that using rudder to correct sideslip is an expected pilot response, sorry, I missed it, but feel free to quote yourself.
            What I noted is you saying "The rudder on a large transport aircraft is there for turn coordination (which often requires no pilot inputs) [and what would be preventing sideslip], moderate sideslip manuevers [what would be inducing sideslip] and thrust assymetry [which would be preventing sideslip].", and none of these are "correcting sideslip".

            But again, whether you had already said or not, no problem. If I missed it before, now I clearly see that you think it is ok, that's enough and we agree.

            Now...

            And it is completely safe to do so UNLESS it is used for RAPID RUDDER REVERSALS to extreme loads.
            If it is so, it is because of the manufacturers proactively self-establishing requirements that are beyond what the FAA requests.
            As you can see in the part that you quoted, there is no requirement for the fin or rudder to withstand anti-sideslip rudder inputs. What is requires is:
            1- Start coordinated.
            2- Suddenly apply full rudder. A sideslip will of course develop.
            3- Keep that full pro-sideslip rudder until the airplane reaches the max overswing angle.
            4- Keep that full pro-sideslip rudder in the max steady sideslip angle.
            5- Suddenly neutralize the rudder.

            At no point is right rudder used in a left sideslip or vice-versa.

            Why am I the only one who actually does any research around here? Have you read the report?
            Yes, at full, but not recently and I don't remember the details of the analysis of the rudder-fin structural design.

            As you know, there are limit loads and then there are ultimate loads. The ultimate loads for certifications requirements are 1.5x the limit loads. When the rudder lug failed on AA587, the load was OVER 2X the limit load. Do you realize this? What Boeing rudder can confidently withstand >2X the limit load?!!
            I don't know, probably it would withstand 2x the limit load required by the FAR.
            As I've said, Boeing has its own internal requirement for the rudder and fin to withstand the limit load of suddenly applying full rudder in one direction and, at max overswing*, suddenly apply full rudder in the opposite direction. I must confess that I read that years ago, some time after the accident, and while I am quite sure it was as stated, I might be wrong in the * part. Maybe the opposite rudder was applied from the steady max sideslip. So although I am almost sure that it was the max overswing, I will take the max steady sideslip which is more conservative for the following analysis.

            In the linear regime, where the lifting surface is not too close to the stall and there is no separation of flow in the flight controls either, the forces can be added arithmetically.
            Probably at high angles of attack existing in a high sideslip condition and with full control displacements, the linear analysis is not super accurate, but the good thing is that approaching to stall and flow separation REDUCES the loads, so the linear analysis is conservative (will yield greater loads than reality).

            So let's add the sidewise force of the rudder deflection and the sidewise force of the fin flying an an angle of attack (that is, an angle of sideslip).

            Fully deflect the rudder and "take a photo" before the plane starts to yaw. Let's call that lateral force Fr.

            Now let the plane stabilize with the rudder fully deflected at the max steady sideslip angle. Note that the lateral force at that point is zero, despite the rudder being fully deflected. If it wasn't the lift on the tail would be creating a yawing moment and the plane would not be steady in yaw (I am neglecting the yawing moment in the fuselage, that is pro-sideslip, and in the wings, that in swept-back wings is anti-sideslip).
            By the linear addition of forces, that means that the lateral force on the fin due to the sideslip (if the rudder was centered), let's call it Ff, must be equal in magnitude but opposite in direction to the lateral force in the fin due to the full rudder deflection. So Ff=-Fr and it is Fr+(-Fr)=0 and voila, the airplane is steady and there is no lateral force on the fin. Note that this point (steady max sideslip with full rudder deflection) is nothing interesting from a structural point of view. Yes, there will be some distribution of forces along and across the fin and rudder, but the overall is zero.

            Now, center the rudder as required by the FAR. Now you have only -Fr left. Basically, you are subjecting the fin as a whole to the same load than when the plane was with no sideslip and suddenly applied full rudder, except that now the load is in the opposite direction. Again, nothing very interesting here. The fin already resisted that, if not we would not have gone so far.

            To end with the FAR analysis, let me add the step that I skipped, the max oversing. If there were no damping forces, the motion of the plane in yaw would be a harmonic oscillation. You start from zero sideslip, the plane is in equilibrium in this state. Now apply full rudder and the equilibrium state shifts to that of max steady sideslip, so zero yaw is not equilibrium anymore and the plane will start to accelerate towards the equilibrium state, but it will overshoot it and will keep increasing the sideslip but now decelerating, it will reach the max overswing where it will reach zero yaw rate, and back we go towards and across the equilibrium point all the way to zero sideslip again, and then back to and across the equilibrium and so on. Basically, the plane will oscillate in yaw between zero sideslip and max oversing forever.

            In a linear model, the max oversing will be twice the steady sideslip, and the force will be Fr+(-2Fr)=-Fr. Again, the same than than when you applied full rudder starting from zero sideslip. We already survived that.
            But it gets even worse (or better). First, if there are nonlinearities (which by the time you reach the maximum oversing considering no damping I expect there will be), the force will be less.
            But then, there WILL be dampening because the lateral motion of the fin due to yawing motion and arm length induces and additional angle of attack on the fin than adds to that of the sideslip. So, for example, when the airplane passes by the angle of steady sideslip with the rudder fully deflected on its way to the max overswing, the force will not be zero as in the steady case but there will be some anti-yaw force. So the max overswing angle will never reach 2 x the steady sideslip angle, which again means less force.

            Basically, although I concede that the loads during the full motion need to be investigated in case there are unexpected effects, basically all the FAR requirements can be reduced to "from zero sideslip, apply full rudder". If it didn't break by then, it will not break in the following steps because they are, at most, equally stringent, if not less.

            Of course, the airplane must withstand than load without structural failure, and 50% more (ultimate load). In that additional 50% certain kind of structural failure (like permanent deformation) is allowed as long as the plane can safely fly to a landing spot and land. But it is not required that the plane must be able to fly again. It can be a write-off, as it happened with the C-130.

            Now let's go to the weak version of the Boeing internal requirement: apply full anti-sideslip rudder from the point of max steady sideslip. Now you have -Fr+(-Fr)=-2Fr. You have just doubled the FAR's ultimate load.
            Let me say once again that, in my opinion, this is a very reasonable requirement. One of the functions of the rudder is to correct sideslip by actively using anti-sideslip rudder, not just by keeping it centered and letting the fin act by itself. And the maneuver speed should be a speed where the pilot should feel comfortable using the controls with liberty. That's the idea. If not why have a maneuver speed at all? Just for the elevator?

            Now, applying this internal rule, Boeing also uses a safety factor, bet because this Boing-internal limit load is already twice the FAR's limit load and 33% greater than the FAR's ultimate load, they use a safety factor of just 20%. So Boeing's ultimate load will be 2.4 times the FAR's limit load and 1.6 times the FAR's ultimate load.

            This is a design criteria for Boeing. I don't know if there is something similar for Airbus.
            I think that the reason why the Airbus withstood 2x limit load is not because of design criteria, but because of the limit load, the 50% safety factor before catastrophic failure, the conservative design and construction, and the fact that the plane was flying slower than Va. But you have better knowledge of the report so I will believe you if you tell me otherwise.

            As I said, I think that the FAR is inadequate.

            [quote]The other point I keep making but is not penetrating the brain-barrier here is about the use of rudder. In almost any upset situation, unless at an AoA where ailerons become ineffective, rudder input is not required on large aircraft.[quote]

            Seriously? That's almost copy-paste.

            Originally posted by Gabriel
            They also modified the syllabus of their upset recovery training saying that aileron is the source of roll control and rudder should be used just as necessary to keep a coordinated flight, except in the high-pitch slow-speed condition that you need to roll the plane, if roll control is ineffective you can use a bit of rudder with caution just to initiate the roll.

            --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
            --- Defend what you say with arguments, not by imposing your credentials ---

            Comment


            • #21
              Originally posted by Evan View Post
              But you're just here to troll. I keep forgetting that.
              Darn...more piles of dog poop...and dead on too, you aren't even clipping them, but hitting them dead center!

              I don't "just" troll. But when I do, I most definitely focus on arrogant, closed-minded individuals who like to accuse well trained pilots- pilots who know more than me, Gabriel and you put together- "mass-produced idiots and cowboys" as you like to call them. And then you can't even understand things when Gabriel tries to explain it nicely. (And in this case, his lack of brevity is his attempt that maybe... maybe, this time it will get through your black and white glasses and thick concrete surrounding your brain.)

              And you think the AA-587 pilot was "improvising"? Never mind, I understand, the concept that he stepped on a rudder pedal to correct yaw is one of those basic cowboy airmanship things that does not compute in the Evan universe. (Nor does the 1" depression and minimal feedback force nor cases of human-induced-oscillation compute in the brain of the perfect one).

              Ok, enough trolling for today. Maybe tomorrow, I'll post something relevant to the rest of the world (as I have done many times before, sometimes invoking a great discussion, sometimes invoking the wrath of wiser pilots whom I respect due to their extensive type-specific training as well as their sound fundamental knowledge.)
              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

              Comment


              • #22
                Originally posted by 3WE View Post
                Darn...more piles of dog poop...and dead on too, you aren't even clipping them, but hitting them dead center!

                I don't "just" troll. But when I do, I most definitely focus on arrogant, closed-minded individuals who like to accuse well trained pilots- pilots who know more than me, Gabriel and you put together- "mass-produced idiots and cowboys" as you like to call them.
                I couldn't help noticing the quotation marks. Is that a direct quote? Or another gross misinterpretation repurposed to put words in my mouth? Or is that the first rule of trolling?

                Comment


                • #23
                  Originally posted by Gabriel View Post
                  So Ff=-Fr and it is Fr+(-Fr)=0 and voila...
                  Phew... ok, as usual, you've gone over my head with the engineering. I appreciate the time you put into it but I have to study up on that a bit more...

                  But I think I get the gist... you believe Boeing may use a greater ultimate load standard than Airbus... and you believe the FAR is inadequate. In 1986, when Airbus tested lateral failure loads on the fin, in failed around 2X of the limit load (see chart attached). That doesn't surprise me. Commercial aircraft are generally overbuilt these days (although the latest designs with their razor-thin margins of efficiency make me wonder if they are becoming less overbuilt). That, combined with correct and adequate pilot training, should be enough to avoid any repeated occurrence of AA587. Add to this the added protections that are the subject of this thread and I don't think we have anything more to worry about (if Boeing also employs them, and the EASA AD has not yet been matched with an FAA AD...).

                  But here's what I know from the report:

                  The report was drawn from analysis done by such experts as the loads and dynamics manager at Airbus and the FAA’s chief scientific and technical advisor for loads and aeroelasticity, I'm certain they have a Gabrielesque grasp on the engineering. It identified exceptional conditions outside the FAR requirement where the fin could fail:

                  The local domain manager for loads and aeroelasticity at Airbus indicated that a return of the rudder from the overswing sideslip angle to neutral or a full rudder movement in one direction followed by a movement in the opposite direction would result in external loads that were “a little bit higher” than those that were developed using the current regulation. He further stated that, if two sets of alternating rudder inputs were performed, a series of dynamic maneuvers would start that could be benign or “could lead the airplane into a severe dynamic situation where, at the proper frequency, this continued application of this surface would allow the motion of the airplane to build up to the point where the sideslip would become excessive and overload the airplane.”
                  So the danger lies with transient loads as opposed to static loads. Specifically, the danger lay in a build-up of trainsient loads over a series of repeated rudder reversals (not a single rudder reversal, not even two subsequent reversals, but a continued occillation until loads exceeded the structure: transient loads + frequency).

                  My point remains that Airbus built the airframe to be structurally sound in any scenario where the pilots are not dangerously abusing it through bad and entirely unnecessary control inputs. In other words, they were assuming competently trained pilots would be at the controls and would be using the rudder accordingly:

                  Airbus established vertical stabilizer loads for specific conditions, as defined by the applicable airworthiness requirements in 14 CFR Part 25. These conditions were the yawing maneuver that results from rudder displacement conditions; an engine failure (the loss of thrust) and the associated pilot corrective action; potential systems failures, in particular, flight control systems; and atmospheric anomalies (for example, a lateral gust).
                  However, pilots were not being trained correctly on rudder. Post-accident investigation revealed that the The American Airlines Advanced Aircraft Maneuvering Program was instructing pilots to use rudder to compensate for wake-turbulence-induced roll, including alternating inputs. Furthermore, the were leaving pilots with the dangerously false impression that, below Va, there was no structural danger and the rudder could be used with impunity, including rapid reversals (a misconception further supported by the presence of the rudder travel limiter):

                  In a postaccident interview, American Airlines’ managing director of flight operations technical stated that most of the company pilots believed that, if the pilot made right, left, and right rudder inputs, the airplane would be protected as long as it was traveling below VA.
                  That, Gabriel, is the crux of the problem. That is what killed AA587, not structural weakness issues. And h o p e f u l l y that misconception has since been deeply corrected in pilot training...

                  Beyond revisions to training and RECURRENT applications of that training, here is what the report suggested:

                  13. Certification standards are needed to ensure that future airplane designs minimize the potential for aircraft-pilot coupling susceptibility and to better protect against high loads in the event of large rudder inputs.
                  I believe this is what you are advocating as well. And that's fine, but how? One way is to create even stronger, and thus heavier structures. The other way is to create system safeguards to prevent what I have heretofore referred to as 'pilot idiocy'. The latter route is the one Airbus is taking. What is Boeing doing about it..?

                  EDIT: I cannot upload attachments with this new forum interface, because it's full of bugs and was a really bad move.

                  Comment


                  • #24
                    Evan, I agree with your last post to an unusual high degree.

                    The math and concepts in my post are really very simple, but I realized while writing it that it was difficult to explain clearly using just text. That's why I added the takeaways that you correctly identified.

                    It is still not clear to me, from your quotes of the report, what was the Airbus fin DESIGNED to withstand (what it actually withstanded is a different thing). The most clear paragraph regarding this is:
                    Airbus established vertical stabilizer loads for specific conditions, as defined by the applicable airworthiness requirements in 14 CFR Part 25. These conditions were the yawing maneuver that results from rudder displacement conditions; an engine failure (the loss of thrust) and the associated pilot corrective action; potential systems failures, in particular, flight control systems; and atmospheric anomalies (for example, a lateral gust).

                    So perhaps the fin withstood (and was designed to withstand) 2x the limit load of the maneuver load requirements, not because of those requirements, but because other requirements were more stringent (like lateral gusts or system failures).

                    That still makes the maneuver loads requirements completely inadequate, in my opinion.

                    The Va misconception was not an issue in AA only. It was generalized in all the aviation industry. Flying clubs, flying schools, PPL instructors, commercial pilots, airline instructors and checkmans.
                    Everybody thought that Va was the max speed at which you could use whatever large control inputs you wanted or needed without the risk of breaking the plane. At speeds above Va you had to use the control inputs with caution.
                    Even myself, an Aeronautical engineer who had to study the structural requirements to pass the Structures and Aerodynamics exams, was confused. Yes, I knew the FAR requirements, but also had the generalized understanding, so I never stopped to read in detail and realize that both concepts were incompatible.
                    WIth the wing the FAR made it very clear. At any speed below Va, the wing will just stall before achieving the limit load. So the wing can never exceed the ultimate load below Va and hence you cannot break the wing from aerodynamic loads below Va no matter what you do. Or can you?
                    Yes, you can. The roil condition requires full deflection of the aileron at Va with the plane at 2/3 of the limit load. IF you are pulling up at 100% of the limit load and apply full aileron, the wing can legally break. This is another unexpected case, that is not being discussed by the industry because an accident didn't happen yet under that condition.

                    Now, first of all, note that there are key speeds that are using for design and certification that the pilot doesn't need to know. Like Vc, Vd, Vmu, Vbe.... These are used to design the plane and the develop the plane's manuals and other speed that are useful for the pilot, like Vne, Vmo, V1, Vr... Now, Va is a key design speed that is ALSO required to be informed to the pilot. So let's make it useful for the pilot. And pilots need that we engineers KISS, because they have too many things, concepts and information to memorize and because they have time of high workload (specially when the fit hits the sham, that is where this knowledge is needed most) where they cannot stop to recall and analyze things.

                    "If you are flying at Va or slower, you cannot break the wing by pulling up or applying full aileron, but not both things at the same time, and you can use full rudder in one direction as long as you start from zero sideslip, and if you are already in a sideslip the max you can do is neutralize the rudder and let the fin do its job, although if you use a bit of anti-sideslip rudder with caution probably you will have no issues either". Seriously? If you believe that is user friendly, don't complain about the new forum interface!!!

                    Then we can blame the pilots for not complying with that. Especially with an overly sensitive rudder where there is barely any difference in pedal deflection and force between nor udder input and full rudder deflection.

                    Again, I agree with all what you said, and it is true that there was a misunderstanding regarding Va, but I think that pilots deserve a better, more user freindly, Va and better rudder control system.

                    We fully agree on the (mis)training issue regarding the use of rudder in upset recoveries.

                    And finally, I think that the best thing that manufacturers are doing is more reasonable rudder systems, where the rudder pedal travel and forces are not limited as the rudder deflection itself gets limited (Airbus solved that in the planes that followed the A300.), and apparently using limit load criterias that are more reasonable than FAR.25.351. I think that if you need to add a "keep your limbs off my controls" warning, there is something wrong with the design quality and pilot quality already and I will not rely on that warning to solve that, as much as a stall warning is of no use you have a crappy pilot or a crappy design (think deep stall).

                    And now I am wondering why this AD applies only to the A320, which is not even a derivative of the A300. I guess that something "special" happens there, that doesn't happen in other Airbus or Boeing models. And I am very curious to learn what it is. I will try to find more info on this AD.

                    Once again, I don't neglect or minimize the pilot idiocy (including components of self-idiocy and industry-induced idiocy, like lack of training in some things, anti-training in other things, misconceptions, and lack of action on the weakness of a particular pilot that had been identified ) that you very well described, but there was more than that that led to this accident. And I will name the FAR limit load criteria and the A300 rudder system among the things that were NOT within the scope of pilot idiocy.

                    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                    --- Defend what you say with arguments, not by imposing your credentials ---

                    Comment


                    • #25
                      Originally posted by Gabriel View Post
                      Evan, I agree with your last post to an unusual high degree.
                      Ditto.

                      Then we can blame the pilots for not complying with that. Especially with an overly sensitive rudder where there is barely any difference in pedal deflection and force between nor udder input and full rudder deflection.
                      But I don't agree with you here. The issue was not rudder pedal sensitivity. It was the technique of repeated rudder reversals, combined with the natural delay effects of the large airframe, that led to increasingly dangerous forces on the rudder attachment lugs. Read that last post again; even a couple of full rudder reversals can be considered "benign". But after a certain frequency the rudder snaps off like a Twix.

                      (I realize that smaller rudder reversal inputs might also be benign, but—this is the core of the issue—even small repeated reversals are indicative of a basic lack of understanding on the role and use of rudder on large transport aircraft.)

                      I do agree that a better rudder control was in order, and as you have pointed out, this was done on the A330, but it wasn't the main issue here. The differences in terms of control forces between the A306 and the A330 are not so large (see page 42 of the final report).

                      Yes, you can. The roil condition requires full deflection of the aileron at Va with the plane at 2/3 of the limit load. IF you are pulling up at 100% of the limit load and apply full aileron, the wing can legally break. This is another unexpected case, that is not being discussed by the industry because an accident didn't happen yet under that condition.
                      But this is what I'm driving at. Here is another example of how you can break a plane by doing something VERY STUPID. We've all come to the conclusion that we have to draw a line between absolute idiot-proofing and practical aeronautics. They could design a wing to withstand this maneuver and also a fin for high-frequency reversals but at considerable cost to efficiency, range, etc. The solution currently in use, to design to the limits of a correctly piloted aircraft and then add a safety margin of 1.5 for transient issues should be sufficient (and I think it would be if pilot training also took this margin approach). Add to this the manufacturers' tendency to overdesign beyond that and we should need to ask no more from them. Although an automated warning, combined with FAC limiting, is welcome.

                      BTW: No competent pilot should ever hear this warning, so perhaps the warning should say: IDIOCY! IDIOCY! LAND IMMEDIATELY AND RETURN TO PILOT TRAINING.

                      And now I am wondering why this AD applies only to the A320, which is not even a derivative of the A300. I guess that something "special" happens there, that doesn't happen in other Airbus or Boeing models. And I am very curious to learn what it is. I will try to find more info on this AD.
                      My only guess is that the A320, being a lighter aircraft with a smaller wing area, it is more susceptible to roll in wake turbulence and CAN possibly get into dangerous attitudes as a result, possibly including a pitch-high attitude requiring rudder for effective roll. But still not ever ever ever repeated roll reversals...

                      Comment


                      • #26
                        I didn't find much useful info.
                        In this link you have the EASA AD and also comments regarding the proposed AD (when it was still a proposed AD, before it became a real AD).
                        http://ad.easa.europa.eu/blob/EASA_P..._2014-0217R1_2

                        Unfortunately, the AD doesn't give any details on the reasons for it beyond what it was published in avherald:
                        During design reviews that were conducted following safety recommendations related to in-service incidents and one accident on another aircraft type, it has been determined that, in specific flight conditions, the allowable load limits on the vertical tail plane could be reached and possibly exceeded.
                        This condition, if not corrected, could lead, in the worst case, to detachment of the vertical tail plane in flight and consequent loss of the aeroplane.
                        To prevent such a possibility, Airbus has developed modifications within the flight augmentation computer (FAC) to reduce the vertical tail plane stress and to activate a conditional aural warning within the flight warning computer (FWC) to further protect against pilot induced rudder doublets.
                        In the comments the airlines are complying for the high cost to do the mods and short time to comply with it.

                        Other than complaining about the cost, Bangkok airlines asks what I would have asked:
                        1. Can you provide more information on the specific flight condition that can lead to exceeding of load limit on vertical tail?
                        2. Can you provide more detail or example of the in-service incidents or accident?
                        To which EASA answer that, well, basically they don't answer:
                        The AD decision is coming from in-service events that led in once case to the loss of the vertical tail on an A300. A significant event was experienced by an A319 followed by a NTSB investigation.
                        They don't say but I think we all know which is the A300 accident they refer to. So my next questions would be:
                        Then why isn't something like that done for the A300?
                        Which is the A319 incident referred?
                        Why only in the A320 family?
                        How can an accident in an A300 trigger an AD in, and only in, the A320 family?
                        What is special for the A320 that is inherited from the A300?
                        Aren't other Airbus or non-Airbus type susceptibles to the potential risk too?
                        Or with other types we can rest assured that rudder doublets will not happen or, if they can happen, they will not lead to structural failures?

                        I expected more from the EASA.

                        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                        --- Defend what you say with arguments, not by imposing your credentials ---

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                        • #27
                          Originally posted by Gabriel View Post
                          They don't say but I think we all know which is the A300 accident they refer to. So my next questions would be:
                          Then why isn't something like that done for the A300?
                          Which is the A319 incident referred?
                          Why only in the A320 family?
                          How can an accident in an A300 trigger an AD in, and only in, the A320 family?
                          What is special for the A320 that is inherited from the A300?
                          Aren't other Airbus or non-Airbus type susceptibles to the potential risk too?
                          Or with other types we can rest assured that rudder doublets will not happen or, if they can happen, they will not lead to structural failures?
                          I know we're getting out of sequence here so I'll report this:

                          My only guess is that the A320, being a lighter aircraft with a smaller wing area, it is more susceptible to roll in wake turbulence and CAN possibly get into dangerous attitudes as a result, possibly including a pitch-high attitude requiring rudder for effective roll. But still not ever ever ever repeated roll reversals...
                          I think pilot training on proper rudder use for all their widebodies, which do not require rudder for roll upsets, may be considered adequate enough.

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                          • #28
                            I confess myself genuinely impressed

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