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  • #31
    Originally posted by Gabriel View Post
    The A300 (and every other certified airliner) can withstand a full, rapid rudder deflection to the mechanical stops (push the pedal as hard as you want) in a single direction at a speed lower than the maneuver speed Va. (aka the limit load) plus a factor of 1.5 (aka the ultimate load)
    just for complete completeness.

    The forces applied to the rudder of aa587 were about 2 times the limit load and well beyond the ultimate load because certification does not require alternating from overswing sideslip angles.

    Because why should it?

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    • #32
      Originally posted by Evan View Post
      he A300 (and every other certified airliner) can withstand a full, rapid rudder deflection to the mechanical stops (push the pedal as hard as you want) in a single direction at a speed lower than the maneuver speed Va (aka the limit load) without permanent deformation plus a factor of 1.5 (aka the ultimate load) in a condition that would allow the plane to keep flying and land perhaps for the last time before becoming soda cans.
      just for total complete completeness.

      ...certification does not require alternating from overswing sideslip angles.

      Because why should it?
      Because...
      - It would be good to have a speed where the pilot knows that he will not break the plane from aerodynamic forces no matter what he does with the primary flight control surfaces (that was believed by many pilots to be the defnition of Va, including myself even when studied the correct definition in the university).
      - It can be done
      - The rudder is precisely intended to be used against the side slip.
      - There are mistakes that can reasonable happen that would induce the pilot to do it (AA was not one of them). Imagine a left engine failure at high thrust (like CLB) and the pilot by mistake steps on the left pedal (this sounds like a ridiculous mistake because it should be so obvious and intuitive, but it happens, to the point that pilots invented a mnemonic rule: dead engine - dead leg). So the plane yaws to the left because of the asymmetric thrust + the wrong pedal depression and pretty quickly (more or less by when the plane is overswinging) the pilot reacts to that yaw by applying corrective rudder input.
      - Because Boeing does require that feature. With a margin of 1.2 instead of 1.5, but still.

      --- 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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      • #33
        Originally posted by Gabriel View Post
        - The rudder is precisely intended to be used against the side slip.
        What?
        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

        Comment


        • #34
          Originally posted by Gabriel View Post
          - The rudder is precisely intended to be used against the side slip.
          I also take issue with this, in one specific sense. When pilots are trained to use the rudder, the circumstances under which they use it are more or less steady-state: countering torque etc. on takeoff, compensating for adverse yaw, sideslipping the aircraft, and straightening the aircraft when landing in a crosswind.

          I can't think of a situation where pilots are trained on repeated alternating application of rudder. I think the most likely need for that in day-to-day operations is in a wake turbulence encounter, but pilots don't get much training on that except that they're told to avoid it.

          The key point (IMHO) being that repeated alternating rudder inputs can easily turn into PIO which is another thing pilots don't get much training on. And PIO is a situation where it's very easy to overstress things.
          Be alert! America needs more lerts.

          Eric Law

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          • #35
            Originally posted by Gabriel View Post
            just for total complete completeness.
            I'm ok with that.



            Because...
            - It would be good to have a speed where the pilot knows that he will not break the plane from aerodynamic forces no matter what he does with the primary flight control surfaces (that was believed by many pilots to be the defnition of Va, including myself even when studied the correct definition in the university).
            - It can be done
            - The rudder is precisely intended to be used against the side slip.
            - There are mistakes that can reasonable happen that would induce the pilot to do it (AA was not one of them). Imagine a left engine failure at high thrust (like CLB) and the pilot by mistake steps on the left pedal (this sounds like a ridiculous mistake because it should be so obvious and intuitive, but it happens, to the point that pilots invented a mnemonic rule: dead engine - dead leg). So the plane yaws to the left because of the asymmetric thrust + the wrong pedal depression and pretty quickly (more or less by when the plane is overswinging) the pilot reacts to that yaw by applying corrective rudder input.
            - Because Boeing does require that feature. With a margin of 1.2 instead of 1.5, but still.
            Airbus jets will stand up to a full reversal (aa587 did) but not repeated, cyclic reversals. Certainly I would like aircraft to be able to withstand all kinds of pilot abuse with an overload factor of 10, but I also want them to perform as efficiently as possible. I think airframers have found that balance (while still leaning towards safety). But according to the reports of previous senior pilots, the pilot of aa587 perceived the rudder as a very active flight control and used it aggressively where it wasn't even needed. The solution to that is not to build the planes stronger.

            Comment


            • #36
              Originally posted by elaw View Post
              ***more or less steady-state:***
              Oh hell no!

              Do you ever sit in the back of the plane in turbulence?

              Have you ever sat anywhere on a plane during a crosswind takeoff?

              I ROUTINELY feel liberal rudder input adjustments and reversals.

              Did you look at my youtube where we see very significant elevator and aileron reversals?

              Ever look out the window and see aileron reversals?

              Rudder reversals occur!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! (Even yaw-damper-generated rudder reversals).

              Regarding AA-587, we will never know what the pilot was thinking...

              Did he fear total wake turbulence upsets and was he doing something akin to AA upset recovery procedures?

              Was he just trying to be a good stick and rudder pilot, and suddenly the low-displacement/low-force mode of the airbus pedals had him slamming the rudder totally back and forth, when he thought he was just doing slight deflections?

              Was he a total dumbass, wantonly slamming it back and forth for no good reason as some people seem to think?
              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

              Comment


              • #37
                Originally posted by 3WE View Post
                Regarding AA-587, we parlour-talkers who cannot be bothered to read the very exhaustive final report will never know what the pilot was thinking...
                The NTSB knows what the pilot was thinking. He was thinking that was how you recover an A300 from roll upset, with the rudder .

                Comment


                • #38
                  Originally posted by Evan View Post
                  The NTSB knows what the pilot was thinking. He was thinking that was how you recover an A300 from roll upset, with the rudder .
                  Accept my apologies. I must have glossed over and misread. I THOUGHT the report said that the data from the cockpit thought recorder was damaged and unrecoverable. My bad.

                  Edit: Actually, I just checked the report. It's pretty clear at the top of page 9 that the both of the cockpit thought recorders were damaged to the point of being unable to recover data...although I understand those things may be prone to bias- particularly Evan's.
                  Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                  Comment


                  • #39
                    Originally posted by 3WE View Post
                    Accept my apologies. I must have glossed over and misread. I THOUGHT the report said that the data from the cockpit thought recorder was damaged and unrecoverable. My bad.
                    Yes, unfortunately we don't have the recording of the F/O stepping on both rudder pedals while saying, "Hey Ed, ya know what I'm thinking?"

                    How about reading all the stuff in there relating his training for wake turbulence, his piloting issues, etc...

                    Comment


                    • #40
                      Originally posted by Evan View Post
                      Yes, unfortunately we don't have the recording of the F/O stepping on both rudder pedals while saying, "Hey Ed, ya know what I'm thinking?"

                      How about reading all the stuff in there relating his training for wake turbulence, his piloting issues, etc...
                      So, riddle me a few quick questions:

                      Were rudder reversals part of AA upset training?

                      Are small, light rudder pedal inputs an oft-used input for the sake of yaw correction?

                      Now, tell me how that extrapolates into such high confidence that he was reversing the rudder to fight off an upset.
                      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                      Comment


                      • #41
                        Originally posted by elaw View Post
                        I also take issue with this, in one specific sense. When pilots are trained to use the rudder, the circumstances under which they use it are more or less steady-state: countering torque etc. on takeoff, compensating for adverse yaw, sideslipping the aircraft, and straightening the aircraft when landing in a crosswind.

                        I can't think of a situation where pilots are trained on repeated alternating application of rudder. I think the most likely need for that in day-to-day operations is in a wake turbulence encounter, but pilots don't get much training on that except that they're told to avoid it.

                        The key point (IMHO) being that repeated alternating rudder inputs can easily turn into PIO which is another thing pilots don't get much training on. And PIO is a situation where it's very easy to overstress things.
                        Where did I say repeated, alternating or PIO? I am talking of an engine failure where the pilot is slow to react, so the plane yaws.

                        Let me ask you a question, if the nose of the plane suddenly goes way left for whatever reason (normal turbulence, wake turbulence, a lateral wing gust or wind shear, engine failure, sudden double engine failure on the 2 left engines in a 747...), what do you think that would be the natural response of a pilot? Well, the plane is not certified for that natural response.

                        The requirement is, at Va (maneuver speed):
                        1- Start from coordinated flight.
                        2- Fully depress the rudder in one direction and keep it there to induce a sideslip.
                        3- Let the sideslip reach the equilibrium sideslip but wait, keep that pedal down. The plane has inertia so it will keep going a bit more.
                        4- Let the plane go past the equilibrium side slip and reach the max overswing angle, but wait, don't release the pedal, keep the pedal down trying your best to keep increasing the sideslip or at least to hold it.
                        5- Since you are at the overswing angle, no matter how hard you try, the plane will yaw a bit back crossing the equilibrium angle again, and will make a few damped oscillations around that equilibrium angle finally stabilizing at the equilibrium sideslip.
                        6- Remember, you are still holding the pedal down. Keep it there... keep it there... a few seconds more... AND NOW YES!
                        7- Neutralize the pedal.

                        For certification, you are not required to EVER use the rudder pedal to counteract yaw or sideslip. Only to increase it or let go.
                        Now, what do you think a pilot will do if, for whatever reason, the plane moves as described in 2, 3 and 4?
                        That is one of the most ridiculous pieces of regulation in FAR 23 and 25.

                        --- 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


                        • #42
                          Originally posted by Evan View Post
                          Airbus jets will stand up to a full reversal (aa587 did) but not repeated, cyclic reversals. Certainly I would like aircraft to be able to withstand all kinds of pilot abuse with an overload factor of 10, but I also want them to perform as efficiently as possible. I think airframers have found that balance (while still leaning towards safety). But according to the reports of previous senior pilots, the pilot of aa587 perceived the rudder as a very active flight control and used it aggressively where it wasn't even needed. The solution to that is not to build the planes stronger.
                          I do not think that the A300 fin will survive a single "well perfromed" rudder reversal.
                          AA was flying slower than Va and likely the pilot didn't apply full rudder exactly at max overswing the first times.

                          The forces applied to the rudder of aa587 were about 2 times the limit load and well beyond the ultimate load because certification does not require alternating from overswing sideslip angles.
                          Certification does not require full anti-yaw rudder from a single overswing sideslip, or partial anti-yaw rudder input from a moderate sideslip, or and anti-yaw reversal. It's like saying "don't use right ailerons if the plane is rolling left".

                          aa587 reached 2 times the limit load at a speed quite lower than Va. Imagine how much easier would be to reach and exceed 1.5 times the limit load at Va.

                          Boeing does have an internal requirement for full and rapid anti-yaw rudder from max overswing. Airbus doesn't (or didn't).

                          --- 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


                          • #43
                            Originally posted by Gabriel View Post
                            - It would be good to have a speed where the pilot knows that he will not break the plane from aerodynamic forces no matter what he does with the primary flight control surfaces (that was believed by many pilots to be the defnition of Va, including myself even when studied the correct definition in the university).
                            That's not how I was taught, and I went to what some would say was (and is) a third-tier school...Va was never taught as a free-for-all speed.

                            Comment


                            • #44
                              Originally posted by ATLcrew View Post
                              That's not how I was taught, and I went to what some would say was (and is) a third-tier school...Va was never taught as a free-for-all speed.
                              To clarify, this is not what I was taught in the university. In the university I had to study from the FAR themselves. At some point I forgot this definition and re-learned the "wrong" one as a pilot.

                              In fact, the definition of Va is the speed that will make your plane stall at the limit load factor (it is the C "corner" of the V-n diagram, also called envelope). Slower than that, you will stall BEFORE achieving the limit load factor. Faster than that, you will can exceed the limit load factor (and brake your wings in the process). So it is pretty much the speed where your wings will not break from aerodynamic loads for example in a steep turn or pulling up from a dive.

                              The other requirements take this already defined Va and say "At Va the plane must withstand this and such control inputs".

                              Click image for larger version

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                              --- 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


                              • #45
                                Originally posted by Gabriel View Post

                                Certification does not require full anti-yaw rudder from a single overswing sideslip, or partial anti-yaw rudder input from a moderate sideslip, or and anti-yaw reversal. It's like saying "don't use right ailerons if the plane is rolling left".

                                aa587 reached 2 times the limit load at a speed quite lower than Va. Imagine how much easier would be to reach and exceed 1.5 times the limit load at Va.

                                Boeing does have an internal requirement for full and rapid anti-yaw rudder from max overswing. Airbus doesn't (or didn't).
                                Hold on... The FARs require the fin to withstand a full rudder deflection in one direction, followed by a return to neutral, followed by a full deflection to the other direction, x1.5 overload. What do you mean by "anti-yaw rudder" if not this? From max overswing (assuming you got there from an engine failure with a neutral rudder) you can still apply the maximum amount of rudder available to correct for that yaw and remove it as needed to cancel out the yaw effect of the thrust asymmetry. Why would you do this to the opposite overswing?

                                Aircraft aren't required to do this cyclically without the neutral centering between cycles, because the only reason that would ever happen is due to very very bad, very very wrong piloting and a completely flawed understanding of the role of rudder.

                                Now there is still the phenomena of aircraft-pilot coupling that needs to be addressed. Should it be done by pilot training or by making the airframes more robust?

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