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  • Originally posted by SmoothAir View Post
    It would seem that the MCAS has far greater authority in movement and angle than the trim ability available to the pilots via trim switches.
    That's absolutely not the case. Whenever the MCAS has any authority, you will have authority OVERRIDING THE MCAS to trim in the other direction by using the trim switches.

    --- 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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    • Originally posted by SmoothAir View Post
      Agreed, it appear to me that the MCAS has full authority to move the stab to the final limits, whereas, the pilot trim can only move to preset limits. WTF ?
      You are totally misunderstanding it. Yes the MCAS may be able to move the trim more nose-DOWN that you can achieve with the thumb switches, but you will always be able to trim UP, AGAINST the MCAS, no matter how far nose-down the the trim has gone for any reason including MCAS.

      --- 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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      • Originally posted by Gabriel View Post
        I disagree.

        First, if you have a failure (not MCAS) that makes continuous nose-down trim inputs (except when using the thumb switch), you will have the same situation and even worse since it will not wait 5 seconds after your last thumb switch input. This can happen in any 737 and it is what happens in the now deleted Mentour video... in an NG sim, not a MAX.


        Second, you are desperately needing to trim nose up, the cutout switch are off because you disconnected it because you had a nose-down trim runaway, you are fighting to turn the trim wheel and not being able to do it, so you decide to reconnect the cutout switches to be able to use the nose-up thumb switch, reconnect the cutout switches, and not use the thumb trim? Because if you do use the thumb switch then the trim will go up, not down, regardless of the MCAS or runaway situation.
        Again, fair points. I just think that, when we get clarity on the real-world experience of this situation, the stealth factor will prove to be a detriment to recovery. If you have an ongoing trim runaway that resumes when you reactivate the electric trim, you would trim up and then immediately cut off the trim again. But MCAS gives you five seconds of false obedience. You might opt to leave the trim engaged, and then it suddenly noses you over again. Again, with perfect SA, you would just trip it off again, but with a high workload and more questions than answers...

        The point is moot. The 737 will never fly again with this stealth factor present. What concerns me now are the (unsubstantiated) revelations about pitch trim authority over the entire 737 line.

        Comment


        • Originally posted by Evan View Post
          I just think that, when we get clarity on the real-world experience of this situation, the stealth factor will prove to be a detriment to recovery. If you have an ongoing trim runaway that resumes when you reactivate the electric trim, you would trim up and then immediately cut off the trim again. But MCAS gives you five seconds of false obedience. You might opt to leave the trim engaged, and then it suddenly noses you over again.
          I agree, for Lion Air (who apparently never went as far as ever using the trim cutout switches in the first place anyway). But but for the Ethiopian pilots this should have been crystal clear, it should not have taken them by surprise. If it did, which I doubt.

          The point is moot. The 737 will never fly again with this stealth factor present. What concerns me now are the (unsubstantiated) revelations about pitch trim authority over the entire 737 line.
          The limitations of the trim wheel (that it can become as hard as to make it impossible to move) seem quite substantiated to me. We have former Boeing engineers, simulations, and even the roller-coaster procedure deleted from the 737 manuals some 40 years ago which explained the condition and how to recover from it: Pull the nose up above the horizon, let go on the yoke (that will unload the stabilizer) and crank the wheel until the nose goes below the horizon, then repeat as many times as needed until the plane is trimmed or the trim wheel becomes light enough that it can be moved without unloading the stabilizer.

          --- 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


          • Originally posted by SmoothAir View Post
            The MAX should be grounded until the engines are moved backwards AND the wings / gear are redesigned - never mind extended mounts and nose gear 8" lift !
            This would defeat the purpose, the purpose being to avoid the new-type certification process. Everything about the Max development was centered around the need to avoid a lengthy and costly new type certification. Boeing learned a hard lesson with the Y2 (B787) development. The B787 may not turn a profit until the mid-2020's. Last I heard, it was still $25B in the red.

            Boeing originally envisioned an all-modern fleet for the new century and called it the Yellowstone Project. The 787 was Y2. The 737 replacement was supposed to be Y1 but they shelved it in 2007 I think. The 737-Max was a crutch solution to compete with the A320neo. Somehow it did, but at a high cost in terms of human lives.

            I guess you can't reheat an omelet without breaking a few eggs.

            Comment


            • Originally posted by Gabriel View Post
              No. I'll try to explain, but let's start forgetting about the elevator, as I understand it has nothing to do with what happens with the trim (rather the opposite).

              The horizontal tail has 2 movable parts. The "fixed" (not so fixed) stabilizer in the front and the elevator in the back. The elevator is controlled with the control column. The elevator is hinged in the front, hinged to the back of the stabilizer, so when it moves the front part of the elevator stays in place and the back moves up and down. Under normal conditions they are hydraulically actuated, so aerodynamic forces cannot move it. But it is easy to visualize that if you deflect the elevator up (to push the tail down and the nose up), there will be aerodynamic forces that will tend to return it to the equilibrium position. Very much like a weather vane.

              The "fixed" stabilizer is really not fixed. It is hinged in the back. So, if you could move it freely, the back part will stay in place and the front of the stabilizer moves up and down. It can be intuitively seen how, if you start from the equilibrium position and displace the front of the stabilizer a little bit say up, and let it go, it will not tend to return to the equilibrium but rather move more up. It's like a weather vane facing back (this is pretty much what happened with the Alaska MD80 accident).

              The stabilizer is controlled by a jackscrew that goes through a bolt in the front of the stabilizer. When the jackscrew rotates in one direction or the other, it moves the nut up or down and the elevator with it. If the jackscrew doesn't turn, the stabilizer won't move, so you will not have the unstable behavior described above. I mean, the stabilizer will try to move farther away from the equilibrium, but the jack screw won't let it. How? Because the threads of the nut will make pressure against the thread of the jackscrew. The stronger the pressure, also the stronger the friction. And when is the pressure stronger? When the loft on the stabilizer is stronger, and that happens the farther away the trim is from the equilibrium position and the faster the airspeed (because the lift is proportional to the angle of attack, of the stabilizer in this case, and the speed squared).

              The trim wheel doesn't control the elevator. It controls the stabilizer. They are mechanically connected to the jackscrew. Rotating the wheels rotates the jackscrew.
              If the stabilizer is very out of trim (very far from the equilibrium position) and the airspeed is high enough, the friction between the jackscrew and the nut can be very high and it can be almost impossible to manually rotate the jackscrew using by manually rotating the trim wheel.

              So from what I read, the elevator has something to do with this as well - if the stabilizer is out of trim (ND), the pilots have to use the elevators to keep the nose level. That leads to forces on the stabilizer. At some point those forces become too strong and it's impossible to trim manually. That's why they say the way to get out of this is to unload the stabilizer by returning the elevators to neutral, and then retrim. That obviously would mean a significant loss of altitude from the point the elevators are centered to the point the stab is retrimed, and is even done incrementally. Please advise.

              Comment


              • Originally posted by Gabriel View Post
                You are totally misunderstanding it. Yes the MCAS may be able to move the trim more nose-DOWN that you can achieve with the thumb switches, but you will always be able to trim UP, AGAINST the MCAS, no matter how far nose-down the the trim has gone for any reason including MCAS.
                Understood, thanks, at least we hope that is working as it should.....

                Comment


                • Originally posted by Gabriel View Post
                  Strange tings going on. I can confirm that the video WAS available. I saw most of it. And the link came from the Leeham news article. The article said that they had been working with Mentour in the sim and that them made a video, and at the end of the article there was a link to the video. Now the article still says that they worked in the sim with Mentour but there is no mention to a video, the link is gone, and the video doesn't exist anymore in the Mentour channel. A guy called Scott, who I don't have the slightest idea who he is, repeatedly said in the comment that the video was removed at the request of Mentour's employer (Ryanair), who seems to have quite a few of MAX in orders.

                  This is Scott Hamilton, the head of Leeham News. Now they are saying Ryanair didn't want it taken down, but a colleague of Mentour suggested it should be taken down.

                  Anyway, for what it's worth, I do find it strange. It was perfectly fine to post videos showing how easy it is to stop a trim runaway, and speculate it should have been easy to stop MCAS if one followed the necessary (very basic) procedure. But the moment a video is uploaded that speculates that it is possible in certain situations to lose the plane even if one follows the procedures, the thing gets taken down in a few hours.

                  Comment


                  • Originally posted by Gabriel View Post
                    That's absolutely not the case. Whenever the MCAS has any authority, you will have authority OVERRIDING THE MCAS to trim in the other direction by using the trim switches.
                    In the Mentour video that was taken down, he did say the trim switches can be effective in only partially returning the trim to where it used to be, and that in the long run, MCAS will be effective in down-trimming, just significantly more slowly than without the trim switches being used. I don't know if it is the case.

                    Comment


                    • Originally posted by Gabriel View Post
                      You are totally misunderstanding it. Yes the MCAS may be able to move the trim more nose-DOWN that you can achieve with the thumb switches, but you will always be able to trim UP, AGAINST the MCAS, no matter how far nose-down the the trim has gone for any reason including MCAS.
                      From the NY Times:

                      Although that move disabled MCAS, it also forced the crew to control the stabilizers manually with wheels at their feet [knees... feet... what's the difference]— a physically difficult task on a plane moving at high speed. A little under four minutes after takeoff, the first officer said the manual method “is not working.”

                      Soon after, the black box data indicates, the crew turned electricity back on and tried two more times to move the stabilizers by hitting the switches. But once they turned the electricity back on, MCAS engaged again, putting the plane into a dive from which it would not recover.
                      Once again, the burning question is: why did they give up on the counter trim? I think we have to reconsider your statement above Gabriel, until we know more, and remain open to the possibility that MCAS might have priority over the trim switches in some way. Since the system is designed to counter pilot error, it sort of makes sense.

                      Comment


                      • On a side note, sources are saying they believe the AoA vane malfunctioned because of a bird strike or some other FOD

                        Comment


                        • Originally posted by Black Ram View Post
                          So from what I read, the elevator has something to do with this as well - if the stabilizer is out of trim (ND), the pilots have to use the elevators to keep the nose level. That leads to forces on the stabilizer. At some point those forces become too strong and it's impossible to trim manually. That's why they say the way to get out of this is to unload the stabilizer by returning the elevators to neutral, and then retrim. That obviously would mean a significant loss of altitude from the point the elevators are centered to the point the stab is retrimed, and is even done incrementally. Please advise.
                          There are 2 factors that can link the elevator deflection to the force in the stabilizer that is transmitted to the jackscrew.

                          One is a direct mechanical force. I would need to understand better how the elevator and stabilizer are geared to understand how strong this force would be or if it would even exist. It will largely depend on where the stabilizer is pivoted.

                          The other one is indirect but it exists with 100% certainty. It is an aerodynamic effect. When you let the elevator go (at least partially) the AoA of the whole plane will reduce, aslo in the stabilizer, and that would reduce the load on the stabilizer even if the elevator was not connected to the stabilizer in any way.

                          Regarding the loss of altitude, it seems that the roller-coaster procedure called for first pitch up well above the horizon, then release back pressure on the elevator and crank the trim wheel and keep doing so until the nose goes below the horizon, then stop cranking, pull up again well above the horizon and repeat as many times as necessary until you achieve the point where you can keep cranking the wheel without needing to unload the stabilizer first. In this way you can not only not loose altitude but perhaps gain altitude in the process.

                          Now, what I say is, if the high speed is a big factor in why the trim wheel became so heavy, and if you have enough elevator authority (and strength) to pull up well above the horizon, then pull up well above the horizon, reduce thrust, let speed go down, and crank the wheel.

                          --- 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


                          • Originally posted by Black Ram View Post
                            In the Mentour video that was taken down, he did say the trim switches can be effective in only partially returning the trim to where it used to be, and that in the long run, MCAS will be effective in down-trimming, just significantly more slowly than without the trim switches being used. I don't know if it is the case.
                            That's not the case. If you want, as soon as the MCAS kicks in and starts trimming down, or at any point later, you can click and hold the nose-up trim for as long as you want. That would stop the MCAS (even for 5 seconds after you RELEASE the thumbs switch) and keep moving the trim in the nose-up direction all the way to the thumb switch nose-up trim limit, wish is VERY DANGEROUSLY nose up so you really don;t want to do that, but just to illustrate ho much up you can trim.

                            Mentour said that, in this interaction between you repeatedly trimming up with the thumb switch and the MCAS repeatedly trimming down every time you release the trim-up thumb switch, the plane will be every time a little bit more trimmed down after each Cycle. But that absolutely doesn't have to be the case and I objected to that in my post about the video.

                            --- 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


                            • Originally posted by Black Ram View Post
                              This is Scott Hamilton, the head of Leeham News. Now they are saying Ryanair didn't want it taken down, but a colleague of Mentour suggested it should be taken down.

                              Anyway, for what it's worth, I do find it strange. It was perfectly fine to post videos showing how easy it is to stop a trim runaway, and speculate it should have been easy to stop MCAS if one followed the necessary (very basic) procedure. But the moment a video is uploaded that speculates that it is possible in certain situations to lose the plane even if one follows the procedures, the thing gets taken down in a few hours.
                              I share your sentiment.

                              --- 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


                              • Originally posted by Evan View Post
                                Once again, the burning question is: why did they give up on the counter trim? I think we have to reconsider your statement above Gabriel, until we know more, and remain open to the possibility that MCAS might have priority over the trim switches in some way. Since the system is designed to counter pilot error, it sort of makes sense.
                                1- I don't trust the NY times on this, or anybody that can't tell knees from feet.
                                2- The FR in the Lion Air crash showed that the MCAS did stop after each thumb switch intervention (even brief clicks), and remained stopped for 5 seconds after that.
                                3- Are there certain cases where that would not be the case? I don;t know. I do remain open. The fact that I make straight statements doesn't mean that I will not immediately change them the moment that I see convincing information otherwise. So far, with the information that I have available, which includes the Lion Air FDR, Boeing's disclosure and description of the MCAS, and Boeing's service bulletin, I am convinced that using the thumb switches will stop the MCAS, let you trim as much as you want, and keep the MCAS stopped for 5 seconds after releasing the thumb switch. That conviction can change at any time, though.

                                One thing that concerns me more is the force issue, It is still not clear for me how you can stop the motorized movement by holding the trim wheel and at the same time there can be circumstances where you will not be able to turn the trim wheel but the electric motor will. My concern is what happens if the jackscrew can get so hard/stuck that neither you nor the electric motor can move it. Then. other than finding a way to slow down or applying the roller-coaster procedure (and for booth you would need to be able to pitch up by pulling back on the yoke), I don't see any way out. And that would not be a MAX thing.

                                I am eager for the preliminary report and the FDR and CVR data. In the meantime, we seem to have a lot of people talking with conviction but possibly with mistakes (including the NYT, Mentour, and myself)

                                --- 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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