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Thread: Breaking news: Ethiopian Airlines flight has crashed on way to Nairobi

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    Quote 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.

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    Quote 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.....

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    Quote 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.

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    Quote 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.

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    Senior Member Evan's Avatar
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    Quote 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.

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

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    Senior Member Gabriel's Avatar
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    Quote 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.

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    Senior Member Gabriel's Avatar
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    Quote 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.

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    Senior Member Gabriel's Avatar
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    Quote 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.

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    Senior Member Gabriel's Avatar
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    Quote 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)

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    Senior Member Gabriel's Avatar
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    Initial report just out. Will take some time to read and analyze...

    http://www.ecaa.gov.et/documents/204...8-d7af1ee17f3e

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    Juan Browne of blancolirio has posted a video walking through the initial report already. It is 24 minutes and he pretty much completely only talks about the report. The reports indicates about 9 seconds worth of nose down trim movement from 4.6 to 2.1 units. This is an exact specification of the MCAS, to move 2 units for about 9 seconds. It is its calling card. That is the initial sign of the problems. One interesting take from this too is he points out the 200 hour pilot made the right calls, which were agreed upon by the captain. But there are other data points that can not be deciphered if it was pilot input or MCAS input. But most of the data can be easily determined if it was human input or done by the planes systems.

    Anyways, way too much for me to go over in this post, have a watch, it is very in depth and looks over the whole accident sequence.

    https://www.youtube.com/watch?v=HBqDcUqJ5_Q

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    Quote Originally Posted by Gabriel View Post
    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.
    I posted somewhere way back in this thread about a 707 (I think) crash that involved a dive with full nose down elevator and trim, where the trim could no longer move the stabilizer upward because the opposing force needed to overcome the aerodynamic force would have compromised the supporting airframe structure, and thus the jackscrew clutch was designed to slip as a safety feature rather than fail the airframe. Maybe there is a similar protection here as well.

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    Quote Originally Posted by Gabriel View Post
    Initial report just out. Will take some time to read and analyze...

    http://www.ecaa.gov.et/documents/204...8-d7af1ee17f3e
    Quickly scanned it. Two things caught my eye...

    • Thrust is near 100% N1 and remains there despite sustained overspeed warning (tunnelling?)
    • The Left AoA value suddenly returns to normal, then fluctuates. This seems to correlate with the transition from climb to final steep descent/dive and the vertical acceleration plot. Was the AoA vane moving by acceleration force instead of aerodynamic force?


    It appears to me that the fatal error here was leaving the thrust at TOGA, despite the overspeed warning. The a/c, now exceeding VMO, beyond its operating speed envelope, could no longer be trimmed manually due to aerodynamic force.

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    Quote Originally Posted by Evan View Post
    Quickly scanned it. Two things caught my eye...

    • Thrust is near 100% N1 and remains there despite repeated overspeed warnings (tunnelling?)
    • The Left AoA value suddenly returns to normal, then fluctuates. This seems to correlate with the transition from climb to final steep descent/dive and the vertical acceleration plot.


    It appears to me that the fatal error here was leaving the thrust at TOGA, despite the overspeed warning. The a/c, now exceeding VMO, beyond its operating speed envelope, could no longer be trimmed manually due to aerodynamic force.
    and as is his wont, evan finds a way to blame the pilots. the fatal error here was boeing's bloodthirst for money and desire to not cede market share to airbus who simply beat them to the punch.

    FUCK!

    a whole shit-ton of boeing people need to spend the rest of their lives in prison...nay, in 4th world hard-labor camps.

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    Quote Originally Posted by Gabriel View Post
    Initial report just out. Will take some time to read and analyze...

    http://www.ecaa.gov.et/documents/204...8-d7af1ee17f3e
    It is unusual (in my experience) that the Table of Contents highlights only paragraph 1.6.2 (Flight Control System) in all capital letters.

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    Quote Originally Posted by TeeVee View Post
    and as is his wont, evan finds a way to blame the pilots. the fatal error here was boeing's bloodthirst for money and desire to not cede market share to airbus who simply beat them to the punch.

    FUCK!

    a whole shit-ton of boeing people need to spend the rest of their lives in prison...nay, in 4th world hard-labor camps.
    Yeah, I have heard several professionals (not just the link I posted) say the pilots acted accordingly and that the MCAS error/malfunction is a hard beast to tame for even the most experienced. You see where I live, not Seattle itself but Boeing brings their planes to my city on tests sometimes. The 787 flew right over my house when it was being tested many times. 737's fly over before they are delivered on pre delivery test flights all the time. One of my favorite vacations is to the Museum of Flight in Seattle. But right now I am not looking to Boeing as a badge of excellence or a point to be proud as far as patriotism for my state goes. I always was prideful they were the leading force in aviation by most accounts. Cue the 787 problems. Hey they were not too bad, hey they fixed it, just a little bump in the road (or turbulence since this is aviation speak). But this is not something I am going to get over. Not just me, far from just me. Sadly this could be bad for the economy of Seattle, and it will trickle down here to Spokane and Moses Lake too. Moses Lake was being eyed for the 797 production. Will they be able to go forward with the 797? Will airlines want to even buy it? Lord knows the brand name is now associated with this fiasco and the 787 isn't a clean slate either.

    I know it sounds funny but I feel betrayed by Boeing even. You can laugh and say thats ridiculous. Go ahead. They have betrayed the common bounds of safety and as such have betrayed the trust the common flyer should put into their newest planes when booking a flight on them, or the pilot that has to figure out how the hell to safely land because another error has happened praying for their life. I don't care if I exaggerate, Boeing didn't exaggerate their safety measures enough and lives were lost, thats the true shameful exaggeration.
    Last edited by KGEG; 04-05-2019 at 04:49 AM. Reason: Spellcheck

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    Quote Originally Posted by KGEG View Post
    After the stab trim is disabled, the aircraft still pitches down intermittently. The expert in this video finds this strange. The expert does not understand why the stab trim has been reactivated causing the final and fatal pitch down.

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    Senior Member Gabriel's Avatar
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    My conclusions after reading and analyzing the preliminary report. I am mixing facts (or things that I consider facts) and opinion. I am putting opinions in parenthesis:

    - The MCAS actuates as designed (as horrible as designed).
    - The trim switches actuated as designed.
    - The electric trim motor was able to move the stabilizer in both directions even when the trim was already way off and the speed was way high, overspeed in fact, and the manual trim wheel was so hard that it was impossible to turn manually (this basically kills my concern that in extreme airspeed and out-of-trim conditions the trim may become so hard that even the electric motor would not be able to turn the jackscrew).
    - The throttles were left in the take-off position since the start of the take off roll and were never moved afterwards.
    - The left AoA sensor had an instantaneous failure a couple of seconds after lift-off that made it go instantly from a normal AoA indication, identical to that of the right AoA sensor, to the ridiculous value of 74 degrees and remain totally fixed there for the remainder of the flight except some fluctuations during the final dive. (The reason for the sudden trip of the left AoA indication may have been a bird strike that moved and bent the vane in a way that got stuck in that ridiculous position or an electronic issue, for the fluctuations in the final dive I don't have any hypothesis).
    - The huge AoA disagree generated a small speed disagree (which remained small throughout the flight) and an initially small altitude disagree which however became very big as the airspeed increased throughout the flight. By comparing with the radio alt, it is evident that the right altitude was correct and the left altitude was wrong (I don't know why the altitude difference becomes so high, old altimeters worked quite well without compensating for AoA. On the other hand, I see how, at 300+ knots, 74 degrees of AoA, if they had been real, can create huge static pressure errors. I don't understand how Boeing doesn't have simple monitoring algorithms to rule out obviously erroneous values. 74 degrees of AoA at 250+ knots is simply impossible. There would be no more plane and no more AoA by then, the plane would have been shredded to pieces by aerodynamic forces).
    - (It is almost certain that the standby speed and altitude would have matched those of the right side instrument, allowing the pilots to quickly recognize which instruments to trust)
    - That erroneous left AoA immediately generated a false stick-shaker on the left side whih remains until the final dive. There is no mention in the report about any reaction or verbal communication of either pilot regarding the stickshaker.
    - With the plane reaching 400 ft AGL, they try to activate the autopilot twice, but the autopilot would not engage and the autopilot disengage aural warning would sound in both occasions. (Activating the AP at 400 ft is standard procedure but I don't think it should be done if you have the stickshaker shaking, not at least until you have deemed the stickshaker false, and then activate the autopilot on the right side. The reason why the autopilot would not engage was likely that they attempted to engage the left-side autopilot and it would not engage with an AoA indication of 74 degrees. If they had activated the right autopilot it would probably have worked and the MCAS would have never activated since it only works in manual flight. During the approach, if they had at least some flaps extended by when they disconnected the autopilot, the MCAS would not have activated either since it activates only with flaps retracted, and if the flaps had not been extended yet the MCAS would have likely activated but it would have been easier to control since the speed would have been slower and the thrust probably about idle).
    - In the next seconds they make a few nose-up and nose-down manual trim inputs with the thumb switch and the trim responds accordingly.
    - (The plane seems to be climbing slowly than normal and accelerating faster than normal. They initially rotated to about 18 degrees, which is a normal initial climb attitude, but after the stickshaker activation the pitch is lowered and held at 8 degrees for several seconds and then lowered again to 4 degrees. Why? Are they following an unreliable airspeed procedure? Are they concerned with the stall due a stickshaker activation? In doesn't look like wither of that since they attempted to engage the AP twice and they are about to do it a 3rd time).
    - Then the try and manage to connect the AP. The left AP, according to the report, which I find surprising since the left AoA is still 74 degrees and the left stickshaker is still shaking.
    - The AP makes several trim inputs, completely normal for the AP, and the trim reacts accordingly.
    - About 20 seconds after engaging the autopilot, they move the flaps handle to the full retracted position. It would take the flaps some 20 seconds to fully retract.
    - About 30 seconds after egging the autopilot and 10 seconds after moving the flaps lever to the fully retracted position, with the flaps still retracting, the AP disengages. There is no mention whether it disengaged by itself or if it was disengaged by the pilots.
    - The flaps reach the fully retracted position. So far, between the pilot and the automatic trim inputs, the trim has moved all the time very close around the 5-units mark. Basically neither the pilot inputs nor the automatic inputs have moved the trim significantly in either direction so far. But this is about to change.
    - As soon as the flaps fully retract, there is a long automatic nose-down trim input. There is no thumb switch intervention during this automatic activation, the trim moves about 2.5 units nose-down from about 4.5 units to about 2 units (full trim down is 0 units). That must have been the MCAS, for the 1st time in the flight.
    - A few seconds after the MCAS stops by itself, the pilots apply briefly nose-up thumb switch bringing the trim about 0.5 units up to the 2.5 units position. (Why they stopped there, I don't know. The plane was evidently still quite out of trim, the MCAS actuation had been much longer than the opposite thumb trim actuation)
    - 5 seconds after they pilots finish their nose-up thumb switch input, there is another long (albeit slightly shorter) automatic nose-down input. The trim goes down some 2 units nose-down to about 0.5 units, almost the full nose-down position.
    - The pilots apply nose-up thumb switch (and that seems to cut the MCAS motion a bit shorter than the previous full time actuation). This time they keep the thumb switch input for about the same length of time that the MCAS had been actuating and the stabilizer returns to about the 2.5 units position. In the mean time, the airspeed increased to about 320 knots (overspeed is 330 knots).
    - (The airplane must have felt terribly out of trim with 2.5 units and 320 knots, again, why did they stop trimming up?)
    - The FO calls trim cutout, the captain agrees and the FO calls trim cutout (surely confirming the action, there is no recording of the actual trim cutout switch position but the next event confirms that they had been moved to cutout).
    - 5 seconds after the pilot finished the last nose-up trim thumb switch command, there is a third automatic nose-down command that lasts about the same than the 1st one. However, there is no trim motion related with this command. That must have been the MCAS trying to apply nose-down trim again but the trim not responding due to the cutout switches having been moved to the cutout position.
    - The plane starts to climb more rapidly and the speed stabilizes at about the overspeed onset.
    - During the next 2 minutes and a half, the recorded position of the trim increases it's nose-down position slightly by just 0.2 units. The report contains no explanation. (I believe it might be due to play and elasticity of the mechanism and structure, with the force on the stabilizer and resisted by the jack screw increasing as the airspeed increases).
    - After the trim cutout calls, there are several conversations in the cockpit about the captain asking for help to pull up (confirmed by records of simultaneous nose-up force in both control columns mentioned in the report but not shown in the FDR graphs), the captain asking the FO to tell this or that to the ATC, and the FO making those ATC communications. For a while, there is no mention of trim for several seconds and no indication that there was any attempt to use the trim wheel.
    - About 1 minute and 10 seconds after the cutout calls, the captain asks the FO if the trim is functional and the FO replays that it isn't (this may be referring the the thumb switch), then the FO offers to try manually (this may be referring to the trim wheel) and the captain agrees. A few seconds later the FO says that the manual trim is not working either (possible meaning that he was unable to turn the trim wheel because it was too hard). The speed by then had increased to about 360 knots.
    - For more than 1 minute there are several conversations in the cockpit, all if them unrelated with the trim. Again, talks about help to pull up, conversations with ATC, changes in the autopilot settings, a master caution that sounded in relation to the left AoA vane anti ice...
    - About 1 minute and 20 seconds after the cutout calls, the pilot once again asks help to pull up and says that the pitch is not enough.
    - While there is no conversation or parameter recorded, at that moment someone must have re-engaged the trim cutout switches beck to the operative position because immediately after...
    - There are 2 very briefs nose-up trim thumb switch clicks and the trim DOES RESPOND by moving up 0.2 units.
    - (If they reconnected the the trim cutout switches o be able to apply nose up trim, why on Earth they did just 2 super-short clicks????)
    - (Sure enough), 5 seconds after the second click we have a long automatic nose-down trim input (hello MCAS again) that moves the trim to almost the full nose-down position. There was no recorded attempt to interrupt the MCAS with the thumb switch or to trim back up afterwards. (Why why why!!!!???)
    - The airplane had achieved 7000 ft over the ground (i.e. they were not so low), but with a minus 3G, minus 40 degrees pitch dive at 400 knots, 20 seconds later they were a hole in the ground.

    My conclusions:
    - I will let Evan and TeeVee discharge his rage on Boeing's design, and I will agree. I want to focus on the pilot's actions GIVEN the airplane design and what was known form the Ethiopian crash.
    - The MCAS and trim system worked as bad as it is designed to but not worse than that. For example, these bad things speculated in the forum did NOT happen:
    --- MCAS acting in conditions other than manual flight with flaps retracted.
    --- MCAS acting when the thumb switch was actuated.
    --- MCAS not stopping when the thumb switch was actuated.
    --- MCAS acting with the trim cutout switches moved to the cutout position.
    --- The electric motor not being able to move the stabilizer in extreme-force conditions.
    --- Limitations on the authority of range of the trim when using the thumb switches.
    --- The stabilizer failing moving in response to the trim thumb switch (with the cutout switch in the operational position).
    - For unknown reasons, the pilots did not re-trim the plane before resorting to the cutout switches.
    - For unknown reasons, for more than 1 minute, when the speed was already high but not so high, the pilots did not seem to attempt to operate the trim wheel manually after disconnecting the cutout switch, even when the plane was severely out of trim. By when they attempted it, the plane was going to fast and the wheel was too hard to move manually.
    - When the situation became desperate, the re-connected the cutout switches. The only logical explanation for that would be that they would attempt to use the thumb switch to re-trim the plane. Yet:
    - They did not apply nose-up trim except for 2 supper-brief clicks on the thumb switch. The stabilized did respond accordingly moving an insignificant little bit nose-up.
    - When the MCAS started to trim down again, they did not attempt to use the thumb switch to stop it. The nose-down trim automatic actuation would have been super evident not only by the turning of the trim wheels but also by the increased force on the control column (with which they amazingly managed to move the control column further back) and the yet uncontrollable quick nose-down pitch.
    - With both pilots pulling hard on the control column together and being unable to control the pitch-down, they didn't attempt to use the thumb switch to trim up.
    - Any of the previous 6 bullets would have saved the flight and the lives. (Also a sound design would have, but I leave that to Evan and TeeVee)
    - All of the previous 6 bullets were known from the Lion Air crash
    (the part that was not known was that it could be impossible to turn the trim wheel manually in extreme conditions)

    --- 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 ---

  20. #580
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by birdguts View Post
    After the stab trim is disabled, the aircraft still pitches down intermittently. The expert in this video finds this strange.
    After the stab trim is deactivated (i.e. the trim cutout switches are put in the cutout position) the stabilizer doesn't move any more except, the report mentions, it moved barely 0.2 units nose-down in 2 minutes and 30 seconds. No mention of intermitency. I suspect this can be reduction of free play and flexibility in the mechanism and structure as the speed kept increasing and hence the aerodynamic forces on the stabilizer kept increasing.

    The expert does not understand why the stab trim has been reactivated causing the final and fatal pitch down.
    I think it is quite clear. They were desperately trying to keep the nose from going down. The captain asks for help from the FO to pull up and the FO helps but the captain says that the pitch is not enough. They desperately needed to trim up and they had already tried the trim wheel manually only to find it impossible to move. They needed to revert to the trim switch. For me the real unanswered question is, why didn't they revert to the trim switch then, after re-engaging the trim cutout switches? They payed for that mistake with their lives.

    --- 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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