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The thing I don't understand about that MCAS system is why it would continue to drive the nose of the plane down even when the pilot is reefing on the stick to

The thing I don't understand about that MCAS system is why it would continue to drive the nose of the plane down even when the pilot is reefing on the stick to pull up. The cruise control in my car automatically disengages when I hit the brakes, it doesn't continue to accelerate to keep the car going 60 while I am jamming on the brakes, in an emergency, I don't have time to find the off switch. Similarly, I would think that in an emergency, it should automatically disengage if the pilot pulls hard on the stick, in an emergency, the pilot doesn't have time to read the manual to find the off switch for the thing. It seems crazy to me that they wouldn't have a safety override like that.

StreamBright 8y ago
Flight augmentation systems work a different way. Those take into consideration all inputs and average it out. Same when two pilots give different instructions.
- piquadrat 8y ago
  > Same when two pilots give different instructions. That's only true for Airbus (and probably contributed to the loss of Air France 447, https://en.wikipedia.org/wiki/Air_France_Flight_447 https://en.wikipedia.org/wiki/Air_France_Flight_447). In Boeing airplanes, the movement of the two yokes is synchronized.
  - neuronic 8y ago
    > In Boeing airplanes, the movement of the two yokes is synchronized. So in case of two equally strong and equally determined pilots you still get averaging of opposing control inputs.
    - wikibob 8y ago
      No, the failure mode is not two pilots consciously fighting against each other. The failure mode is two pilots who are UNAWARE they are commanding contradicting inputs. Airbus negligently averages these inputs. This is legacy tech debt because the physical design didn’t leave room to link the two controls. Now they are stuck because they feel they can’t change and have some planes that (dangerously) average the inputs, and some that are physically linked. When controls are physically linked, if one pilot thinks he should nose up, and the other tries to nose down, they can yell “what are you doing” at each other and this create an accurate mental model of what the other is intending. Think of this every time you fly Airbus. It’s very similar to debugging a production technology issue. Imagine you decide to restore the database to a snapshot from 1 hour ago, and at the same time someone else tries to restore to an earlier snapshot from 12 hours ago. Should the system average these inputs and restore from 6.5 hours ago? No, obviously not.
      - SmellyGeekBoy 8y ago
        > Think of this every time you fly Airbus. Why? They're not falling out of the sky so the system obviously works. Stop being a fanboy.
      - foldr 8y ago
        >Now they are stuck because they feel they can’t change and have some planes that (dangerously) average the inputs, and some that are physically linked. This is a really weird way to construe the situation. The accident report doesn't conclude that the handling of dual inputs was a factor, and it's clear from the transcripts that it wasn't. Airbus has no reason to change this. Merely linking sidesticks would be pointless in any case, since sidesticks don't have an identifiable position (but are used with brief movements away from center). It's hardly any easier for a pilot to passively observe the movements of his own sidestick than it is just to look over at the other pilot's stick.
        neuronic 8y ago
        The commenter appears to have a vested interest in making Airbus look bad, otherwise the comment cannot be logically understood. Is there a case where the averaging of inputs has been proved catastrophic?
        notfromhere 8y ago
        Air France 447
        foldr 8y ago
        Not according to the official accident report. This is a meme spread by an article in Popular Mechanics that came out long before the official report.
      - deleted 8y ago
        [deleted]
      - benhurmarcel 8y ago
    - kissickas 8y ago
      It sounds far more intuitive if you have a feedback mechanism in the form of a tangible demonstration of what the force is actually averaging out to.
    - philpem 8y ago
      If they're mechanically linked, the captain would at least feel the first officer countering their inputs -- giving a prompt that they need to communicate their intentions more clearly! I'll defer to a pilot to say whether they are mechanically linked -- it sounds like they are on Boeing aircraft but possibly not on Airbus?
      - WalterBright 8y ago
        They're mechanically linked on Boeing aircraft. On the 757, and I believe the rest, the mechanical link has a detent. With enough force, the pilot can move it out of the detent and move the controls independently of the copilot. The reason for this is if one side or the other is jammed, the pilot can still fly it.
      - foldr 8y ago
        No, the captain would not, because only one pilot has their hands on the controls in any given instance (with the possible exception of rotation at takeoff).
        ceejayoz 8y ago
        Clearly, that’s not always the case, given at least one plane falling out of the sky from dual control inputs. Mistakes happen. Part of aviation safety is preventing that from becoming fatal.
        foldr 8y ago
        >Clearly, that’s not always the case, given at least one plane falling out of the sky from dual control inputs. That's not what the accident report concludes. There's a clearly audible "dual input" alarm to prevent dual inputs. Dual control inputs only occurred for brief moments during the AF flight.
        ceejayoz 8y ago
        It's quite clear that wasn't sufficient. "Brief moments" during critical points in an emergency can matter enormously. From the recorder transcript: > As the plane approaches 10,000 feet, Robert tries to take back the controls, and pushes forward on the stick, but the plane is in "dual input" mode, and so the system averages his inputs with those of Bonin, who continues to pull back. The nose remains high. 02:13:40 (Robert) Remonte... remonte... remonte... remonte... Climb... climb... climb... climb... 02:13:40 (Bonin) Mais je suis à fond à cabrer depuis tout à l'heure! But I've had the stick back the whole time! 02:13:42 (Captain) Non, non, non... Ne remonte pas... non, non. No, no, no... Don't climb... no, no. 02:13:43 (Robert) Alors descends... Alors, donne-moi les commandes... À moi les commandes! Descend, then... Give me the controls... Give me the controls! > Bonin yields the controls, and Robert finally puts the nose down. The plane begins to regain speed. But it is still descending at a precipitous angle. As they near 2000 feet, the aircraft's sensors detect the fast-approaching surface and trigger a new alarm. There is no time left to build up speed by pushing the plane's nose forward into a dive. At any rate, without warning his colleagues, Bonin once again takes back the controls and pulls his side stick all the way back. The crash occurred less than a minute later.
t0mas88 8y ago
The same applies to the trim in a 737 and any other transport class aircraft. If you move the control column in the opposite direction of trim, the electric trim and autopilot trim will cut out. The same applies to the autopilot in all types I've flown, it disconnects if you override it manually (except for CWS mode, but that's known and expected) I don't fly the 737, so I haven't read the aircraft specific books and systems overviews, but I expect this override behavior is very confusing in the MCAS scenario. Because any pilot would counteract a nose-down force by pulling on the yoke. The MCAS would then stop moving the trim, and the pilot would think (as would be true in any other situation) that the override worked. But MCAS didn't stop because of the override, it stops because it always does that after a few seconds and will re-activate 10 seconds later (or 20, didn't read a definitive number on this). The confusing thing is that 95% of the unwanted trim movements are caused by the autopilot, so at this point the pilot would think the situation is under control and probably disconnect the autopilot if the movement didn't already, The other 5% (or probably less) of unwanted trim movements are trim runaway, which is either a stuck switch (2 actually, you depress two switches at once to make it move, pressing 1 does nothing) or an electrical problem. In both cases pulling back works and stops it. If it stays "stopped" you think autopilot and leave it disconnected. If it keeps moving continuously you think trim runaway and flip the trim cutout switches or pull the circuit breaker (this one is usually marked bright orange so you can find it quickly in planes without cutout switches) But in the MCAS case, and without MCAS knowledge (which apparently nobody had) you would not expect it to start moving again later. You would have concluded "autopilot" of the above two scenarios, and after disconnect would think you're safe. So now when it starts moving again, your first idea is that the autopilot did not disconnect which leads to even more confusion in the cockpit. The LionAir flight that was saved by the jumpseater got lucky in the sense that this guy was not flying, not debugging the autopilot, and did see the trim move because he has no instruments to scan and the trim is right in front of his face (in the middle between the two pilots). If you conclude that the issue is the trim, then any pilot would decide to go for the cut-out switches. It's just that concluding the trim is the issue, without knowing MCAS, under high pressure and with the wrong (autopilot) conclusion already on your mind is not very likely.
- mlindner 8y ago
  Why didn't the pilots adjust the trim manually? I don't understand how any pilot would consider increasing control pressure to be normal and not automatically respond by manually adjusting the trim.
LoSboccacc 8y ago
> why it would continue to drive the nose of the plane down even when the pilot is reefing on the stick to pull up well this other crash was caused by a pilot puling up a plane all the way to its ceiling and stalling it, so there's that, so things can go wrong whether if you do and if you don't allow for easy control overrides. https://en.wikipedia.org/wiki/Air_France_Flight_447#Third_interim_report https://en.wikipedia.org/wiki/Air_France_Flight_447#Third_in... the trick seems to find the right balance to relinquish control only when appropriate, so it seems that proper documentation, checklists, training and maintenance are crucial. instead looks like to have had deficiencies in all those elements, one way or another (except the checklist itself which wasn't followed).
- _archon_ 8y ago
  The AF 447 crash is one of the reasons MCAS was created. It's the exact scenario MCAS was designed to prevent: High altitude, high-speed, nose-up stall. This was more likely than the NG and dinosaur generations of 737 due to the engine relocation forward and up (causing a pitch-up moment due to thrust). AF 447 should never have happened. The copilot crashed the plane, and the pilot and pilot on break had no indication or feedback that he was holding the stick back (crashing the plane by causing a stall). There was no feedback, and the controls deferred to his inputs when the pilot was trying the opposite command (which would have saved the flight). On top of this, the stall warning buzzer was turning off when they were so deep into the stall that the airspeed dropped below a certain threshold, and every time the nose was pitched down and airspeed increased, the buzzer came on again. Airbus has a controls feedback, stall buzzer parameters, and training problem. 447 should never have gone down.
  - refurb 8y ago
    This is a valuable comment. Most people ask “why have MCAS at all?”. Well, AF477 shows that sometimes you need it.
  - TheHypnotist 8y ago
    I think you mean low-speed & nose up.

StreamBright 8y ago

Flight augmentation systems work a different way. Those take into consideration all inputs and average it out. Same when two pilots give different instructions.

piquadrat 8y ago
> Same when two pilots give different instructions. That's only true for Airbus (and probably contributed to the loss of Air France 447, https://en.wikipedia.org/wiki/Air_France_Flight_447 https://en.wikipedia.org/wiki/Air_France_Flight_447). In Boeing airplanes, the movement of the two yokes is synchronized.
- neuronic 8y ago
  > In Boeing airplanes, the movement of the two yokes is synchronized. So in case of two equally strong and equally determined pilots you still get averaging of opposing control inputs.
  - wikibob 8y ago
    No, the failure mode is not two pilots consciously fighting against each other. The failure mode is two pilots who are UNAWARE they are commanding contradicting inputs. Airbus negligently averages these inputs. This is legacy tech debt because the physical design didn’t leave room to link the two controls. Now they are stuck because they feel they can’t change and have some planes that (dangerously) average the inputs, and some that are physically linked. When controls are physically linked, if one pilot thinks he should nose up, and the other tries to nose down, they can yell “what are you doing” at each other and this create an accurate mental model of what the other is intending. Think of this every time you fly Airbus. It’s very similar to debugging a production technology issue. Imagine you decide to restore the database to a snapshot from 1 hour ago, and at the same time someone else tries to restore to an earlier snapshot from 12 hours ago. Should the system average these inputs and restore from 6.5 hours ago? No, obviously not.
    - SmellyGeekBoy 8y ago
      > Think of this every time you fly Airbus. Why? They're not falling out of the sky so the system obviously works. Stop being a fanboy.
    - foldr 8y ago
      >Now they are stuck because they feel they can’t change and have some planes that (dangerously) average the inputs, and some that are physically linked. This is a really weird way to construe the situation. The accident report doesn't conclude that the handling of dual inputs was a factor, and it's clear from the transcripts that it wasn't. Airbus has no reason to change this. Merely linking sidesticks would be pointless in any case, since sidesticks don't have an identifiable position (but are used with brief movements away from center). It's hardly any easier for a pilot to passively observe the movements of his own sidestick than it is just to look over at the other pilot's stick.
      - neuronic 8y ago
        The commenter appears to have a vested interest in making Airbus look bad, otherwise the comment cannot be logically understood. Is there a case where the averaging of inputs has been proved catastrophic?
        notfromhere 8y ago
        Air France 447
        foldr 8y ago
        Not according to the official accident report. This is a meme spread by an article in Popular Mechanics that came out long before the official report.
    - deleted 8y ago
      [deleted]
    - benhurmarcel 8y ago
  - kissickas 8y ago
    It sounds far more intuitive if you have a feedback mechanism in the form of a tangible demonstration of what the force is actually averaging out to.
  - philpem 8y ago
    If they're mechanically linked, the captain would at least feel the first officer countering their inputs -- giving a prompt that they need to communicate their intentions more clearly! I'll defer to a pilot to say whether they are mechanically linked -- it sounds like they are on Boeing aircraft but possibly not on Airbus?
    - WalterBright 8y ago
      They're mechanically linked on Boeing aircraft. On the 757, and I believe the rest, the mechanical link has a detent. With enough force, the pilot can move it out of the detent and move the controls independently of the copilot. The reason for this is if one side or the other is jammed, the pilot can still fly it.
    - foldr 8y ago
      No, the captain would not, because only one pilot has their hands on the controls in any given instance (with the possible exception of rotation at takeoff).
      - ceejayoz 8y ago
        Clearly, that’s not always the case, given at least one plane falling out of the sky from dual control inputs. Mistakes happen. Part of aviation safety is preventing that from becoming fatal.
        foldr 8y ago
        >Clearly, that’s not always the case, given at least one plane falling out of the sky from dual control inputs. That's not what the accident report concludes. There's a clearly audible "dual input" alarm to prevent dual inputs. Dual control inputs only occurred for brief moments during the AF flight.
        ceejayoz 8y ago
        It's quite clear that wasn't sufficient. "Brief moments" during critical points in an emergency can matter enormously. From the recorder transcript: > As the plane approaches 10,000 feet, Robert tries to take back the controls, and pushes forward on the stick, but the plane is in "dual input" mode, and so the system averages his inputs with those of Bonin, who continues to pull back. The nose remains high. 02:13:40 (Robert) Remonte... remonte... remonte... remonte... Climb... climb... climb... climb... 02:13:40 (Bonin) Mais je suis à fond à cabrer depuis tout à l'heure! But I've had the stick back the whole time! 02:13:42 (Captain) Non, non, non... Ne remonte pas... non, non. No, no, no... Don't climb... no, no. 02:13:43 (Robert) Alors descends... Alors, donne-moi les commandes... À moi les commandes! Descend, then... Give me the controls... Give me the controls! > Bonin yields the controls, and Robert finally puts the nose down. The plane begins to regain speed. But it is still descending at a precipitous angle. As they near 2000 feet, the aircraft's sensors detect the fast-approaching surface and trigger a new alarm. There is no time left to build up speed by pushing the plane's nose forward into a dive. At any rate, without warning his colleagues, Bonin once again takes back the controls and pulls his side stick all the way back. The crash occurred less than a minute later.

t0mas88 8y ago

The same applies to the trim in a 737 and any other transport class aircraft. If you move the control column in the opposite direction of trim, the electric trim and autopilot trim will cut out. The same applies to the autopilot in all types I've flown, it disconnects if you override it manually (except for CWS mode, but that's known and expected) I don't fly the 737, so I haven't read the aircraft specific books and systems overviews, but I expect this override behavior is very confusing in the MCAS scenario. Because any pilot would counteract a nose-down force by pulling on the yoke. The MCAS would then stop moving the trim, and the pilot would think (as would be true in any other situation) that the override worked. But MCAS didn't stop because of the override, it stops because it always does that after a few seconds and will re-activate 10 seconds later (or 20, didn't read a definitive number on this). The confusing thing is that 95% of the unwanted trim movements are caused by the autopilot, so at this point the pilot would think the situation is under control and probably disconnect the autopilot if the movement didn't already, The other 5% (or probably less) of unwanted trim movements are trim runaway, which is either a stuck switch (2 actually, you depress two switches at once to make it move, pressing 1 does nothing) or an electrical problem. In both cases pulling back works and stops it. If it stays "stopped" you think autopilot and leave it disconnected. If it keeps moving continuously you think trim runaway and flip the trim cutout switches or pull the circuit breaker (this one is usually marked bright orange so you can find it quickly in planes without cutout switches) But in the MCAS case, and without MCAS knowledge (which apparently nobody had) you would not expect it to start moving again later. You would have concluded "autopilot" of the above two scenarios, and after disconnect would think you're safe. So now when it starts moving again, your first idea is that the autopilot did not disconnect which leads to even more confusion in the cockpit. The LionAir flight that was saved by the jumpseater got lucky in the sense that this guy was not flying, not debugging the autopilot, and did see the trim move because he has no instruments to scan and the trim is right in front of his face (in the middle between the two pilots). If you conclude that the issue is the trim, then any pilot would decide to go for the cut-out switches. It's just that concluding the trim is the issue, without knowing MCAS, under high pressure and with the wrong (autopilot) conclusion already on your mind is not very likely.

mlindner 8y ago
Why didn't the pilots adjust the trim manually? I don't understand how any pilot would consider increasing control pressure to be normal and not automatically respond by manually adjusting the trim.

LoSboccacc 8y ago

> why it would continue to drive the nose of the plane down even when the pilot is reefing on the stick to pull up well this other crash was caused by a pilot puling up a plane all the way to its ceiling and stalling it, so there's that, so things can go wrong whether if you do and if you don't allow for easy control overrides. https://en.wikipedia.org/wiki/Air_France_Flight_447#Third_interim_report https://en.wikipedia.org/wiki/Air_France_Flight_447#Third_in... the trick seems to find the right balance to relinquish control only when appropriate, so it seems that proper documentation, checklists, training and maintenance are crucial. instead looks like to have had deficiencies in all those elements, one way or another (except the checklist itself which wasn't followed).

_archon_ 8y ago
The AF 447 crash is one of the reasons MCAS was created. It's the exact scenario MCAS was designed to prevent: High altitude, high-speed, nose-up stall. This was more likely than the NG and dinosaur generations of 737 due to the engine relocation forward and up (causing a pitch-up moment due to thrust). AF 447 should never have happened. The copilot crashed the plane, and the pilot and pilot on break had no indication or feedback that he was holding the stick back (crashing the plane by causing a stall). There was no feedback, and the controls deferred to his inputs when the pilot was trying the opposite command (which would have saved the flight). On top of this, the stall warning buzzer was turning off when they were so deep into the stall that the airspeed dropped below a certain threshold, and every time the nose was pitched down and airspeed increased, the buzzer came on again. Airbus has a controls feedback, stall buzzer parameters, and training problem. 447 should never have gone down.
- refurb 8y ago
  This is a valuable comment. Most people ask “why have MCAS at all?”. Well, AF477 shows that sometimes you need it.
- TheHypnotist 8y ago
  I think you mean low-speed & nose up.