5 ms·

From what I read purpose of MCAS is to make stick give same feedback as on 737 NG, i.e. certain linear force increase would correspond to deflection amount on t

From what I read purpose of MCAS is to make stick give same feedback as on 737 NG, i.e. certain linear force increase would correspond to deflection amount on the elevator. With MAX it was no longer true due to air frame geometry and engine changes, and at certain high angles same amount of force applied to stick would result in more deflection. All this had to be maintained in order to not have re-certify all the pilots from NG but be allowed to only give minimal training I think.

speedplane 8y ago
We don't even know the real problem, how can we make speculation like this. Every article seems to cite "faulty sensor data", but is this really true? Would making the sensor more reliable or having an additional backup sensor have prevented the issue? Is it possible for the software to even detect when the sensor is faulty? Maybe the sensor worked fine, and the MCAS worked fine, but the plane itself has faulty aerodynamics. At this point, it seems clear that MCAS had something to do with the crashes, but it's far too early to point to the root cause.
- markdown 8y ago
  > Would making the sensor more reliable or having an additional backup sensor have prevented the issue? Is it possible for the software to even detect when the sensor is faulty? Maybe the sensor worked fine, and the MCAS worked fine, but the plane itself has faulty aerodynamics. The sensor on the Lion Air flight was 20 degrees off. While it wouldn't have "prevented the issue", a simple disagree light in the cockpit would have told the pilots that the AOA sensor was "wrong" while they were still on the runway. Simply put, the plane was unfit to fly and had this light been installed on that plane, the flight would have never left the ground and the crash wouldn't have happened. Boeing charged extra for this light. Since this has come to light, Boeing have announced that the disagree light will now come as standard on all 737MAX's at no additional cost.
  - ams6110 8y ago
    There is no AOA on the ground. it requires airflow. You would not have seen an AOA disagree until the aircraft was in the air or at least moving at a pretty good speed down the runway.
    - markdown 8y ago
      I read that in another heavily upvoted comment on here in an earlier discussion a week or so ago. I really shouldn't assume that randoms on the internet know what they're talking about. Anyway, thanks for the correction.
- pas 8y ago
  To supplement the other comments: http://nrg.cs.ucl.ac.uk/mjh/lionair.png http://nrg.cs.ucl.ac.uk/mjh/lionair.png
jacquesm 8y ago
> From what I read purpose of MCAS is to make stick give same feedback as on 737 NG No, the MCAS system is not in any way responsible for feedback to the control stick. It engages the stabilizer (tail plane) trim motors based on AOA sensor readings, the state of the autopilot and whether the flaps are engaged or not. It has a limit of 2.5 degrees change but this limit is reset every time the system is reset in effect allowing it to fully deflect the tailplane until the limit stops kick in.
- joosters 8y ago
  Same thing, surely? Altering the trim means that pulling a certain amount on the stick doesn't make the plane pitch up as much as it would otherwise, hence you need to pull more on the stick (than a plane without MCAS). So the (intended) end effect is the same, right?
  - sokoloff 8y ago
    Yes. Stick force curve certification compliance is a primary reason for MCAS to exist.
  - inferiorhuman 8y ago
    No, because what changes is how far you move the yoke not necessarily how hard you have to pull (on the penultimate Lion Air flight the elevator feel system was non-op which meant that the yokes were probably extremely light for the whole flight). There will also come a point where you cannot overcome the stabilizer with the elevator.
  - jacquesm 8y ago
    The bit you put between brackets '(intended)' is the key here. What they intended was one thing, what they got was an entirely different thing. The stabilizer being as large as it is and the degree of authority afforded the MCAS system could cause the tail plane to deflect to a degree that the stick could not overcome it at all, the elevators are simply not large enough to counteract a tailplane that is trimmed purposefully the wrong way to the maximum deflection, in part because trimming the tailplane wrong also limits elevator effectiveness.

speedplane 8y ago

We don't even know the real problem, how can we make speculation like this. Every article seems to cite "faulty sensor data", but is this really true? Would making the sensor more reliable or having an additional backup sensor have prevented the issue? Is it possible for the software to even detect when the sensor is faulty? Maybe the sensor worked fine, and the MCAS worked fine, but the plane itself has faulty aerodynamics. At this point, it seems clear that MCAS had something to do with the crashes, but it's far too early to point to the root cause.

markdown 8y ago
> Would making the sensor more reliable or having an additional backup sensor have prevented the issue? Is it possible for the software to even detect when the sensor is faulty? Maybe the sensor worked fine, and the MCAS worked fine, but the plane itself has faulty aerodynamics. The sensor on the Lion Air flight was 20 degrees off. While it wouldn't have "prevented the issue", a simple disagree light in the cockpit would have told the pilots that the AOA sensor was "wrong" while they were still on the runway. Simply put, the plane was unfit to fly and had this light been installed on that plane, the flight would have never left the ground and the crash wouldn't have happened. Boeing charged extra for this light. Since this has come to light, Boeing have announced that the disagree light will now come as standard on all 737MAX's at no additional cost.
- ams6110 8y ago
  There is no AOA on the ground. it requires airflow. You would not have seen an AOA disagree until the aircraft was in the air or at least moving at a pretty good speed down the runway.
  - markdown 8y ago
    I read that in another heavily upvoted comment on here in an earlier discussion a week or so ago. I really shouldn't assume that randoms on the internet know what they're talking about. Anyway, thanks for the correction.
pas 8y ago
To supplement the other comments: http://nrg.cs.ucl.ac.uk/mjh/lionair.png http://nrg.cs.ucl.ac.uk/mjh/lionair.png

jacquesm 8y ago

> From what I read purpose of MCAS is to make stick give same feedback as on 737 NG No, the MCAS system is not in any way responsible for feedback to the control stick. It engages the stabilizer (tail plane) trim motors based on AOA sensor readings, the state of the autopilot and whether the flaps are engaged or not. It has a limit of 2.5 degrees change but this limit is reset every time the system is reset in effect allowing it to fully deflect the tailplane until the limit stops kick in.

joosters 8y ago
Same thing, surely? Altering the trim means that pulling a certain amount on the stick doesn't make the plane pitch up as much as it would otherwise, hence you need to pull more on the stick (than a plane without MCAS). So the (intended) end effect is the same, right?
- sokoloff 8y ago
  Yes. Stick force curve certification compliance is a primary reason for MCAS to exist.
- inferiorhuman 8y ago
  No, because what changes is how far you move the yoke not necessarily how hard you have to pull (on the penultimate Lion Air flight the elevator feel system was non-op which meant that the yokes were probably extremely light for the whole flight). There will also come a point where you cannot overcome the stabilizer with the elevator.
- jacquesm 8y ago
  The bit you put between brackets '(intended)' is the key here. What they intended was one thing, what they got was an entirely different thing. The stabilizer being as large as it is and the degree of authority afforded the MCAS system could cause the tail plane to deflect to a degree that the stick could not overcome it at all, the elevators are simply not large enough to counteract a tailplane that is trimmed purposefully the wrong way to the maximum deflection, in part because trimming the tailplane wrong also limits elevator effectiveness.