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Comment by jacquesm | Hacker News Reader

> Boeing was worried that #1 would cause the plane to nose up during high angles of attack

Your #2 is flawed. Shifting the center of gravity forward would cause the plane to nose down relative to the older model. The problem is that the engines are more powerful, mounted higher and further forward shifting the point of thrust to a location where the plane would rotate further (nose 'up'), that is why MCAS attempts to push the nose down by trimming the tail plane if it senses the plane to be in a too high angle of attack.

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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.

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.

> 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.

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.

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.

To supplement the other comments: http://nrg.cs.ucl.ac.uk/mjh/lionair.png

> 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.

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?

Yes. Stick force curve certification compliance is a primary reason for MCAS to exist.

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.

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.

This.

The engines are further away from the plane's fulcrum, and this means that they can now cause the plane to tilt up more efficiently.

Yes both points 1 and 2 seem the wrong way round. Presumably the centre of lift has moved relatively further forward than the centre of gravity to create a greater pitch up moment (which apparently exists on older models).

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