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Comment by _ph_ | Hacker News Reader

All aircrafts with engines below the wings have the pitch up tendency at high angles of attack. The only problem with that is, that it is required that the stick force the pilot needs to apply need to increase with high angles of attack. Therefore, most airplanes have some way of augmenting the stick forces in those domains.

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All airliners have a cylindrical body projecting forwards from the wing, which produces a pitch-up moment at high angles of attack: it is called the fuselage [1]. Normally, the effects of all such destabilizing aspects of the design are compensated for by the correct positioning and sizing of the wing and horizontal stabilizer.

Most airliners now flying do have additional augmentation of their handling for a variety of reasons. Earlier versions of the 737 already had the elevator feel and centering unit and mach trim, but they are much less aggressive than MCAS, and, most importantly, their presence and full scope of effect was not obscured in order to avoid the cost of training pilots for their failure modes. Furthermore, in other cases where handling and stability augmentation is powerful, adequate redundancy is provided (even the 737 feel and centering unit has more redundancy than MCAS.) The presence of handling augmentation devices on other airliners does not justify the way Boeing introduced MCAS.

[1] https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stabil...

I really appreciate the increasing levels of technical accuracy the further this thread grows. Hacker News draws in subject matter experts like moths to a flame.

Reminds me of the old futurama quote- technically correct is the best kind of correct.

Using those Positive, Neutral, and Negative stability terms from PuffinBlue's comment, is the problem that we want positive stability, but the MAX without MCAS exhibits negative stability at a certain part of its flight envelope?

That is, if the pilot is holding the airplane with a constant amount of force and the airplane is climbing at a certain angle, some turbulence or other disturbance could cause the nose to pitch up, and then continue pitching up?

No. The Max exhibits positive pitch stability in all flight regimes. What it fails to do is have sufficiently linearly increasing stick forces (required by certification) at high pitch.

It’s still aerodynamically stable (not neutral or divergent).

Just trying to understand the nuances of terminology here, but does increasing thrust count as a 'disturbance' in defining the stability regime?

Meaning if the plane is under increased power and continues to pitch up to the point of stalling, is that an indicator of a negative stability regime?

If it would continually increase pitch without bound, that would be described as negatively stable (a ball sitting on top of an upside down capital U. If it "didn't care", that would be neutrally stable (a ball sitting on a horizontal plane). If it tended to reach stability (even if at a different pitch than originally), that's positive stability (a ball sitting in an upright U).

The 737-Max does pitch up with increased thrust (as does any jet airplane with a low center of thrust), but it still reaches a stable pitch.

The problem in hand flight is that the controls on the Max don't get "progressively stiffer" in a linear fashion with increasing pitch. They still require positive force, but the force required to pull 2.0 G is less than double the force required to pull 1.5 G (as an example; I don't know the particulars of the Max stick force gradient curve).

I have not read anything to suggest that a normally trimmed, MCAS-disabled Max would pitch itself into a stall from thrust application. That would be bad, but is also almost surely far from the actual situation.

Thanks for the clarification. Do you know if there are rules/regulations regarding the control inputs? I.e. is linear feedback necessary or just a preferred/expected feel? Without much knowledge of the system, wouldn't a non- linear input be the "expected" feel of a purely mechanical system because the drag increases proportionately to the square of air speed?

Yes, there are certification requirements spelled out in FAR 25.175.

https://www.law.cornell.edu/cfr/text/14/25.175

The requirements have the form of "In configuration <XYZ>, the stick force curve must have a stable slope at all speeds within a range which is the greater of <range definition> above and below the trim speed."

It is this linear stick force curve requirement that was failing that MCAS was implemented to address. The airplane is still aerodynamically stable.

I am fairly sure I recall reading somewhere that because the engines on the Max are mounted farther forward than previously, there is a region of high AoA and high thrust where the pitch-up force increases with AoA.

I can't vouch for this. It's possible I misunderstood it; it's possible the source was misinformed, though I seem to recall it was written by a pilot. But I have trouble imagining what lesser problem would have required such a heavy hand as MCAS to fix. If it were really just a matter of the controls going light, surely that could have been cured with motors that were not so powerful as to be almost impossible to overcome.

> I have not read anything to suggest that a normally trimmed, MCAS-disabled Max would pitch itself into a stall from thrust application. That would be bad, but is also almost surely far from the actual situation.

?? Most aircraft will do this. Even docile trainers.

https://www.youtube.com/watch?v=DZfOtvjJR-I [edit: watch this instead https://youtu.be/6RtVdmsx4qU?t=71 ]

There's been a couple 747 crashes due to this exact scenario. Or regarding the 737 in particular, to quote "Mike734" in 2007:

> In the B-737 too much nose up trim can make a go-around very exciting. The under wing engines create a very large pitch up when adding full power for a go-around. The pilot has to really push hard to stop the jet from pitching too far nose up.

and "Cac737":

> Now in the Boeings for example, B737 and bigger with underslung engines, this situation is aggravated even more because when you push the power levers forward for go-around thrust, due to where they are and their thrust lines, that act alone will cause the nose to rise very noticeably, and if you are light you can actually find yourself pushing forward on the control column on a go around. now trim "back" as you say and forgetting you did so, can find yourself in a very nosehigh attitude if not careful.

https://www.airlinepilotforums.com/hangar-talk/17446-trim-la...

> ?? Most aircraft will do this. Even docile trainers.

That might be, but the video doesn't demonstrate it - the pilot flying pulls the elevator back quite a bit to induce the stall after he puts in full power.

I agree it's a poor demonstration-- he helps it the last bit of the way to the stall. But look at how much it pitches first...

Unfortunately there's two videos I found of the trim stall demonstration on youtube-- this one which shows most of it with a full cockpit view, and one which I think is a bit better technically but has the camera shaking everywhere.

See FAA AFM 4-12 for official training materials. https://www.faa.gov/regulations_policies/handbooks_manuals/a...

Here is a better demonstration:

https://youtu.be/6RtVdmsx4qU?t=71

The plane goes way above the normal climb attitude. He doesn't take it to the full stall, but has to push heavily on the control column to arrest it-- he never pulls.

(Really, it's pretty astounding how hard you have to push, and how high the nose rises anyways-- even if you weren't fully trimmed for glide, and even in mild-handling airplanes... I tend to fully trim for whatever flight condition I'm in, --except glide-- and then am holding light backpressure just to control this).

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