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.

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).

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.