[ReGenGen]

We don't know the MAX is inherently unstable. That's still conjecture at this point. MCAS may have altered fight characteristics to match older 737 to avoid additional pilot training. If the MAX is inherently unstable, then this scandal is much bigger and far reaching.

[halter73]

I heard the MCAS was required to avoid a situation where the MAX, after reaching a certain pitch, would continue pitch up into a stall even if both pilots released the yoke. I don't know if that's considered "inherently unstable", but this is against FAA regulations for any commercial airplane from what I heard. That's probably a big reason why Boeing made it so difficult to completely disable the MCAS system.

My understanding is that MCAS altered fight characteristics not only to match older 737s and avoid additional pilot training. MCAS altered fight characteristics so the FAA would approve the MAX as a commercial aircraft period. The fact they could match older 737s flight characteristics for a more speedy approval from the FAA was just gravy.

[cyphar]

The article is making the point that the MAX is inherently unstable because of the larger engines causing the "pitch up" problem with an increasing angle-of-attack:

> Pitch changes with increasing angle of attack, however, are quite another thing. An airplane approaching an aerodynamic stall cannot, under any circumstances, have a tendency to go further into the stall. This is called “dynamic instability,” and the only airplanes that exhibit that characteristic—fighter jets—are also fitted with ejection seats.

So arguably the existence of MCAS in the first place indicates that the aircraft design is dynamically unstable (otherwise MCAS wouldn't have been necessary).

[ReGenGen]

There are several respected industry professionals that believe the MAX is inherently unstable (and maybe they have insider information...) but this is not considered "fact" right now. (AFAIK)

MCAS was needed to maintain original 737 type specification which allows 737 pilots to fly any 737... significant operational flexibility & cost savings for airlines.

Commentary from blancolirio who is a current 777 pilot. https://www.youtube.com/watch?v=zGM0V7zEKEQ&t=0s

[cyphar]

I'm not in any way an authority when it comes to aeroplanes.

However, my understanding is that the reason why MCAS was needed to maintain the original 737 specification is because of the "pitch up" behaviour on increased AOA (which is what is being described as "dynamic instability" in TFA).

The video you linked doesn't disagree with this -- though it's phrased as being primarily there to "replicate the same feel as earlier versions of the 737, by giving a little bit of nose-down trim". The article claims that being dynamically unstable means that at high-AOA you get nose-up lift (I'm not a pilot or aeronautics expert, so this might be an incorrect definition -- but I've not seen anyone disputing that definition nor disputing it's against FAA guidelines).

If you need an additional system to "replicate the feel" of not having nose-up lift at high-AOA that tells me that your plane design must therefore have nose-up lift at high-AOA. The guy in the video then goes on to say that it's an inherently stable design, but he doesn't really qualify it (other than saying that all other 737s are stable designs) and goes on to say that "the nose goes a little bit light".

Obviously we should hold back judgement until we know all the facts, but "the 737 MAX is an inherently stable design" is not someone holding back judgement.

[yason]

But wasn't this explained in the article, already. Dynamically stable plane doesn't aggressively rotate along any of its axis, for example, when you increase or decrease throttle. The location of MAX engines generates additional forces to increase pitch when engine power is increased, and even more so when the plane is already pitched high. Earlier 737's could do without MCAS because they were designed with smaller engines in lower locations in order to be dynamically stable.

[pps43]

You are referring to thrust asymmetry, not dynamic stability. Thrust asymmetry is actually not that different between MAX 8 and NG. MAX 8 engines have more thrust, but they are also mounted higher (closer to the centerline), reducing torque.

Dynamic stability is a tendency of the plane that flies straight and level to maintain this straight and level flight. MAX 8 still has this property, MCAS or not.