My father-in-law is the former chief test pilot for the RAF. And he was Head of Flight Test and Evaluation for large parts of the Eurofighter programme (now retired). He has decades of pilot, engineering and aircraft design experience.

We have had numerous discussions on this topic and any pilot with an understanding of the definition of the term will tell you that the 737 Max is inherently stable. My comments parrot his words.

Many here have to understand that learning about these things is not compatible with a 5 minute HackerNews comment. There is a good deal to educate yourselves about. One excellent series to do so comes from Juan Browne - a current 777 pilot:

https://www.youtube.com/watch?v=s3LrsvaCUoo&list=PL6SYmp3qb3...

Yes, that's a playlist, yes watching all of them is necessary to understanding the true nature of the problem. If understanding this was a simple 5 minute thing then you can expect pilots with years of direct training to be able to deal with it. The fact something has gone this catastrophically wrong should give sufficient indication that this is a complex and difficult subject.

There is also clearly a massive misunderstanding about what stability is. This site has a good overview:

https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types...

To misunderstand this means one will misunderstand what went wrong with MCAS. MCAS is about trimming the aircraft (moving the horizontal stabniliser) in order to recreate the exact same handling characteristics as the previous generation 737's.

On the 737 Max (and most aircraft actually) there is a pitching moment that results in the engine nacelle producing lift. Most aircraft will do this to some extent - more power = increase in altitude for many aircraft. The 737 Max is not unusual or unstable in this regard. The important thing to know is that the pitching moment on the 737 Max was different enough due to design and engine changes that the aircraft handled slightly differently in certain phases of flight and in certain flight configurations - namely high angle of attack.

Because the aircraft handled differently to previous generation 737 it weould have needed a new type rating and that would have likely cost billions of dollars in certification and pilot retraining. Boeing saught to avoid this through the use of MCAS.

The important things to know are:

1) If an aircraft model retains the same type rating as previous generations then pilots don't need retraining, this saves money.

2) The 737 Max engines and mounting design produced a slightly higher pitching moment in certain phases of flight and under certain flight configurations than previous 737 designs. This was enough to make the handling different - not unstable, but different. In general terms, this would require slightly different trim level to 'balance' the aircraft under certain flight configurations (notably high angle of attack). Balance does not relate to stability/instability, it relates to balanced aerodynamic forces on flight control surfaces and ensures pilots don't have to continually input control stick movement to counteract an unbalanced setup. Trimming an aircraft is piloting 101 and noting to do with inherent stability.

3) To keep the same type rating, Boeing decided to create an automated system that adjusted the trim on the 737 Max automatically in order to cause the aircraft to handle in the exact same fashion as previous generations 737 under all phases of flight. This automated system allowed the 737 Max to retain the same type rating as pilots flew an aircraft with the exact same characteristic thanks to the automation provided by MCAS.

What went wrong:

MCAS had too much authority and took input of flight data from too few sources. This meant that when those inputs were erroneous, MCAS was able to apply greater nose down trim than could be overcome with other flight control surfaces. In order to recover from this pilots would have to pull the stabiliser trim cutout switches and manually trim the aircraft. Again, this is nothing to do with stability/instability and is piloting 101 stuff.

The problem was compounded further because the failure mode of this runaway trim condition was not clear to the pilots. It presented as a series air speed indication and other warnings with a the highly disconcerting addition of automatic nose down trim. This multi-failure presentation can quickly overwhelm a pilot and make identifying the correct course of action difficult.

Further, stabiliser trim cutout is a memory item, it's not contained in a checklist. So human factors, overwhelming error indications and the fact MCAS was not even explained fully in early manuals meant that pilots didn't immediately know what the issue was. When there is just a few seconds to identify the problem before crashing, these factors combine into a fatal outcome.

Excellent reply, thank you.

Again, this is not aimed at you but the general posting without underpinnings that goes on far too much.

My problem is that when this plane is discussed there are some strange posts that I don't trust (they try to spread blame to the pilots, to question other countries' flight safety, smear things out). Also there are claims from people who may have just done some reading, and kudos to them but they aren't experts. Without more info a post like yours may fall into either of these camps, or you may be an actual expert, or quoting one. Clearly the latter here, so I can trust your info now.

> about these things is not compatible with a 5 minute HackerNews comment

Actually they are. If a verified expert they can present a lifetime of experience in a short sentence, then do. With any luck these threads will become a fraction the length and much more informative.

I upvoted your original comment. You raise a valid and important point that I should have taken time to address without prompt. As such you added critical value to the discussion.

Overall excellent posts on this thread.

> stabilizer trim cutout is a memory item, it's not contained in a checklist

All memory items are contained (in written form) above the dashed line on the checklists. Memory item means crews are expected, trained, and tested to execute those actions prior to referencing the written checklists. It does not mean they aren’t on the checklist.

Screenshot: https://i.stack.imgur.com/cRG4J.jpg

Ah, excellent addition thank you.

> If an aircraft model retains the same type rating as previous generations then pilots don't need retraining, this saves money.

It also makes the airplane safer if pilots can transfer their experience and training from one aircraft to another, as it reduces the likelihood they'll do something right for one airplane but wrong for another (there have been multiple crashes due to this sort of mistake, even when they were correctly trained for each type).

Yes, the commonality of pilot experience is an important safety consideration, and something that Boeing has been praised for in the past, but it is the the breaking of that principle (or, more precisely, the attempts to downplay the differences introduced by MCAS) that is the issue in this case.

"On the 737 Max (and most aircraft actually) there is a pitching moment that results in the engine nacelle producing lift. Most aircraft will do this to some extent - more power = increase in altitude for many aircraft."

Minor side note: More power is supposed to increase altitude. (An aircraft is trimmed for a given airspeed; adding power injects more energy into the system. Then, it doesn't go faster, it goes up. To go faster, lower the nose. To go faster without losing altitude, you lower the pitch and increase power.)

For aircraft with engines below the center of mass, thrust tries to rotate the aircraft upwards around the center of mass. In un-accelerated flight condition, this is counteracted by other forces. But adding thrust increases the force and causes the aircraft to pitch up to a new angle based on the new thrust. This is aside from the more power = more altitude effect.

A second-order effect is that the engine nacelles act aerodynamically. (They're wings, albeit poor ones.) When pitched up, they generate more lift. Because the 737 MAX's nacelles are in front of the wing, their increased lift causes another pitching up force.

None of these forces affect stability; the aircraft will arrive in another stable state shortly after you change the power. However, they can lead to other effects like the control force changes you clearly describe in other comments.

Edit: Source: I slept in a Hilton last night.

Yes: the distinction between pitching force from engine thrust, and pitching force from engine nacelle aerodynamic lift, is essential and, here, frequently elided.

MAX has more powerful engines with center of thrust lower than in NG, and they are both physically bigger and mounted such as to exaggerate an already increased aerodynamic pitching force.

The distinction is frequently neglected in discussion here. Both effects contribute to MAX's different handling characteristics, but differently in different flight regimes.

The result is that it is tricky to make a MAX fly like an NG, particularly while trying to conceal the system used to do it.

That's a really good summary of the issue. Does your father-in-law have an opinion on if the MAX should fly again and what it would take to make it safe?