My Letter to the Editor of New York Times Magazine

When you're talking aeronautics, you should use the definition used by aeronautical engineers.

I'm stretching to the limit of my understanding of the matter here but I'll try outline why that statement is misleading.

To put it in basic terms, you want to avoid a situation where an aircraft can enter an 'accelerated stall'. I believe this to be the correct term.

In the original documents to the FAA I think the exact quote of the wording was:

> MCAS “was added to address potential nose-up pitching moment at high angles of attack at high airspeeds outside the normal flight envelope.”

Specifically this was in a banked downward spiral, known as a 'wind-up turn' where instead of maintaining constant resistance on the control column, at a certain point the resistance to pulling back would lessen and this could allow entering an accelerated stall situation.

NOTE - from there I think MCAS ballooned into other areas of control, but I believe that the original intent of MCAS.

The reason this possibility arose was because the engines were moved forward, altering their thrush relationship to the center of gravity of the aircraft and, as I understand it, by the design of the engine pods themselves (flat bottomed and can add lift).

Under normal flight conditions an airliner would never enter a wind up turn, but it is a test that must be passed as part of the certification process, as far as I understand it.

So yes, there are circumstances where the characteristics of the 737 Max can make it easier to enter a dangerous nose up pitch. Note only 'easier' i.e. less resistance on the control column so pulling back with the same force in the same circumstance through the duration of a wind-up turn could allow it to be easier to enter a stall state.

But in normal flight and under conditions an airliner would fly, the nose up pitching moment caused by the engines would be entirely 'normal' and in line with what many airliners do (i.e. control column resistance would remain linear) but would have caused the handling to be different is very specific circumstances and so would have needed a new type rating and pilot training to address.

MCAS sought to avoid the need for that type rating by automatically applying trim to prevent any situation arising, even outside the normal flight parameters, that would cause different handling characteristics.

Again, none of this relates to stability/instability as was my original point.

Sources:

I read about this level of detail here quite a long time ago:

https://www.seattletimes.com/seattle-news/times-watchdog/the...

In looking up this article I found this aviation.stackexchange.com answer that also comments on this:

https://aviation.stackexchange.com/questions/66799/what-is-m...

https://news.ycombinator.com/item?id=21249017

Not for nothing, how is this dismissal of terms helping forward understanding? This is in part a technical discussion with intelligent individuals. If avionics takes ‘stable’ to have a special meaning, then denying the term’s special meaning undermines credibility of those who are working so hard to hold Boeing to account.

http://bit-player.org/2019/737-the-max-mess

That malfunction manifested as the plane pitching down consistently. No matter what the input of the stick, it'd point to the floor.

Now, if a big flashy light+warning buzzer had been sounding (like with a stall warning) then this would have never happened. But it didn't, and because the pilots were not trained for this specific feature (cause it had the same rating) They didn't know what to do.

Its nothing to do with stable. the MCAS silently pointed the plane at the floor regardless of user input.

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.

For example.

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

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

Both things can be true: 1) Many pilots are trained in assembly-ling training 2) Boing did not do adequate testing.

Often discorse seems to follow the lines of bracket tournaments: only one thing can be true.

SpaceX's (almost) entire existence is centered on a cost cutting measure: reusing rockets. But they didn't approach it in a way that killed a bunch of people.

Unfortunately, accounting methods and markets don't distinguish between these, especially where benefits (or profits) are immediately evident, but costs are occult and obfuscated. This is a situation which appears frequently, repeatedly, and notoriously in technological contexts. I'm seeing it increasingly as a fundamental conflict and paradox of market economics.

The issue can work in reverse as well: measures (practices, products, technologies, services) with immediately evident costs, but distant, obscure, or covert benefits, can be a very hard sell. Maintenance. Employee training. CO2 mitigation.

My understanding of dutch roll is it takes only a small perturbation to push it off of equilibrium and into oscillation, the oscillation is sickening to people, and it doesn't return to equilibrium without active correction (by the pilot or an active system). A small perturbation means it happens more or less continuously. Correcting the oscillation rather than making it catastrophically worse is something only a well trained pilot can do successfully.

The stable point is not straight and level, it's lurching around sickeningly. I mean that literally, at Boeing I was told about a 707 crossing the Atlantic when the yaw damper failed. The pilot didn't crash it, but the airplane was full of vomit by the time it arrived.

To be technical, it is not stable flying straight and level. The stability point is dutch roll, like a carbureted car does when the engine rpm "hunts" at idle.

They make mistakes, and in particular some of these are mistakes that are more likely when you're flying for your own reasons (usually leisure) rather than because it's a job. "Get-there-itis" is an example, an airline pilot sees the weather at Nashville looks too dodgy, they confer with their colleague, then divert. "Sorry ladies and gentlemen, looks like Nashville's having some attrociously bad weather this evening, we're going to get you on the ground at... Charlotte and make alternative arrangements". But a private pilot father who promised to attend his daughter's performance this evening is damned if he'll divert and miss it, he's going to land at his intended destination. When he realises he might miss the performance by being dead, it's too late.

Of course unexpected catastrophic failures do occur, but I've not seen any statistics that claim that they are a prevalent cause of general aviation crashes.

All that being said, in the case of the MAX, I find it unfathomable to blame pilot error, given the lack of training on the MCAS system.

If there is a lack of manual flying ability it is entirely due to culture and not the aircraft. Pilots can and do turn off the autopilot if they want to practice, and most landings are done manually because it is softer.