[VBprogrammer]

> The short answer to your first sentence is that MCAS was needed in order to make the MAX so similar to the NG versions that it could be claimed that no additional training was needed.

I think that places the emphasis in the wrong place in a tabloid headline kind of way. The reason for MCAS is so that the 737 Max passes the certification requirement that the pitch controls cannot get lighter on the approach to a stall. It's correct in that clearly the previous 737 were certified as meeting this standard, but it's wrong in that even if the pervious 737 didn't exist this would still be a requirement of certification.

[mannykannot]

Yes, I had realized my mistake and corrected that, apparently while you were composing this reply. I think the current version covers your objection.

Bjorn's Corner in Leeham's has a lot of information on the topic.

[VBprogrammer]

It's probably splitting hairs at this point but I don't think that MCAS was nessasary to avoid having different training requirements between the NG and the Max.

The Max may not have been certified without MCAS.

The existence of MCAS certainly seems to have been brushed under the carpet. It's probably fair to say the reason was to avoid creating additional training requirements.

[mannykannot]

Perhaps the way to put it is that MCAS was necessary for certification, to correct a handling change caused by the installation of larger engines. Separately, the 737 MAX, in its as-produced form (which included MCAS), was considered similar enough to the NG that additional training was not thought necessary. Once Boeing had ruled out design choices such as a longer undercarriage with the engines moved back, or a larger stabilizer (if the latter would, in fact, have helped), the MAX without MCAS was not an option.

[cmurf]

I follow all of this, but something doesn't pass the smell test. Disclaimer, I'm a pilot, former CFII, but I don't have much knowledge about aircraft certification requirements.

I cannot comprehend feature XYZ that helps achieve aircraft certification, that can also be disabled by the pilot. Either feature XYZ is mandatory for certification or it isn't.

I can imagine a feature that provides better handling behavior or safeguarding. But if it can go crazy in a way that it's routine to disable such a feature, it must be mandatory the pilot know about the feature's operation, and they must demonstrate competency at handling the aircraft when the feature is enabled and disabled.

And all of that tells me I don't know the full story yet.

[VBprogrammer]

I don't think disabling MCAS (or electric stab trim) should be routine. Given most airliners have given up putting trim wheels in the cockpit I'm sure the reliability of electric trim is very high.

Reading between the lines this system was added as a bit of an after thought. There are plenty of systems which have control of trim so I think they probably didn't give it the respect due to stabilizer trim.

There is always the possibility we haven't got the full story. I'm going to check the full preliminary report from Lion air but from what I've read there is some strange behaviours on the trim system that aren't fully explained yet, even by this half baked fix.

[cmurf]

I don't see how MCAS obviates a positive static stability requirement, because it can be disabled. But I admit I'm not familiar with FAA requirements in this area.

If the airplane has substantially different pitch behavior, that usually means there'd be a type rating requirement on the pilot, not a lack of airworthiness certification for the airplane. So I'm not really clear on what behavioral requirement MCAS is mitigating. And further I'm not clear how something that can be disabled can help with either aircraft certification or a obviate a separate pilot type rating.

e.g. fly by wire airplanes have various layers of safeguards in place, and pilots type rated for a particular airplane (or models in that same type) are required to understand those safeguards and how the airplane behaves when they aren't in place.

In the 737 MAX case, it's very weird to me that MCAS is somehow a requirement on the one hand, but then it can be disabled without pilots understanding the alternate behavior on the other hand.