canopus27

I know .... not poking fun at the messenger (you) ... I'm just expressing the profound relief that I'm sure we all feel about the good news that the 737 MAX does not have any design issues.

Resurrection

Nice to know AC rolled the dice with pax safety to save a few dollars. At least they bought both optional "safety" features, as it has just been revealed the sensor that was supposed to warn pilots of faulty data from the angle of attack sensor would only work with those optional safety features (which wasn't disclosed to buyers):

https://www.washingtonpost.com/natio...=.e41a62dea5fb

bimmerdriver

Here's the latest video by Juan Brown:
He has requested an opportunity to fly the MAX simulator, as a credentialed member of the press who happens to have held a 737 NG type rating.

Refer to around 10:15 where he talks about what he refers to the myth that the MAX is not stable and MCAS is designed to make it stable or prevent it from stalling. He says what I said above, which is that the purpose of MCAS is to allow the MAX to be the same type rating as previous versions of the 737.

canopus27

I've never doubted that he said that. He might even believe it (which is not to imply that he's lying, just that part of a journalists job is to simplify explanations of complex topics for the viewer.)

None of that makes it any more true that the purpose of the MCAS system is really type consistency.

bimmerdriver

He's a commercial pilot, flying a 777 for AA. Journalism is his hobby. I find him credible and objective, much more so than anyone on this board. I also think he's probably forgotten more about airplanes in general and the 737 in specific than the combined knowledge of the people on this board. YMMV.

BTW, the use of the word "myth" is his, not mine.

WildcatYXU

I really, really hope that bimmerdriver is wrong and type rating consistency wasn't the most important driver behind the MCAS implementation The idea of 300+ pax paying with their lives for some airline executive's desire not to pay for pilot training makes me nauseous.

canopus27

To my knowledge, Boeing has not ever formally stated why they implemented MCAS. We do have, of course, the CEO's recent confirmation that it wasn't a flawed system {yes, I'm cynical} but that's all.

@bimmerdriver has found his source, and as I wrote above, it's clear that Juan Brown has an opinion about the design motivation, and as a journalist he is competent at making his case.

My sources are not nearly as cleanly packaged as a Juan Brown video, but to me they feel more credible. YMMV.

For the record, I have no doubt at all that Boeing absolutely wanted type consistency between the MAX and the other 737 variants, and I'm equally convinced that reducing the MAX pilot training and transition requirements were all key goals of the MAX. Whether or not those factors were causal to the implementation of the MCAS is not publicly known.

The argument that I believe, is that MCAS was introduced to provide compliance with the Federal Aviation Regulation (FAR) § 25.173 "Static longitudinal stability". You can read the regulation here:
https://www.govinfo.gov/content/pkg/...-sec25-175.pdf
Note specifically the section "Demonstration of static longitudinal stability". My read is that it's this required test that the MAX failed without the MCAS:
The section in question is quite readable .... but the key question ("how did the 737 MAX test aircraft actually perform and demonstrate compliance [or not] with that section") is unknowable without lots of detailed test information that Boeing have not make public.

I was initially pointed at this issue (compliance with 25.173) by discussion on pprune. As I said, Boeing has never formally discussed the official reason for the MCAS, so there's no definitive smoking gun that I've found.

The most detailed analysis I've seen is here: https://www.satcom.guru/2019/03/regu...n-systems.html
And the cleanest (most concise) summary I've seen is here: 737 MAX - MCAS

RangerNS

If the MAX was not in compliance with § 25.173, it wouldn't be able to get any type certificate, then it obviously couldn't maintain a common type certificate as the rest of the 737s. It isn't an either/or question.

canopus27

One additional comment:

Even if my interpretation is correct, and the MCAS is required in order to provide compliance with the regulations -- there is still plenty of opportunity for you to feel nauseous about the implementation.

The satcom link that I provided, starts with a discussion of a Seattle Times article - which, in turn, claims that the reason the MCAS was implemented using only a single sensor, was because the alternative (using multiple sensors) would have led to a chain of events which in turn would have resulted in pilots needing simulator training - and that's something Boeing wanted to avoid.

Yes, there's speculation and extrapolation in the article .... but even so, it's getting harder to imagine a scenario that doesn't make you nauseous about short cuts and compromises that Boeing took.

You know, this Boeing: Boeing CEO says 737 Max was designed properly and pilots did not 'completely' follow procedure

canopus27

Agreed.

But the claim is that the motivation for MCAS was to provide type consistency. My belief is that (as you said) type consistency was a key goal of the MAX, but it's not the case that the pre-MCAS MAX was compliant with the regulations and failing only on type consistency; thus, the MCAS was introduced to provide regulatory compliance, not type consistency.

RangerNS

Yeah, but the regulation was not some arbitrary irrelevant point. The Venn diagram of design options are concentric.

§ 25.173 essentially is "must fly and be controlled reasonbally like a passenger aircraft".

The type consistency demand makes that more stringent to be "must fly and be controlled reasonably like other 737s".

It is not logically possible to design something that "flies like a 737" that doesn't also meet "flies like a PAX aircraft".

canopus27

We agree.

Of course, it is possible to build a plane that is fully compliant with all FAR regulations, but doesn't fly like a 737. We also know that the MCAS was a late add to the MAX, designed to solve a problem. Thus, there are two options about what that late problem was:

• The pre-MCAS MAX was fully compliant with all regulations, but did not fly like other 737's. The MCAS was added to correct that.
• The pre-MCAS MAX was not compliant with the FAR's (which implies, as you write, also not consistent with previous 737's). The MCAS was added to correct that.

My belief is that it's the second scenario.

Juan Brown, in his latest clip

You Tube

, states
There are two flaws here. First, it's ambiguous when he uses the term "an inherently unstable aircraft". That term is not formally defined, and so I'm choosing to use a more precise term, "not compliant with the FAR regulations". Secondly, his argument is that because the MAX passed certification, therefore it's stable (compliant), and therefore MCAS was not required to provide that stability (compliance). The problem is that his argument does not prove the pre-MCAS aircraft was also stable (compliant). Perhaps it was, perhaps it wasn't ... but you can't use the fact that the post-MCAS aircraft passed certification, to prove that the pre-MCAS aircraft was inherently stable (compliant).

My reading of his words, makes it clear to me that he believes the MCAS system is not related to the 737 stability, and (by implication) the MCAS system is not related to compliance with the FARs. In other words, he's asserting that the first of my above options is the one that's true.

OK, but based on my understanding of the facts, I disagree; I think it's the second of my two options.

skybluesea

delete

JustSomeGuy1978

you think (or expect) walmart employees check all the electronice (either the product iself or the certification, or both) that enter their store??? Yeah right.

maybe the analogy wasnt so great given that there are literally millions of electronic products covered under CSA vs the smaller production rates and grester complexity of an aircraft.

would you rip apart your new home (probably a much closer analogy) after the builder deemed it passed inspections just to ensure that everything was indeed built to code and standard. If you would might i suggeat that you are a bit more thorough/ocd than most people.

RangerNS

MCAS is an integrated feature. It isn't an option. The warning light option tells the pilots that the AoA sensors disagree, nothing more. It does not follow that that option being available would trigger a line of questioning to asking about a buried standard feature.

I can understand why more cost conscious airlines would skip the option. It doesn't detect disagreement with, say, the artificial horizon, or other sensors that can provide a sanity check on AoA sensors, or for that matter, the pilots eyes and body, simply looking outside.