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Why would Boeing, or anyone, ever jeopardize the aerodynamic integrity of an airliner, just to change the way a pilot feels the controls? The more I learn about it, the clearer it becomes that that is really all it is. The reason why MCAS was installed, never made an airplane crash, or even stall for that matter. People who should know, keep on saying on this site it wasn't a safety issue. Then why turn it into a safety issue by installing a solution that rules over the entire horizontal tail section?

Why take such a big risk to solve such a trivial problem? Why take any risk at all? And why would the FAA approve of that?

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    $\begingroup$ If solution A is chosen, but solution A turns out to be bad due to "faulty assumptions" (per the Congressional report), what else is there? I mean, what is missing that you'd like to know? Were mistakes made? Yes; it should be common knowledge at this stage and is covered in other posts here. I fear the question needs more focus. Right now it's a "rant in disguise." $\endgroup$
    – ymb1
    May 16 at 0:52
  • $\begingroup$ @ymb1 That is not what the question is about. I flagged your comment. This is harassment. Why change the way an aircraft flies when that is not a problem? is a perfectly valid question. $\endgroup$
    – Berend
    May 16 at 1:01
  • $\begingroup$ @ymb1 If you read the congressional report it becomes clear it was more than just faulty assumptions. There were faulty assumptions alright , but those were concerning MCAS itself. Not concerning the problem it was the solution for. With all the trouble Boeing had to go through and the risks they had to take to get MCAS flying, the question only becomes more valid. $\endgroup$
    – Berend
    May 16 at 1:46
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    $\begingroup$ Does this answer your question? What is MCAS trying to fix on B737 MAX? $\endgroup$
    – Abdullah
    May 16 at 3:20
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    $\begingroup$ Why solve a control control problem with an aircraft control solution? -- John K's and other excellent answers on this site make it clear that Boeing was solving a business problem, not a control problem. From reading your post history on this site you already seem to be aware of this: The sole purpose MCAS serves is the prevention of costs for extra certification and training. --Berend, May 2, 2021. Therefore, what do you hope to gain by asking this question? $\endgroup$
    – Steve V.
    May 16 at 7:52
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It would make sense if you've ever been involved in a development program. It's important to understand the trade-offs Boeing was faced with. It was either the band-aid, MCAS, or kibosh the program. The "proper" solution to the problem was doing something physical to aircraft; limiting aft C of G, enlarging the tail, stretching the fuselage, or moving the engines back to the original position and making the gear taller. If you had any experience of development programs you would understand the design merry-go-round from follow-on consequences that starts when you make physical changes - round and round it goes, and where it stops, nobody knows (for example; moving the engines forward lol).

Any of those options would drive development costs to the stratosphere and made the program unfeasible from a business case perspective, and it would have been cancelled had the band-aid not been developed. So there weren't really any other options from a practical perspective. MCAS was to address behavioural issues in a little corner of the flight envelope. Had the band-aid been implemented properly, with a proper fault tree analysis, risk assessment and risk mitigation strategy resulting in the proper architecture (like dual AOA sources), MAX's would be flying around to this day with MCAS systems that might in some rare flight configuration actually do something, but 99.9 % of the time just sit there doing nothing.

Like a stick pusher.

A stick pusher is a very similar band aid on lots of airplanes, and they have been around for eons. The pusher is there because the natural stall behaviour is dangerous (typically with supercritical airfoils, and T tails; or even worse, with both) and it intervenes if the pilot gets too far past the shaker. Which means 99.9999 percent of the time, the pusher just sits there and never gets used (although if it does activate, the pilot will know it).

The 787-8 has an even crazier band-aid. It was having wing vibration problems, a kind of low frequency, flapping up and down oscillation (not quite flutter) near the tips because of a lack of stiffness in the structure. The "proper" fix would have been to make the wing box stiffer. The actual fix was to employ the inactive-in-that-regime FBW ailerons to actively counteract the wing motions in the background, without the pilots being aware they were even doing anything. They worked out the software changes to the FBW computers, went and flew it to get the wings wiggling, flipped a switch, and the wiggling stopped.

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    $\begingroup$ That's been covered numerous times. The engine change moved the neutral point forward and Boeing's analysis indicated the stability effects would be negligible, but in specific flight regimes (low speed flaps up) the static stability was a bit too close to neutral (nose drifting up) and caused certification problems and threatened the common type rating. They probably could have just added some tip extensions to the horiz tail to compensate the engine move, but at that late stage such a change would've killed the prog with all the added costs. $\endgroup$
    – John K
    May 16 at 14:21
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    $\begingroup$ Your question suggests a person who is knowledgeable in the aviation community but who isn't familiar with development and certification pitfalls, and the choices and pressures involved. I've had a ringside seat to this sort of thing as an insider (minor supporting role) for many years with a different OEM and when I read about the MCAS fiasco I could almost picture in my mind the internal struggles and battles going on between the engineering and program factions over the decisions that were made that led to the system as developed. $\endgroup$
    – John K
    May 16 at 14:29
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    $\begingroup$ You aren't getting it. Because the alternatives were out of the question from a program feasibility perspective. The massive redesign required to implement any physical fix would have killed it. And as I pointed out, this sort of thing is done all the time. FBW lets you cover the entire airplane in a band aid since behaviour can simply be programmed. The problem was with the decisions made over the architecture. The fatal error was someone was allowed to prevail in making a loss of AOA input a "minor" event (vs major, hazardous, catastrophic) which drove the redundancy of architecture. $\endgroup$
    – John K
    May 16 at 16:27
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    $\begingroup$ The same MCAS software working on the stick pusher rather than on the speed trim can't have been significantly differently priced and requires even less additional training and certification. The vigor applied to make it work as it is, would have worked for that solution as well. The congressional report reveals clearly Boeing broke quite a lot of rules to get the MAX flying. Talking vigor, I have worked on projects that were eventually cancelled for lack of funds. We also broke rules, but robbing a bank was never an option. There's always a limit. $\endgroup$
    – Berend
    May 16 at 16:56
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    $\begingroup$ They made some very bad assumptions and justifications, and the FAA was too passive and didn't challenge them. There has to be a level of trust between the OEM and Regulator or nothing would get done because the Regulator would have to review and challenge every little bit of engineering material submitted, and they don't have the staff for that. And they got burned by it. Anyway, believe me you wouldn't want to use the stick pusher lol. The stick flies out of your hands when it fires. MCAS was supposed to make subtle trim changes in the background to mask the low static stability problem. $\endgroup$
    – John K
    May 17 at 0:51

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