Comment by chx | Hacker News Reader

Comment by chx | Hacker News Reader

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Facts:

1. Boeing was running multiple studies to replace the 737 -- and possibly the 757 with the same plane -- https://web.archive.org/web/20090615055919/http://www.flight...

2. Boeing Chairman President and Chief Executive Officer Jim McNerney in 2011 February said

> We’re gonna do a new airplane. We’re not done evaluating this whole situation yet, but our current bias is to not re-engine, is to move to an all-new airplane at the end of the decade, or the beginning of the next decade.

For source, put that quote into Google, it's everywhere.

3. Less than a year later, Southwest Airlines bought 150 of this very re-engineered plane. The 737 replacement plane disappeared. The 757 still has no replacement, twenty years after it was canned. The hole this created for the above 200 seats segment is well visible: https://i.stack.imgur.com/Qc47G.jpg (I posted this image to SE after digging it out of some ancient blog post)

4. The MCAS system exists because SWA insisted on no retraining

> Not only did Southwest management not want their pilots who flew the earlier model 737 NG to have to train in a flight simulator for the MAX, they insisted to Boeing that even classroom training be off the table, the filing shows. Southwest insisted on a clause in the sales contract stipulating a penalty of $1 million per airplane delivered if that standard wasn’t met.

https://www.seattletimes.com/business/boeing-aerospace/legal...

Rumor:

It was Southwest Airlines threatening Boeing to buy Airbus instead if it delivered anything but a 737 to them that created the 737 MAX in the first place.

Now you know how far a single-plane airline would go to avoid buying a different plane.

> The MCAS system exists because SWA insisted on no retraining

This is somewhat inaccurate (and appears to trade on the oft-repeated but completely incorrect misunderstanding that the MCAS acts as a sort of “normal 737 emulator”).

In fact, the MCAS exists because without it the MAX would be 100% uncertifiable for commercial flight.

The central problem is that, due to the lopsided aerodynamics of the MAX, as the nose of the MAX moves up and approaches a stall, the forces on the pilot’s stick “invert” such that it becomes easier to continue to pull into the stall than push away from the stall.

Commercial aircraft are not allowed to have inverted stick forces at any angle. When Boeing realized this, they decided to implement a kind of hack that would use the trim to put pressure on the nose downwards to counteract the forces on the stick such that they never invert. The MCAS is the system that commands this downwards trim.

The desire to avoid retraining led Boeing to conceal the existence of the MCAS, but it is not why it exists. The plane flatly is uncertifiable without it. No amount of training would change this, as inverted stick forces are a failure of fundamental airworthiness requirements.

The plane should have also been uncertifiable as MCAS lacked sufficient redundancy in air data or computing for so much control authority. It's not clear whether this was deliberately concealed by Boeing or if they had just lost the institutional memory to know that this solution was inherently unsafe.

> The plane flatly is uncertifiable without it. No amount of training would change this, as inverted stick forces are a failure of fundamental airworthiness requirements.

I didn't know that. In that case though, would it even be allowed to disable MCAS as part of any SOP? As far as I recall the documentation did mention one obscure process indicating to the pilot to disable MCAS when it acted up like it did during the accidents.

But I can't imagine it would be allowed to even have this as an operating procedure this when it puts the plane in a situation where it can't be certified?

The whole thing is such a mess... It's clear they should have just redesigned the whole airframe to modern standards IMO.

As was mentioned earlier, MCAS exists to modify the stick forces in certain circumstances. Those circumstances never occur in anything near normal flight. And if they do occur it is not like the stick force deviation from what regulations allows makes the plane not flyable. I've seen it described by pilots who have tested it as it makes it have a slightly softer feel for stall recovery. When the European Union Aviation Safety Agency was testing it for return to service they did full flights tests with and without MCAS and found that it was fine both ways [1].

So if something goes wrong with MCAS during a flight making it interfere with normal operation of the aircraft, then disabling it is quite reasonable. The odds that you will end up later in a situation where MCAS actually has to do something are extremely low, and the odds are even lower that the pilots won't be be able to handle it without the help from MCAS.

And that's assuming you are in a stage of flight where it would even be used. MCAS is only active when flaps are up and the autopilot is disengaged, so most of the time for most flights it is out of the picture.

MCAS seems to be one of those things that needs to have great care taken in its implementation because the danger from it mistakenly activating when it should not seems to be orders of magnitude higher than amount of safety benefit it provides when it correctly activates. They did not take that great care.

[1] https://theaircurrent.com/aircraft-development/mcas-may-not-...

>But I can't imagine it would be allowed to even have this as an operating procedure this when it puts the plane in a situation where it can't be certified?

Anything can be allowed with enough omission, lawyers, scapegoating, campaign donations, and enough tolerance for blood on one's hands. It's called regulatory capture and it's the fundamental conflict-of-interest between most engineering orgs and their in house Quality programs, with the exacerbating effect that if you can coerce the external regulatory forces to give you a pass, there's really nothing your Quality people can do but resign in a gesture of "I won't build that".

You will never see the regulatory apparatus work as designed if it isn't willing to just say "No" to that which clearly violates prescriptive certification criteria.

The response to a pitch trim runaway (MCAS or otherwise) is anything but an obscure process to anyone sitting forward of the cockpit door on a 737. It’s a memory-item checklist, demonstrated in initial and recurrent training. (The pitch trim wheels are prominent in the cockpit and make a clanging noise while running as well.)

> In fact, the MCAS exists because without it the MAX would be 100% uncertifiable for commercial flight.

My understanding is that they had to change some things about the plane due to efficiency regulations, which should have resulted in a new plane which would require flight lessons. They kept the things that they had to to escape FAA definitions of "different plane" and MCAS made the resulting monstrosity airworthy. If, so we are back to MCAS being a "737 emulator" required by SWA's mandate to receive the exact same airplane for all eternity.

Not that this is an excuse.

> My understanding is that they had to change some things about the plane due to efficiency regulations, which should have resulted in a new plane which would require flight lessons. They kept the things that they had to to escape FAA definitions of "different plane" and MCAS made the resulting monstrosity airworthy.

Again, not really. They wanted to put more efficient engines on the 737 to compete with the A320Neo, but keep the 737’s basic height in order to be compatible with the huge number of gates at old regional airports that are essentially built for the exceptionally low shape of the 737. Compatibility with those airports is essentially a competitive advantage for Boeing, as it’s what’s been keeping Airbus from eating their lunch with more efficient planes.

But the new engines were essentially too big for the low slung 737, so they moved them super far out from under the wing, making the plane basically unstable. Yeah, if they’d solved this by redesigning the plane to have more room under the wings it would have moved them out from under the 737’s grandfathered approval and thus required a lot more certification certainty and new training – but it would also have messed up the fundamental sales pitch of “buy this instead of the A320 Neo so that you don’t have to renovate your gates at every regional airport you fly to”.

the big engines moved so far forward creates the inverted stick forces that aren’t a training issue – no amount of training would have permitted them. It’s a Fundamental Airworthiness Requirement that they not occur, and the MCAS exists explicitly to make them not occur.

eg)

> Engineers observed a tendency for the plane’s nose to pitch upward during a specific extreme maneuver. After other efforts to fix the problem failed, the solution they arrived at was a piece of software — the Maneuvering Characteristics Augmentation System (MCAS) — that would move a powerful control surface at the tail to push the airplane’s nose down.

Again, that “tendency for the plane’s nose to pitch upward” isn’t a training issue – it’s flat out illegal for commercial planes to have that characteristic:

https://www.law.cornell.edu/cfr/text/14/25.175

So the MCAS is just an airworthiness issue caused by the legacy market Boeing was trying to continue to be compatible with. They were careful not to call attention to the MCAS to avoid the FAA adding training requirements that would have upset Southwest, is the main thing where that factors in.

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