[andy_ppp]

As I understand it the aircraft has a quite complex relationship with trim because of fundamental aerodynamic flaws (engines are too far forward). Am I right in saying other aircraft e.g. an A320 would handle this sort of failure without as much risk to the plane?

[geocrasher]

MCAS was only installed to cover up aerodynamic changes (not flaws) that would have required greater recertification and retraining of pilots. To save that expense, they covered it up with MCAS so they could say "See, it's just another 737!" which caused all the issues we know so much about.

[andy_ppp]

So you’re saying without MCAS the plane flys just as safely but differently which would have required retraining. That’s interesting, thanks!

[geocrasher]

Yes @andy_ppp that's what I'm saying :)

[cratermoon]

No, the plane does not fly safely without MCAS. Without the MCAS there the aircraft has a tendency to pitch up, resulting in a stall situation.

[manigandham]

That's incorrect. All planes have a relationship with thrust and pitch based on multiple factors (size, weight, speed, wing configuration, altitude, etc). The aircraft was perfectly sound.

The problem was software designed to alter the flight profile automatically to minimize the differences and new training required (and was allowed to override manual inputs). Any pilot with full training of this specific plane without MCAS would have no problem flying it.

The planes didn't crash because they pitched up but because bad software mistakenly, and forcibly, pitched them down.

[salawat]

What the poster was more likely referring to was the uncertifiable behavior that would occur without MCAS during a wind-up turm or during descent whereby stick control forces would slacken on the way to stall instead of requiring steadily increasing pressure on the control column to bring the plane to a stall due to extra lift from the forward nacelles. While technically a pilot could deal with it, aircraft that demonstrate said behavior cannot be certified as civil transport aircraft without appropriate mitigations.

[cratermoon]

It seems to me that the folks here arguing that the 737 Max is safe have a definition of "safe" that would satisfy a military test pilot. I'm sure that even without MCAS a top-notch pilot could fly the Max without incident, but when it comes to passenger aircraft safety, the bar is considerably higher.

[avereveard]

Both incorrect. The stability system to correct pitch to avoid training is only one side of the story. The other side is there wasn't a linear relationship between pitch, aoa and stick forces, and that would not have passed the faa certification.

[geocrasher]

LOTS of aircraft have a tendency to pitch up or down based on throttle. This isn't unique to the 737 MAX.

A quick google on "thrust line pitch moment" gave me this: https://www.homebuiltairplanes.com/forums/threads/thrust-ang...

  " I want to adjust the thrust angle from where it was (0° with respect to the chord line) to reduce the downward pitching moment when thrust is added (or more importantly the upward pitch moment when thrust is reduced)"

[cratermoon]

Not many passenger aircraft have a tendency to pitch up to the point of stalling, though.

[manigandham]

It does not just pitch up randomly. It does pitch more than other 737s due to engine size and placement, but that's just the specific behavior of this plane.

Stalls are not a serious problem, they're basically the first thing pilots learn to solve. And there are plenty of aircraft that are more challenging to fly. That's what training is for.

The real issue is that pilots should have full understanding of the behaviors of the airframe instead of relying on software to change it. Especially when they don't have full understanding of the software either, and the system can both override manual inputs while being susceptible to faulty sensors.

[geocrasher]

Neither does the 737 MAX.

[cratermoon]

No, they are legit flaws. https://www.business-standard.com/article/international/boei...

[geocrasher]

I respectfully disagree. A news article calling them flaws does not make them a flaw. Here's the issue: At lower speeds such as you'd see during a landing approach, the throttle setting of the 737 Max directly affects its pitch. If the pilot were to suddenly reduce throttle, the nose would pitch up. This is due to the engine's more forward position.

The forward position of the engines was done so that they wouldn't have to re-engineer half the plane to fit the larger engines. High bypass turbo fans are big, and they couldn't maintain ground clearance without either moving the engines up higher or making the landing gear longer. To move the engines higher they had to move them forward. So they did.

The new behavior of the aircraft required retraining and making sure pilots knew how the throttle and pitch were related. Mind you, a lot of aircraft have such relationships, so this is not in itself a flaw.

The real flaw came when they decided to replace training with software, and then conveniently forgot to tell anybody about the robo-pilot that they put in the cockpit. THAT was the failure. The airplane itself, even without MCAS at all, would be a bit more of a handful to fly, but nothing terrible. With a properly functioning MCAS, and proper training about how to disable it in case of a problem, the issue is solved. And that's what the FAA believed happened when they recertified the 737 MAX to fly again. But by now the reputation has been tarnished so badly that we're hearing news about unrelated failures because it happened on a 737 MAX.

So you can see the real failure isn't aerodynamic, its pretty much everything else.

If I got any details wrong, I apologize. I'm flying by the seat of my pants on this layman's analysis after a long day.

[na85]

>At lower speeds such as you'd see during a landing a approach, the throttle setting of the 737 Max directly affects its pitch.

Any conventional airliner has this "flaw". If you are at low airspeed and you push the throttles forward, an A320 will exhibit nose-up pitch, too. It's a direct consequence of having giant engines slung kinda-sorta-underneath the wings of a low-wing monoplane.

[geocrasher]

That's my point. The engine positioning is not an aerodynamic flaw, it's just a design change that makes the aircraft behave differently than the 737-shaped planes that came before it.

[chipsa]

Most airliners have the engines under the wings (and therefore under the CG). Thus, most airliners will pitch up when thrust is applied. The 737 Max has a higher thrust line (which means closer to the CG) than prior 737s. The nose will pitch up less than other 737s when applying power. The problem isn't pitching up when applying power.

The problem is that at high angles of attack, the nacelles start to produce lift, and with the more forward position, they cause a greater pitch up force than previous versions.

[geocrasher]

This sounds very plausible, but I haven't heard this before. Do you have a reference? I'm genuinely curious- not calling you out, so to speak.

[waiseristy]

So I actually went and found this fabled 245-page report :

https://transportation.house.gov/imo/media/doc/2020.09.15%20...

And this, I think, is where the whole faulty/flawed thing gets introduced :

> Faulty Design and Performance Assumptions.

> Boeing made fundamentally faulty assumptions about critical technologies on the 737 MAX, most notably with MCAS. Based on these faulty assumptions, Boeing permitted MCAS—software designed to automatically[...] It also expected that pilots, who were largely unaware that the system existed, would be able to mitigate any potential malfunction. Boeing also failed to classify MCAS as a safety-critical system, which would have attracted greater FAA scrutiny during the certification process.[...]

So, I would agree that Boeing's design flaw here was not alerting the pilot with a huge red warning light (and being cheeky about re-certification). But I don't think that this report goes so far as to say that the airframe was flawed and therefore necessitated MCAS.

[ilrwbwrkhv]

Yup never flying Boeing again. Airbus all the way.

[airhead969]

It's a feature, not a bug.

[na85]

>As I understand it the aircraft has a quite complex relationship with trim because of fundamental aerodynamic flaws (engines are too far forward).

That's not really correct. The engines being in a different position means the aircraft doesn't meet a very specific criterion of the FARs (positive stick force gradient). The 737 Max has the exact same relationship with trim as any other airliner.

>Am I right in saying other aircraft e.g. an A320 would handle this sort of failure without as much risk to the plane?

No. TFA explicitly states this issue was not related to MCAS. It's likely an analogous failure on an A320 or a 737NG would still have necessitated aborting the flight.

[slacktide]

One thing that has always bothered me about how MCAS was implemented - if it truly was a stick force gradient issue, why not make the change in the Elevator Differential Feel Computer, which already manipulates the stick force gradient during approach to stall, rather than physically moving a control surface? Aside note, the elevator feel computer is a mechanical / non-electronic computer that is stuffed full of aneroids and solenoids and cams and followers and servovalves. Straight out of the 1950’s.

[VBprogrammer]

Where have you been all through the 737 Max debacle! It's refreshing to read someone who genuinely understands this issue.

And for what it's worth I've pondered that before and my only conclusion was as others have said here, probably a lack of detailed organisational knowledge on the design of the pitch feel computer. Perhaps there are confounding factors, it has its own pitot tube but not a AoA vane for example. Or perhaps changing it may have triggered more regulatory oversight than MCAS did (unfortunate given events which followed).

[salawat]

>Aside note, the elevator feel computer is a mechanical / non-electronic computer that is stuffed full of aneroids and solenoids and cams and followers and servovalves. Straight out of the 1950’s.

That's likely why they didn't. That type of change is hard to conceal and far more difficult to keep out of the documentation. A software change could potentially be handwaved. Full on revamping of said system would probably have edged a regulator enough to have raised an objection or a deeper dive into the nature of the reconfiguration.

[tremon]

A software change could potentially be handwaved

This to me seems a much greater process failure.

[bonzini]

Maybe that's why, I don't blame engineers who didn't want to touch it...