[geocrasher]

"- A component of the main electric trim system became inoperative. Our pilots ran the appropriate checklist, which included manually trimming the aircraft. They returned to MIA and landed uneventfully. The issue was not related to MCAS."

Indeed. This was a part failing on an aircraft in flight. It landed without incident, and was likely never in any sort of danger. Losing electric trim is an annoyance but also trims safety margins by, as I understand it, making autopilot impossible. So, it's good that they returned.

If this had happened on a 767 or A320 we'd never have heard about it.

[phire]

Not just the autopilot, but if pitch trim has failed then the Speed Trim System is disabled too.

What is Speed Trim? Well imagine MCAS, but instead of moving the stabiliser near the edge of the flight envelope, it moves the stabiliser all the time (when the speed is below mach 0.68). And instead of being introduced with the 737 Max, it was introduced 35 years ago with the 737 classic.

At least it's mentioned in the manual.

[martimarkov]

I agree but it’s like the galaxy phone that had the battery issue. It just killed confidence with the consumer.

The investigation (and what we learned form it) into Boeing also didn’t help with confidence levels.

After a few new iterations nobody talks about the battery anymore. I’m not so sure how Boeing can turn this around relatively quickly.

[jryle70]

Southwest just placed a large order for 737MAX-7 two days ago - [0]. 100 firm orders plus 155 options. They may get a great deal from Boeing for all we know, but placing a large order during this terrible time for the airlines is a bet on the future of MAX as well as the industry recovery.

[0] - https://www.cnbc.com/2021/03/29/southwest-airlines-adds-100-...

Meta note, this relatively frequent aircraft incident gets a lot of votes here on HN, yet the Southwest's order falls off the crack. Objectivity is as scarce on HN as on any other media frequently criticized here.

[rualca]

> They may get a great deal from Boeing for all we know, but placing a large order during this terrible time for the airlines is a bet on the future of MAX as well as the industry recovery.

Is this supposed to mean anything? To me it reads like an airline managed to get a massive discount on inventory that a struggling aircraft manufacturer hasn't been able to move for about a year due to their repeated problems with safety and accountability, which led the whole world to drop their orders.

[sharken]

For starters they can reiterate what has been said here, that a common error occurred and not the original MCAS problem.

Not being open about it will have the opposite effect.

[HeWhoLurksLate]

Speaking of which, I have a friend that still runs a Note7

[fsckboy]

weren't those banned from flying on 737MAX's? (and all other aircraft)

[berkut]

With respect to the 737-8 MAX though, although in this case it's nothing to do with MCAS, doesn't the 737-8 MAX have smaller radius trim wheels than previous models (so less leverage)?

So in theory, trimming might be more difficult on the MAX than previous 737 models, especially at higher speeds (where there's more force on the control surfaces that would need to be counteracted in the case of runaway trim).

[slacktide]

The trim wheel on the 737 MAX is identical, down to the part number, to the trim wheel on the 737 NG, which has been in service since 1997. The trim wheel on the 737 NG was reduced in diameter by approx 1/8 of an inch compared to the 737 Classic, which has been in service since 1982, in order to give adequate clearance to the “new” CDU (data entry keyboard for the flight management system). Since the trim wheel is around 10” in diameter, a 1/8” change would have a negligible effect on trim forces.

[jjtheblunt]

Why re 767 or A320? (i'm guessing because they're not recently in the news.)

[gfiorav]

737 max had two fatal crashes due to automated trimming issues (MCAS) and the fleet had been grounded until very recently.

[ulfw]

It's a new aircraft not one 20years in service like your average 767 or A320!

[TylerE]

That makes it less surprising, not more.

Component failures follow a bathtub curve. Mid-life is the lowest failure rate.

[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.

[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.

[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...