Incident: American B38M near Nassau on Mar 29th 2021, pitch trim issue/failure (opens in new tab)

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.

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.

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).

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

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

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?

Aerospace engineer here.

From TFA:

>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.

That statement is from the airline, not from Boeing, so I'm more inclined to trust it. Additionally, if the airline lies and it turns up on an airworthiness audit (air maintenance organizations are subject to regular audits) then the penalties are quite severe.

In any case, per the airline's statement it was an issue unrelated to MCAS. Aircraft break literally every day in a myriad of ways that are often invisible/imperceptible to people riding on that very aircraft. In this particular case it was some unspecified component of the electric trim system.

You can look up the Master Minimum Equipment List for the B737[0] and see for yourself just how granular the approved maintenance program gets for aircraft like this. Everything on the MMEL is essentially an item that can be broken and the aircraft can still take off legally. Note that this is a different (and more rigorous) standard than "can be broken and aircraft can take off/operate safely".

I don't know exactly what broke here but I suspect it is a part that has broken on 737s hundreds if not thousands of times in the past, with similar outcomes.

I merely dabble in software but to further your analogy: This situation is when you've been dreaming about that bug for weeks and you get the call from the boss thinking it has recurred but instead it turns out it was a similar-smelling failure caused by some intern's microservice not failing gracefully when confronted with a network outage that brought down the system anyways.

[0] https://fsims.faa.gov/wdocs/mmel/b-737%20r55a.htm

the good old programming testing rule: the number of bugs you will find in a piece of software is proportional to the number of bugs you already found

FTA: "The issue was not related to MCAS"

It's not the engineers pain, as it wasn't an engineering decision to mount the engines higher and forward of the wing. Precipiting a pronounced nose-up aspect. All the same, I admire your attempt to deflect blame from the management at boeing.

Boeing did not get into this position overnight. After the massive cost and schedule overruns on the 787 program, they were far too cash-poor to fund the development of a clean-sheet design, hence the re-warmed 737. The 787 itself was a huge experiment in outsourcing both the costs and risks of aircraft development. It turns out you can outsource cost, but the prime contractor always owns the risk. Dr. John Hart-Smith published an excellent white paper warning of this while the precursor to the 787 was still in development, shortly after the Boeing-McDonnell Douglass merger.

https://www.google.com/search?q=Outsourced+Profits+%E2%80%93...

What has really ruined my trust for Boeing is not necessarily the incidents with the 737 MAX model specifically, but more the change in that company's culture that these incidents have made me aware of. I can recommend this video[1], but the short of it seems to be that Boeing over a period of decades has replaced its previous "engineering first" culture with what I can only describe, for lack of better wording, as the "suits taking over", with predictable results.

[1]: https://www.youtube.com/watch?v=y_zn_x2JK5Q

I have no opinion about flying on this particular plane.

It's not so much MCAS, electric trim, this, or that.

The bigger issue is it's too complicated, has too many features, and the FAA can't adequately oversee testing and certification all the way down the engineering stack. Furthermore, because of pressures to save a buck, Boeing is willing to cut corners and sacrifice safety by slapping a plane together without properly engineering or testing it. Because of this behavior, it's difficult to know how many other problems are lurking around like in the 787.

I have taken a similar stance myself (not that I'm flying anywhere these days) but, out of curiosity: do you also refuse to be a passenger in automobiles? The 737MAX is quite a bit safer than those.

Almost every airplane except the absolute most basic have trim controls for flight surfaces, especially the elevator. Even little two seat Cessnas get trim controls.

Trim is changed for each phase of flight. Electric trim is just a motor spinning the manual trim control.

Most airliners will be doing constant trim adjustment.

If you want a real change of pace check out Airbus’ control system. It does a lot more intervention than anything Boeing does. Depending on the state of the aircraft the control stick will respond to input in entirely different ways (3 ‘laws’ that contain no less than 5 submodes). Sometimes it will act as you would expect a stick to act, other times it will intentionally limit what the pilot is asking of the plane, sometimes it will average what the two pilots are asking. Confusion about how the system works has caused at least two crashes I can think of (AF447, QZ8501). It’s killed more people than the Max, but it was written off as pilot error since it was operating as designed in both cases. It just happens to be a design that will change the way the plane is controlled when things go wrong. A few of the modes do in fact include automatic trim adjustments.

Yeap. The electric trim systems tend to be automatic with speed and altitude to approximate what the pilot wants via trim up/down controls. Without trim, planes would be pitching and diving all over the place, or at least require yoke/sidestick back-pressure to hold steady. Some planes can be trimmed through enormous deflections of the horizontal stabilizer. If configured incorrectly and under the wrong conditions, incorrect trim can lead to dangerous flight characteristics like pitching up or down beyond what the elevators can handle at full deflection. A broken jackscrew (part that moves the horizontal stabilizer for the trim commands) can cause this type of dangerous configuration or even worse.

Even gliders have trim. It's not technically considered a primary flight control because you can survive without it, but at the very least it's annoying not to have it.

The plane doesn't really NEED to use MCAS. Systems like MCAS or Speed Trim and others do not make the planes flyable, they make them predictable. Aerodynamically, even the 747 or the A380 are normal planes operating on the same principles as the tiny Cessna 172, they're just much bigger, fly faster and react a lot slower.

Airliners are packed with systems that make them dull and predictable to fly, because things can break on a sunny day over Texas, but also at midnight in the rain over the Atlantic, and the last thing you want your pilots to care about when they're stressed out, disoriented and working through complicated checklists is whether this particular plane has a weird tendency to pitch up at these particular conditions.

Nope. “The EASA flight tests confirmed that MCAS was needed to provide full compliance but also that the loss of this function does not preclude the safe flight and landing of the aircraft; i.e. the 737 MAX remains stable following the loss of the MCAS function.”

https://www.easa.europa.eu/sites/default/files/dfu/B737_Max_...

Thankfully, our standards for aviation safety are not low enough for an anecdote of a safe landing to be a valuable data point.

Anecdotal evidence but glad you survived.