Ethiopian Airlines Crash; Another 737 Max

[kayoh190]

I’m curious if anyone outside of Boeing actually knows whether or not Stab Trim Cutout actually stops MCAS from providing further inputs.

Normally I'd tell you that the switches will completely disable the electric motor, thereby making it impossible for MCAS to do its thing, but Boeing made me feel like such an idiot for not even knowing about MCAS to begin with - I admit that I no longer have much trust in my manual these days. That said, I find it much more likely that the pilots restored power to the motor in a last ditch effort to trim the nose up than MCAS being able to run with the switches in the off position.

 

[sourdough44]

I take a little issue with trying to engage the autopilot at low altitude(or at all) with the erroneous instrument indications present. Just saying, it’s a time to be flying the plane, more the ‘old fashion’ way.

 

[TCABM]

View attachment 73091

No electric stab trim with switches in cutout. One has to use the manual wheel.

That’s not quite the question I’m asking.

Is there, anywhere on the aircraft, a breaker (or breakers) that actually controls the power flow, which means the Cutout input could have an instance in which it is bypassed.

 

[TCABM]

The report explains that. It removes power from the motor.
...
Stabilizer Trim
The STAB TRIM PRI cutout switch and the STAB TRIM B/U cutout switch are located on the control stand. If either switch is positioned to CUTOUT, both the autopilot and main electric trim inputs are disconnected from the stabilizer trim motor.[/I][/INDENT]

No, it doesn’t. It specifically says AP and main electric trim inputs are disconnected. Words mean things, if there is a ‘main’ electric trim input, it implies there is an alternate. It also specifically omits mentioning how MCAS commands are routed and if they are impacted by the Cutout switch.

 

[dmspilot]

No, it doesn’t. It specifically says AP and main electric trim inputs are disconnected. Words mean things, if there is a ‘main’ electric trim input, it implies there is an alternate.

You removed the part of my post where I quoted the MCAS activation having no effect on the trim position after the cutout switches were placed in the cutout mode.

 

[Matthew]

I have a question for software people. Why wouldn’t the MCAS system look at all the available flight info such as speed, altitude, VSI, etc and piece together something is wrong? Why can’t the system determine its getting bad info from one system that doesn’t have info that’s matching what other systems are providing? That’s what we ask pilots to do - I don’t see why a computer processing alot of info very fast can’t piece this puzzle together. Especially when the aircraft is speeding toward the ground. I am not a software engineer or aircraft designer - just seems odd that a safety feature of sophisticated software can’t resolve what we want the pilots to resolve in 6 mins.


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I'm not a flight control or avionics guy, so I don't know if there are any FAA requirements on s/w:

Software, like any thing else, can be slapped together or carefully designed. For life critical or similar applications there are generally thorough specs and requirements. Why the s/w was not designed to handle multiple inputs before making a decision was probably discussed in a conference room with a lot of people as they worked out things like "likelihood of failure, different failure modes, how bad would things get if it did fail, ..." The second part of that is "how much time will it take to implement, and how long will it take to test." That's sometimes followed by, "Well, if we skip that part, now how long will it take?"

I don't know how compressed the schedule was between when MCAS became a requirement and when it was delivered. It's likely that all decisions were done in good faith and carefully thought out.

 

[kayoh190]

There aren't any Max simulators in the U.S. and the pilots got trained with iPads so I think that answer is no.

This is correct. I will say that I never felt the training was inadequate, as we were led to believe that this thing was essentially an -800 with new engines and bigger displays. From a pilot perspective it was all supposed to be really minor stuff, and indeed all of the differences we were aware of fit on one side of an 8.5x11" laminated cheat sheet. The MAX certainly handled like an -800, if not a little better. So you can imagine that some guys felt betrayed when we found out about MCAS only after the Lion Air crash. To my training department's credit - they didn't know about MCAS either.

 

[sourdough44]

One needs the handle out on the manual trim wheel. Was the handle found out in the crash?? Yes, could of been totally destroyed or moved.

 

[TCABM]

Normally I'd tell you that the switches will completely disable the electric motor, thereby making it impossible for MCAS to do its thing, but Boeing made me feel like such an idiot for not even knowing about MCAS to begin with - I admit that I no longer have much trust in my manual these days. That said, I find it much more likely that the pilots restored power to the motor in a last ditch effort to trim the nose up than MCAS being able to run with the switches in the off position.

And that’s where I stand on this as a past event. There’s just enough missing links to make me really question exactly what’s been implemented and how.

So, to prevent future accidents of this nature, we kind of have to understand what exactly occurred.

 

[AKiss20]

View attachment 73092 One needs the handle out on the manual trim wheel. Was the handle found out in the crash?? Yes, could of been totally destroyed or moved.

Look at the state of the DFDR and CVR in the photos of the report. Those are intended to survive. I don’t imagine the trim wheel survived well enough to give any useful information, but who knows I suppose.

 

[TCABM]

You removed the part of my post where I quoted the MCAS activation having no effect on the trim position after the cutout switches were placed in the cutout mode.

Because it’s an assumption.

MCAS is an FCC output. Is it possible the stab trim switch cuts out control column input, but does not de-energize the circuit, potentially allowing for bypass or downstream FCC inputs to still occur?

Or some similar flow of queertrons being able to make their way to one of the trim motors.

 

[dmspilot]

Because it’s an assumption.

MCAS is an FCC output. Is it possible the stab trim switch cuts out control column input, but does not de-energize the circuit, potentially allowing for bypass or downstream FCC inputs to still occur?

Or some similar flow of queertrons being able to make their way to one of the trim motors.

It's an assumption highly likely to be correct. Considering they used the manual electric trim again, they must have reactivated the stab trim, which is what caused MCAS to reactivate and move the stabilizer; not queertrons.

 

[TCABM]

https://www.prnewswire.com/news-rel...-flight-302-preliminary-report-300825008.html

Interesting parts of the statement. Boeing acknowledged

“...the release of the preliminary report of the Ethiopian Airlines Flight 302 accident investigation, it's apparent that in both flights the Maneuvering Characteristics Augmentation System, known as MCAS, activated in response to erroneous angle of attack information.
... As pilots have told us, erroneous activation of the MCAS function can add to what is already a high workload environment. ...
We're taking a comprehensive, disciplined approach, and taking the time, to get the software update right. ...
This update, along with the associated training and additional educational materials that pilots want in the wake of these accidents, will eliminate the possibility of unintended MCAS activation and prevent an MCAS-related accident from ever happening again.”

 

[Tantalum]

trimming manually

*electric* trimming manually. By engaging electric trim it seems they reactivated the defective system, hence the returning nose down inputs

The pilots couldn’t get the nose up so they **probably** tried electric trim

..I wonder, why would they reengage a faulty system?

Why can’t the system determine its getting bad info from one system that doesn’t have info that’s matching what other systems are providing?

That's where a big part of the critique lies. Even without fancy computer algorithms just have it use both AoA inputs and that way a fault is easily connected

361 total
207: 737
58: Max 8

That's horrifying, and frankly borderline criminal

FO explicitly tried manual trim and reported “it wasn’t working”

Sounds like a panic, that wheel takes a lot of turns to, they probably gave it a few turns and panicked. Have one guy hold the yoke back, and the other manually trim it up

does anyone (including Boeing) know what kind of forces are needed to manually trim the stab nose up?

That's a great question. I was wondering the same. I would assume (hope?) that within the flight envelope the control forces would not be so great as to overwhelm an otherwise normal, healthy adult

 

[AKiss20]

Sounds like a panic, that wheel takes a lot of turns to, they probably gave it a few turns and panicked. Have one guy hold the yoke back, and the other manually trim it up

I don’t think we have evidence of that yet (without hearing the CVR it’s hard to tell). Given the question of the aero forces and the previous accounts of how difficult it is to re-trim (there was a previous reply in this thread describing how it required unloading the stab to actually turn the wheel) it’s possible the FO simply couldn’t turn the wheel manually and reported that as “not working.”

 

[Bill]

that wheel takes a lot of turns to, they probably gave it a few turns and panicked.

If they could turn it at all.

 

[Larry in TN]

Normally I'd tell you that the switches will completely disable the electric motor, thereby making it impossible for MCAS to do its thing

MCAS is part of the STS. The STS, and all electric trim, is disabled with the switches in cutout. This was confirmed in the data from the Ethiopian DFDR preliminary report. The MCAS commanded nose-down trim but the trim did not move after the switches were in cutout.

If they could turn it at all.

The fold-out handles are 90° (of rotation) apart on the two trim wheels. This is so that the two pilots can work together and at least one will always have leverage at any point in the rotation.

 

[Tommar98]

Why so quick to assume “panic?” The whole flight lasted 6 mins and they fought with the plane the entire time. I really don’t understand a software system that essentially takes control of the aircraft from the pilot to avoid a stall and then doesn’t properly assess ALL the systems on the plane to determine if the corrective action has worked. How does a plane head down at 600 mph from low altitude and not have a corresponding software recovery? It seems to me (and I have no experience in this) that the system just never accounted for a faulty AOA reading as possible. Hence no mechanism to address it.

The Air France disaster was a classic case of software not accounting for something they didn’t think could happen. The AOA was soo high in that case the software simply dealt with the info as faulty and ignored it. Hence the pilots got stick shake when they nosed down and nothing when it went up. Bad design.




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[Sluggo63]

..I wonder, why would they reengage a faulty system?

Because it wouldn’t work manually. If they did re-engage the electric trim, I think it was probably a last ditch effort to do something that may work.

does anyone (including Boeing) know what kind of forces are needed to manually trim the stab nose up?

Yes. Sort of. I talked about it in one of my previous posts.

So now that the runway has stopped, you have to get the airplane back in trim using the manual trim wheel. This part was my the actual point of this whole post. Trimming and out-of-trim jet like this is definitely a two person operation. And it's difficult. If the PF just hold the nose level with all that pressure, there is NO WAY that stab wheel is going to turn. I'm telling you, you can't get it to budge due to the heavy air loads on the stabilizer. What has to be done is the pilot flying has to raise the nose a couple of degrees then, let the nose fall, thereby unloading the stabilizer, allowing the PM to turn the wheel as fast as they can to try to relieve some pressure. Once the nose lowers, the PF is now adding back pressure to keep the plane form getting too nose-low, jamming the manual trim wheel again. It usually took 3-4 of these oscillations to get the plane where is was back to a somewhat normal trim feel.

Rereading that post, when I typed it, it made it sounds like we only did this in the simulator, but we actually did it in the aircraft as well to show new pilots to the KC-135 the yoke forces created by an out of trim airplane. We’d fly on downwind, all trimmed up, and then slow about 20 knots and let the student feel how heavy the plane was. Then we showed how impossible it was to move the trim wheel without relieving the yoke forces. The plane was just a few units out of trim and it was a lot of coordination to get the plane back in trim.

Edit: Again, this is all base on experience in a different airplane. Maybe the 737 manual trim wheel has some sort of mechanical advantage that we didn’t have in the 707. For us, it was just a sprocket and chain from the wheel back to the stabilizer jack screw.

 

[Sluggo63]

Sounds like a panic, that wheel takes a lot of turns to, they probably gave it a few turns and panicked. Have one guy hold the yoke back, and the other manually trim it up

You could never move the manual trim wheel if you were holding pressure against the stab. If one guy was holding that much pressure against the trim, that wheel is not moving (IMO). The guy flying would have had to actually release pressure (i.e. let the nose fall) to get the wheel to move.

 

[Tommar98]

You could never move the manual trim wheel if you were holding pressure against the stab. If one guy was holding that much pressure against the trim, that wheel is not moving (IMO). The guy flying would have had to actually release pressure (i.e. let the nose fall) to get the wheel to move.

I didn’t post the panic remark. I was trying to quote it and respond. Anyway others have pointed out the huge forces at play and the inability to crank that trim wheel.


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[Sluggo63]

I didn’t post the panic remark. I was trying to quote it and respond. Anyway others have pointed out the huge forces at play and the inability to crank that trim wheel.


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Gotcha. Sorry. Fixed it.

 

[Tantalum]

Because it wouldn’t work manually

But what does "wouldn't work" manually mean? They physically couldn't move it? Was there a mechanical defect? Or they were spinning it but nothing was happening

You could never move the manual trim wheel if you were holding pressure against the stab

Thanks, can you elaborate? Are the aerodynamic forces too great? Or is there a mechanical cut out? Also, this doesn't make any sense.. are you telling me that you can only manually trim a plane if you basically let go of the yoke? I imagine that would make smooth pitch changes and trimming the plane nearly impossible. I assume we're speaking in hyperbole here

I still wonder, why was the other crew then that survived able to deactivate the system and manually trim it?

Lastly, we on PoA know this, so I assume the pilots did, but if MCAS is only active with AP off, wouldn't they have benefited from engaging the AP and having that fly the plane? Giving them a chance to troubleshoot?

Armchair quarterbacking I know.. but this whole thing is bonkers

 

[Tantalum]

Anyway others have pointed out the huge forces at play and the inability to crank that trim wheel

But another crew on a previous flight was able to deactivate the system and manually crank the wheel.. so what made this aircraft and crew different? I'm sure some engineer somewhere at Boeing at some point ran the figures to determine aerodynamic manually trim forces.. if for nothing else to work out the gearing and mechanical requirements of the manual trim system..

 

[Tantalum]

**If it is actually true that manually trimming the plane may have aerodynamic forces that are too great then that is a massive design issue. My whole point this whole thread has been "shut the system off, manually trim it, as any other trim runaway situation calls for" <- but if that is not actually a realistic option.. then we are in deed talking about a major problem. I would love to know what the theoretical max loads required are would be

 

[chemgeek]

If they could turn it at all.

And Boeing has acknowledged that under certain airspeed conditions it may be impossible to overcome air loads with the manual trim wheel. So MCAS can potentially put you in a position where you have excessive nose down trim and may not be able to correct it manually? Wouldn't you like to be figuring that out while trying to regain control of an airliner under control of HAL the friendly computer?

 

[Piperonca]

so what made this aircraft and crew different?

An additional pilot.

 

[chemgeek]

This is where I take fault with Boeing as well, but only to the degree that "it's a stupid design" - not to the degree that it is a lethal design. The pilots are not helpless passengers along for the ride to press a few buttons.. they're the captains and in charge of the ships they command. The very fact that some crews crashed while others didn't (with the same issue) shows that this is a survivable event

I think it may depend on how fast you get into a pickle with uncommanded nose down trim inputs. Once you get past a certain point and airspeed builds it may be impossible to overcome air load with the manual trim. That leaves you re enabling electric trim and fighting the MCAS? I don't envy the pilots trying to figure out why they couldn't regain control of the aircraft. All because the software can't, or wasnt enabled to, figure out one of the AOA sensors is hosed and desist with uncommanded corrections before things spiral out of hand. Makes no sense from an engineering safety standpoint to avoid an in flight redundant crosscheck when one is readily available.

 

[Greg Bockelman]

I take a little issue with trying to engage the autopilot at low altitude(or at all) with the erroneous instrument indications present. Just saying, it’s a time to be flying the plane, more the ‘old fashion’ way.

The problem with that is it takes brain power to manually fly the airplane that could otherwise be used to troubleshoot the problem. Typically one wants to get the autopilot on so one can work on the problem.

But, as usual, there are exceptions.

 

[Greg Bockelman]

Edit: Again, this is all base on experience in a different airplane. Maybe the 737 manual trim wheel has some sort of mechanical advantage that we didn’t have in the 707. For us, it was just a sprocket and chain from the wheel back to the stabilizer jack screw.

I’ll bet it’s the exact same system. Relatively speaking, the two airplanes were designed about the same time.

 

[sourdough44]

Yes, the one AOA probe was feeding bad info, that’s not good.

The thing is, the plane never started the flight out of trim, nose down. This all happened over a period of time, maybe not a whole lot, but incrementally. When the trim started to get out of whack, that’s the time to get in front of it. Yes, a bit of armchair flying. I do admit, the stick shaker would be annoying.

 

[TCABM]

...Maybe the 737 manual trim wheel has some sort of mechanical advantage that we didn’t have in the 707. For us, it was just a sprocket and chain from the wheel back to the stabilizer jack screw.

I’ll bet it’s the exact same system. Relatively speaking, the two airplanes were designed about the same time.

I think the mechanical systems are similar/same as the E-3 is a 707-200 base platform and that part is familiar.

What changed with the classics, NG, and Max’s is the amount of automation. There’s things as early as the -300 classics, automation and display wise that we didn’t have. I mean, we’re still flying with panel Navs and FEs.

I can’t speak the what changes occurred when the -135s got their motor and cockpit upgrades.

 

[SkyDog58]

Apparently, which in itself is odd. Thirty years in airline maintenance from 727’s to 787’s and I can’t recall the last time I replaced a faulty AOA vane, they just don’t fail very often in general.

Same here.

 

[Bell206]

What will be interesting is how the experience level of the crew will be disseminated in the final report. In the Lion Air report it showed the Captain was able to handle the automatic nose down trim inputs via electric trim 21 times, keeping the aircraft level. It wasn't until he handed control over to his less experienced co-pilot that after 4 attempts to counter the nose down inputs that the aircraft was lost.

Regardless, it appears the Boeing CEO has owned up to the MCAS issues in a statement after this prelim report was released.

 

[SkyDog58]

I heard a news report this morning where the pilots turned off the MCAS in an effort to fix the problem, that didn't work, and then they turned it back on again. Maybe thinking it would reset itself.

And how exactly did they turn off MCAS? To my knowledge there is no MCAS ON-OFF switch. You can turn off the switches for the electric pitch trim but that kills it for all inputs, not just MCAS. Contrary to its name, it is not a new system with all new componentry of its own but rather an added algorithm (lines of computer code) that uses existing sensors as inputs and the existing pitch trim system as an output. And it basically manifests itself as all of the other computer algorithms that output to the pitch trim. And the overriding of it is the same as those other algorithms. This has been covered rather extensively. Of course it has also been mischaracterized rather extensively as well. Carry on.

 

[Doug F]

I apologize if this has already been posted but I didn't read the entire 15 pages of this thread. There have been several questions about the forces required to manually change stab trim. While this video doesn't provide a numerical answer, and they're in a simulator, it does appear that it's not all that easy and 2 arms make it work better (after watching this I want my crew to have them a set of guns). I have no idea if this guy is legit but he does appear to have access to actual aircraft and simulators so I'm guessing he's not totally talking out his rear end.

There's another vid I just noticed that is specifically about the Max 8.

I'd love to hear from actual Max 8 pilots how close these presentations are to reality.

 

[Greg Bockelman]

In the Lion Air report it showed the Captain was able to handle the automatic nose down trim inputs via electric trim 21 times, keeping the aircraft level.

Seems like after 8 or 10 or a dozen such events one would figure out something was wrong. Wonder what took him so long to figure out something was wrong.

 

[Ravioli]

Normally I'd tell you that the switches will completely disable the electric motor, thereby making it impossible for MCAS to do its thing...

Today some "expert" was saying if you turn something off, you leave it off. Is that right?

I think if a breaker pops, you reset it ONCE. If it pops again, you go without. Same thing with cycling other circuits.

Is it different for the big planes? <- actual question.

 

[Greg Bockelman]

I'd love to hear from actual Max 8 pilots how close these presentations are to reality.

I didn’t watch all of either one of those videos, but he was accurate on what I did see.

 

[Greg Bockelman]

Today some "expert" was saying if you turn something off, you leave it off. Is that right?

I think if a breaker pops, you reset it ONCE. If it pops again, you go without. Same thing with cycling other circuits.

Is it different for the big planes? <- actual question.

Yes that is correct, and we do not reset breakers unless a procedure specifically tells us to. We got away from one reset.