Ethiopian Airlines Crash; Another 737 Max

Larry in TN said:
But it is stopped by the primary electric trim.

If the nose is "heavy", don't you trim it back up? Doing that stops MCAS.
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These guys were probably screwed by doing the memory item too quickly. Just like Boeing and their new training told them to do. The plane was probably out of trim when they turned off the electric stabilizer trim.
 
kayoh190 said:
Random data point (which is probably mentioned somewhere, but I didn’t see it): it takes about 30 revolutions of the trim wheel to move the stab 2.5 degrees.
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I can see someone abandoning that procedure when there is no apparent progress ...
 
Let's be clear. As manufactured and delivered, we have an aircraft that relies on pilots to recognize and very quickly react to a single point failure mode that can apparently rapidly and aggressively put the aircraft into an unrecoverable flight regime, where the aircraft does not provide clear and unequivocal annunciation of the source of the failure mode (a single failed, active AOA sensor). This chain of events, the precipitating event which is apparently far from rare based on current events, could be easily and cheaply interrupted by requiring redundancy checks from two available sensors, better annunciation of failure modes, and less aggressive automated maneuvering that does not exceed the capability of the manual or other backup trim system. This basic engineering oversight, in retrospect of course, should be a hard lesson for all aircraft manufacturers to absorb. Why were maximal safety and automation crosschecks not implemented when it was easily achievable? The events leading to these crashes was certainly not intended or anticipated, but the initial response to these events...essentially blaming the operators to a large extent instead of critically analyzing the role of the automation...has not been confidence inspiring. American engineering should aspire to better.
 
TCABM said:
But 21 times worked although 5 didn’t. That’s a duration of input vs number of inputs problem.

The problem I see with both instances is you have trust the plane is doing what you want it to do, up until it isn’t.

And I’m really trying understand what’s the indicator for that.
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Seems like, once the nose takes a dive on you, that's a pretty good indicator and a guy is going to want to put his coffee down and get ahead of that ****.
 
Palmpilot said:
Larry pointed out that the five times that didn't work were because the copilot didn't re-trim to a trimmed condition each time, as the captain had done the previous 21 times.

https://www.pilotsofamerica.com/com...ucwD_Ny4wSL_ZuNAVbAiBVljuBWY1SOk#post-2699904

(See Item 9 on the list.)
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I’ve read that, and again I’ll stipulate being small brained, but my question still hasn’t been answered. What indicates an abnormal event?

If ANY amount of MCAS input has to be countered, because you don’t know how much you’re going to get until after you’ve already got it, and by then the input may have put you into an unrecoverable configurations re: trim speed, then every activation of MCAS is an abnormal event.
 
jesse said:
Seems like, once the nose takes a dive on you, that's a pretty good indicator and a guy is going to want to put his coffee down and get ahead of that ****.
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By then, it may be too late.
 
TCABM said:
I’ve read that, and again I’ll stipulate being small brained, but my question still hasn’t been answered. What indicates an abnormal event?

If ANY amount of MCAS input has to be countered, because you don’t know how much you’re going to get until after you’ve already got it, and by then the input may have put you into an unrecoverable configurations re: trim speed, then every activation of MCAS is an abnormal event.
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I assume that you are writing of an improperly fed MCAS, not one that is working as designed. I'd guess that it's been invoked perhaps millions of times with no incident.
 
Silvaire said:
Why do people dwell on this when it was not, as far as the evidence shows so far, a factor?
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"At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and First Officer confirmed stab trim cut-out." Agreed. In fact, from the prelim report the FO was the FIRST to call stab trim cut out.... the appropriate procedure. Probably fresh in his mind from training. The fact that he had low hours does not seem to have made any difference in this case. I think we have to be careful when jumping to conclusions about someone who perished in the crash, especially considering it appears he certainly was acting as a competent crew member trying to troubleshoot.
 
Kenny Phillips said:
I assume that you are writing of an improperly fed MCAS, not one that is working as designed. I'd guess that it's been invoked perhaps millions of times with no incident.
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I simply would like to understand how one differentiates whether result of the the designed commanded input is going to help you or kill you.
 
TCABM said:
I simply would like to understand how one differentiates whether result of the the designed commanded input is going to help you or kill you.
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I believe that you simply won't notice its presence if it's working as designed. If the nose starts pointing down (or up) with no apparent reason, time to suspect a trim runaway (which is what a misfed MCAS is, in essence.)
 
LongRoadBob said:
The other takeaway from this, the progammer or folks reviewing the code never changed his comical error message to something like "interrupt from X and Y received, DEADLOCK!" and that also should never happen, but often does.
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Yes, I can't count of the number of times I've added a code comment that says "/* Should never get here...how? */" only to have a support guy call me 3 months later say "we got into that section of code".

It's subtle and the interactions between seemingly unrelated things is astounding. You jiggle a stick over here and over there half the roof collapses.
 
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Sluggo63 said:
Excellent technical explanation and theory as to what happened and why. Better than anything I’ve seen to date.

https://leehamnews.com/2019/04/05/bjorns-corner-et302-crash-report-the-first-analysis/#more-29839
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Interesting comment to that article:

Erroneous AOA Results in calculation errors of airspeed that is displayed to the pilot on the side with the failed AOA. It also sends a signal to the stall warning system to alert the pilot he is approaching stall (stick shaker only on the side the side that has the failed AOA). Differences in airspeed between the 2 sides of the cockpit cause an airspeed disagree alert. Hence, the pilot flying calls for and the pilot not flying executes the Unreliable Airspeed non normal checklist.

That checklist drives the crew to set pitch and power, in this case with flaps down that initial pitch target is 10 degrees nose up and 80% N1. No place in the checklist does it direct the crew to raise the flaps. MCAS can not fire with flaps down.​
 
kayoh190 said:
All I can do is understand them at whatever level given by my manual. But until now, at least they were *in* my manual! :)
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I can definitely appreciate the sentiment. I just believe in a majority of emergency situations especially when flying a highly complex airplane that the more important thing is to know what to do in reaction to a set of symptoms and not necessarily worrying about what caused them. At least not until you have the plane under control. What this means to me is if the problem is manifesting itself as runaway trim then take the required action for that which should be a memory item. It really doesn’t matter at the time which of the various underlying functions, algorithms, or systems is causing it. At least not at the moment.
 
Wow. I just watched the Mayday episode on Adam Air flight 574. It’s amazing how a crew can totally screw the pooch when they fail to just fly the airplane.
 
TCABM said:
So, how does one determine when an abnormal event is occurring, considering MCAS only provides input during certain phases of flight?
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I don't know your background but will answer this so that (I hope) it will make sense to any non-pilots who are reading.

When you are hand-flying an airplane the nose (pitch) will frequently become either too-heavy or too-light. This is an indication that you are out of trim. The pilot responds by adjusting the trim to return to a neutral condition where the nose (pitch) will stay where the pilots wants it to be. This is a normal part of hand-flying an airplane which occurs many dozens of times, if not hundreds of times, on every flight. On larger airplanes, and all transport jets, this trim input is via the primary electric trim switches on the outboard yoke horn of the pilot-flying's control wheel.

With an unscheduled MCAS activation the MCAS will add nose-down trim input for 9.3 seconds (2.5 units of trim) then pause for five seconds. After the pause, the cycle will repeat if the triggering condition still exists.

The pilot will be climbing out when the nose starts getting heavy. The natural response is to trim back up until the airplane is back in trim. Doing so stops the MCAS activation and starts the five-second pause.

If the triggering condition (bad AoA singal) persists, the MCAS will start trimming down again and the pilot will apply nose-up trim to return the airplane to a trimmed condition. MCAS stops trimming and starts another five-second pause. As we learned from the Lion Air DFDR data, this process can continue for an extended amount of time. They did it for 21 activations over several minutes.

Normally, by the third or fourth cycle you should release that something is wrong with the trim and start the runaway stabilizer procedure. With the stick shaker, and other distractions, it might take four, five, or even six cycles but you still should figure it out fairly quickly.

That is how you would determine that an abnormal event is occuring.

Sluggo63 said:
These guys were probably screwed by doing the memory item too quickly. Just like Boeing and their new training told them to do.
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I don't understand what you're saying. Why would Boeing tell them to do the memory items too quickly?

The plane was probably out of trim when they turned off the electric stabilizer trim.
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It certainly looks that way. That wouldn't be because they did the runaway stabilizer procedure too quickly, it would be because they didn't do it before they allowed the trim to progress to a far nose-down position.
 
Larry in TN said:
I don't know your background but will answer this so that (I hope) it will make sense to any non-pilots who are reading.

With an unscheduled MCAS activation the MCAS will add nose-down trim input for 9.3 seconds (2.5 units of trim) then pause for five seconds. After the pause, the cycle will repeat if the triggering condition still exists.

That is how you would determine that an abnormal event is occuring.
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That makes sense theoretically, but it’s described as “up to 9.3 seconds”, not every activation will be 9.3 seconds.

I have a couple thousand hours in E-3s from more then a decade ago.

I’m having a hard time imagining that big ol’ trim wheel spinning away during a critical phase of flight and knowing when to trust it and when not to.

Because prior to MCAS, an extended run of trim was bad. But with MCAS, it can run for up to 9 seconds, whether commanded/scheduled or not and it’s supposed to be normal (when it wasn’t before) since it’s there because control feel won’t be right unless it’s working.

And one abnormal (or non-normal) condition results not in a trim feel condition difference, but apparently in these two cases, the shaker going off, which isn’t what I’d expect if the trim is running away, UNLESS we’re now normally flying that close to the ragged edge during initial climb.

If we’re doing that, then it goes back to any automated trim inputs should be countered not later than what I feel (normal), despite what the plane’s telling me and I have to make a real-time decision every time that wheel starts to move during climb when I’m going to hit the cutout. And I need to seriously consider if the shaker is going off, and I’m down low, I need to potentially pull power and push the column to help unload the forces so me and guy next to me can make that wheel turn manually.

To me, that’s not helpful automation.
 
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Does the 737 have a trim position indicator to see what point of the trim cycle you’re in? I can look down at my console in my bug smasher and see where my trim is. Same in a jet?
 
It sounds like using manual trim is impossible at certain speeds. Given that, I’d say the fact that electric trim must be disabled to disable MCAS to be entirely unacceptable.

Wouldn’t it be a lot safer to simply have some kind of stick pusher that could be overpowered by the pilots in the event it went wacky?

Also, if the failure of a single AOA probe causes uncommanded nose down trim... how the hell did that pass certification?
 
ircphoenix said:
I don't quite understand how anyone can be okay with 21 uncommanded pitch down maneuvers by an automated system and then blame the pilots. "Well they corrected for the system that tried to kill them the 21 times prior. Clearly the next 5 were sheer incompetence."
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Well, wouldn’t you think that after the first few the crew would think they might have a trim problem? I don’t hold the crew completely blameless.
 
dans2992 said:
It sounds like using manual trim is impossible at certain speeds. Given that, I’d say the fact that electric trim must be disabled to disable MCAS to be entirely unacceptable.

Wouldn’t it be a lot safer to simply have some kind of stick pusher that could be overpowered by the pilots in the event it went wacky?

Also, if the failure of a single AOA probe causes uncommanded nose down trim... how the hell did that pass certification?
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Since MCAS uses the electric trim system, it is entirely acceptable to disable it with an MCAS problem.

And good question on your last one.
 
Greg Bockelman said:
Well, wouldn’t you think that after the first few the crew would think they might have a trim problem? I don’t hold the crew completely blameless.
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Yeah, there’s culpability on their part, with the Lion Air Crew. Then again the world didn’t know about MCAS.

——-

As for Ethiopian, I can see where it’d get really overwhelming as soon as your airborne.

Looking at the timeline the the Ethiopian prelim, the plane is yelling at them about the AOA problem 4 seconds after rotate at 05:38:44.

“At 05:38:44, shortly after liftoff, the left and right recorded AOA values deviated. Left AOA decreased to 11.1° then increased to 35.7° while value of right AOA indicated 14.94°. Then after, the left AOA value reached 74.5° in 3⁄4 seconds while the right AOA reached a maximum value of 15.3°. At this time, the left stick shaker activated and remained active until near the end of the recording.”

That’s not the end world, but it sucks, so you’ve got to deal with it. So it’s flaps up and hit the AP, which seems to be drunk, then kicks itself off. MCAS has no idea what’s going on, but it kicks in and you get half your nose up trim taken away.

“At 05:40:00 shortly after the autopilot disengaged, the FDR recorded an automatic aircraft nose down (AND) activated for 9.0 seconds and pitch trim moved from 4.60 to 2.1 units. The climb was arrested and the aircraft descended slightly.”

35 seconds later, stab trim is cutout, without any more successful firings of MCAS.

Let that chain of events sink in for a second.

Because for the next four minutes you’re going to do everything you can to get enough units of trim back manually to unf*ck what the airplane just did to you.
 
So why didn’t they trim out what MCAS was doing like the Lion Air crew did? After all, they should have been able to use the electric trim up until it was cut out, just like the Lion Air crew did.
 
Greg Bockelman said:
Since MCAS uses the electric trim system, it is entirely acceptable to disable it with an MCAS problem.

And good question on your last one.
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Why not a discrete power circuit with a manual electric trim button to be used in such emergencies. A dpdt on the cut-out?
Better yet, why not just redesign the tail section so it can actually control pitch in all corners?
 
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Greg Bockelman said:
So why didn’t they trim out what MCAS was doing like the Lion Air crew did? After all, they should have been able to use the electric trim up until it was cut out, just like the Lion Air crew did.
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I believe it was because the FO identified the abnormal, executed the book procedure to hit the cut-outs and neither the captain or FO were aware they were in the portion of the envelope where the jack screw would be locked up and the manual trim wheel would be insufficient to allow them to fly the airplane. I don't believe the procedure says to wait to execute the runaway trim procedure until the aircraft is in relatively neutral trim. As stated earlier in the thread, once off, the crew followed the procedure to leave them off, until they ran out of options.

Since identifying the portion of the corner of the envelope where aerodynamic pressures overcome the ability of the pilots to manually overcome them is not part of the training syllabus as far as I know, I have a hard time finding fault with the crew's performance. Quite frankly, I believe they came damn close to pulling it out, had they just cycled the cut-outs back off after they got the last ANU.
 
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Greg Bockelman said:
So why didn’t they trim out what MCAS was doing like the Lion Air crew did? After all, they should have been able to use the electric trim up until it was cut out, just like the Lion Air crew did.
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Bingo....my thoughts exactly. ;)
 
Kenny Phillips said:
I assume that you are writing of an improperly fed MCAS, not one that is working as designed. I'd guess that it's been invoked perhaps millions of times with no incident.
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There has not been a single MCAS activation in the US fleet since the int roduction of the MAX. Not one.
 
Checkout_my_Six said:
Bingo....my thoughts exactly. ;)
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Because you're using a system with a known abnormal to do it, very close to the ground. Wouldn't it stand to reason that this thing might work now, but not next time the aircraft tries to kill you? Wouldn't you think going to the manual system might be a safer bet than continuing with the known faulty system? Besides, isn't going manual what procedures tell you to do after identifying a runaway trim?
 
Chip Sylverne said:
Because you're using a system with a known abnormal to do it, very close to the ground. Wouldn't it stand to reason that this thing might work now, but not next time the aircraft tries to kill you? Wouldn't you think going to the manual system might be a safer bet than continuing with the known faulty system? Besides, isn't going manual what procedures tell you to do after identifying a runaway trim?
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Full disclosure....I don't know what happened nor have I talked with Boeing, nor the Ethiopian investigators...

Na....prior crews with this problem survived. I'll bet they followed the runaway trim procedures....from memory. The key to their survival, IMHO, was time and altitude. They responded quickly and had altitude to recover.

This crew ran outta options because they didn't respond correctly and quickly....and yes, Boeing shoulda done a better job with this band-aided design.....but, others were able to work with it....and there were zero incidents in the US. What's up with that?

I've never cranked the trim under full load to neutral....but, I understand it's doable. I did roll a 777 sim both directions then landed it with nothin broke.....does that make me an expert? :D
 
Checkout_my_Six said:
Bingo....my thoughts exactly. ;)
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Wait so first the crew is blasted for not following the runaway trim procedure and just activating the trim cutout and manually re-trimming to counter MCAS, but when it comes out that they in fact did do exactly that and it turns out they likely couldn’t manually re-trim, they are blasted again but this time for activating the trim cutout and not using the electric trim to counter MCAS ?
 
Checkout_my_Six said:
I've never cranked the trim under full load to neutral....but, I understand it's doable.
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We’ve heard testimony from Sluggo that it is in fact very difficult and requires unloading the stab. In fact such a procedure used to be in the 737 manual during the Jurassic generation (see the image in he below link) but from all that I’ve seen such a procedure was not included in the Classic, NG, or MAX generation manuals. Why was such information removed from the manuals? It seems pretty fair to me that if the crew is not trained to recognize that they can’t manually re-trim in certain regimes without unloading the stab, that when the FO went to re-trim manually and couldn’t he would report it “not working” and try something else.


https://theaircurrent.com/aviation-safety/vestigal-design-issue-clouds-737-max-crash-investigations/
 
Checkout_my_Six said:
Full disclosure....I don't know what happened nor have I talked with Boeing, nor the Ethiopian investigators...

Na....prior crews with this problem survived. I'll bet they followed the runaway trim procedures....from memory. The key to their survival, IMHO, was time and altitude. They responded quickly and had altitude to recover.

This crew ran outta options because they didn't respond correctly and quickly....and yes, Boeing shoulda done a better job with this band-aided design.....but, others were able to work with it....and there were zero incidents in the US. What's up with that?

I've never cranked the trim under full load to neutral....but, I understand it's doable.
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I don't believe the other incidents commenced seconds after the airplane rotated off the runway. As far as your trim crank comment, try doing it pulling on a control column with 50 or 100 lbs pressure at the same time.

I can imagine the field day you all would be having if they had left the system on, but the flight had the same outcome because of some other compounding reason. "oh my God, they left the cutout engaged for FOUR MINUTES !!. IDIOTS! yadaYada."

As far as I can tell they followed procedure with very little time and very few options in a difficult physical environment. The procedures and the airplane failed them and their passengers.
 
word around the water cooler and my friends who worked the field and Boeing say.....poor ground handling causes those AoA vanes to fail. Could be why no US carriers have reported this failure mode.
 
Chip Sylverne said:
I think zero incidents in the US is a red herring. By all accounts this was a normal take-off roll and rotation. If a bird strike took out the AOA providing the MCAS data, that could have happened anywhere.
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Agreed, is there any evidence that such erroneous AOA readings that caused the improper MCAS activation ever occurred in the US? Everything I’ve been reading is that AOA sensors are normally incredibly reliable so many people in the know were even surprised the bad AOA condition came up. I’m very much willing to believe it has, just haven’t seen any particular evidence of it yet.

Honestly, I do find it pretty disgusting how quickly some people in the US Aviation community jumped to the assumption that because these incidents occurred outside of the US or Western Europe that it was “obviously” due to poor piloting and training. Some were condemning the crews before even the barest amount of information was available. I’m not directing this at anyone here in particular, just what I have seen on various aviation communities.
 
Checkout_my_Six said:
word around the water cooler and my friends who worked the field and Boeing say.....poor ground handling causes those AoA vanes to fail. Could be why no US carriers have reported this failure mode.
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I'm sure you mean that as opposed to US Customs agents using pitot tubes as a stepladder.
 
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