Ethiopian Airlines 737 MAX Investigation Preliminary Report
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Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#2This shouldn't be marked as a dupe since it's the actual report.
It sounds like they did everything right. There are two fateful moments:
>At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working
At this point they were in stable flight and the trim wasn't that extreme. An inability to manually adjust trim is a huge design flaw unless it was pilot error.
>At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units. >At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down. The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording.
They re-activated electric trim control but only slightly adjusted it. MCAS kicks in to send them nose down. There's no further pilot correction recorded. Again, unclear here if there is an airplane issue preventing them from adjusting trim or a pilot error.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#3Indeed looks like they did disable MCAS using the stabilizer trim cutout switches, but the aircraft was trimmed nose down when they did so. Aerodynamic loading prevented manual trimming using the trim wheels. They eventually turned electric trim back on, as they didn't really have any other options, but didn't trim back enough, and left it turned on. At this point the right airspeed indicator (which should be more reliable) reads 365 knots. VMO is 340, so they're already in big trouble. 5 seconds later, MCAS kicked back in, and then it was all over. Very sad.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#4This isn't only MCAS issue anymore... If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. Interesting to find out was that an issue with older 737 models, just without severe runaway trim events.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#5Indeed looks like they did disable MCAS using the stabilizer trim cutout switches, but the aircraft was trimmed nose down when they did so. Aerodynamic loading prevented manual trimming using the trim wheels. They eventually turned electric trim back on, as they didn't really have any other options, but didn't trim back enough, and left it turned on. At this point the right airspeed indicator (which should be more re…
So MCAS put them in a hard to recover position which required electric trim to recover, and as soon as they turned on the electric trim MCAS turned on again and forced them into a crash?
Then this is much worse than initially suggested, it means a pilot can be aware of MCAS and still be put in a extremely dangerous position due to the extreme control authority of the module.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#6Indeed looks like they did disable MCAS using the stabilizer trim cutout switches, but the aircraft was trimmed nose down when they did so. Aerodynamic loading prevented manual trimming using the trim wheels. They eventually turned electric trim back on, as they didn't really have any other options, but didn't trim back enough, and left it turned on. At this point the right airspeed indicator (which should be more re…
So MCAS put them in a hard to recover position which required electric trim to recover, and as soon as they turned on the electric trim MCAS turned on again and forced them into a crash? Then this is much worse than initially suggested, it means a pilot can be aware of MCAS and still be put in a extremely dangerous position due to the extreme control authority of the module.
Per the article a combination of things put them in that situation. Not just MCAS. Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Complex systems that have millions of man hours spent making them safer don't usually have back to back failures because of one faulty subsystem. We've incrementally refined commercial air travel to the point where single source accidents are basically nonexistent.
I'm not saying that MCAS isn't mostly to blame but the fact that the maximum inputs from the pilot is still lesser than the control authority from the trim raises some serious questions MCAS or not. Had "extend flaps" been at the bottom of the checklist that would have disabled MCAS and likely prevented this crash.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#7> The crew performed runaway stabilizer checklist and put the stab trim cutout switch to cutout position and confirmed that the manual trim operation was not working.
This is very damning to Boeing. It's precisely the procedure they prescribed and intended to mitigate possible MCAS caused trim runaway. In light of the fact that they put this in place as a justification for moving the engines forward without reclassifying the plane, I don't see how they can get around fault here.
What a tragedy.
I can't imagine the emotion those pilots must have felt after attempting to do exactly what they were told to do to keep the aircraft in the air, only to have the nose lurch back downwards. Godspeed, gentlemen. It wasn't your fault.
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#8Earlier quoted context omitted.
So MCAS put them in a hard to recover position which required electric trim to recover, and as soon as they turned on the electric trim MCAS turned on again and forced them into a crash? Then this is much worse than initially suggested, it means a pilot can be aware of MCAS and still be put in a extremely dangerous position due to the extreme control authority of the module.
Per the article a combination of things put them in that situation. Not just MCAS. Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Complex systems that have millions of man hours spent making them safer don't usually have back to back failures because of one faulty subsystem. We've incrementally refined commercial air travel to the point where single source accidents are ba…
MCAS issues nose-down, speed increases and nose pitches down. Nose never pitches back up. Plane crashes.
Sure, maybe some complex combination of maneuvers could have helped them out of this but there's really everything pointing the blame at this one system. Pilots did what the boeing manual suggested (even included at the end of this report).
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#9I really feel as we're entering an era where full automation is the only solution. This current paradigm of the plane telling the pilots "I don't know what to do here - handing it back to you" doesn't make much sense. Do we just need better multi-modal and redundant sensors so the plane isn't flying blind?
Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report
#10This isn't only MCAS issue anymore... If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. Interesting to find out was that an issue with older 737 models, just without severe runaway trim events.
> If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem.
It's not so much a "problem" as it is "physics". If the stabilizers are forcing the nose down, and the pilot pulls back on the stick (engaging the elevators) to pull the nose up, the forces exerted on the stabilizer are such that, in extreme cases, human force is insufficient to move the manual wheel.
This is extremely (EDIT: No longer) well documented by Boeing going back decades, and it presents solutions of either: 1) reengaging the electronic stabilizer trim and trimming electronically back to a point where the forces are manageable, then cutting electronic trim again, or 2) relaxing the stick, letting the nose go the way the stabilizers want it to, which slackens the force, and makes cranking the wheel manually doable. Great if you're stabilizers are trimmed severely nose down at a height of 40,000 feet; less great at a height of 1,000 feet. (EDITED TO ADD: Apparently this information & procedure was removed from Boeing's manuals after the 1980s, and pilots no longer train on it, as the issues that led to its necessity were thought solved. The MCAS apparently has reintroduced the need for them.).
This is compounded by a new change in the 737-MAX: in prior 737 series aircraft, there are two stab trim cutout switches: one to cut the autopilot's ability to command the stabilizer trim, the other to cut electronic power to the stabilizer trim adjustment entirely, so you could stop the autopilot from changing the trim but still retain electronic control and not need to worry about the manual crank forces. The two switches are MAIN ELEC and AUTO PILOT.
In the MAX, this changed, and it's now an all or nothing setup: you have to kill electronic power to kill autopilot commands to the stabilizer. The two switches are now PRI and B/U. Both must be cut out to stop any stabilizer runaway, including MCAS runaway.
This all seems to add up to Boeing's narrative of "The MCAS is safe, pilots just need to know to hit the stab trim cutout" being grossly insufficient. They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control.