6 ms·

> And, the crew left the power setting at climb throughout almost the entire sequence, so they oversped the airframe. I believe that's correct given that they

> And, the crew left the power setting at climb throughout almost the entire sequence, so they oversped the airframe. I believe that's correct given that they had an Unreliable Airspeed Indicator warning (due to the malfunctioning AoA sensor), per the memory items on the UAI checklist. It makes the forces on the trim wheel much worse, which explains why they couldn't turn it, but it's the correct checklist response to the conditions observed, and the Boeing/FAA directive completely failed to take the impact of this response into account.

cjbprime 8y ago
I think they couldn't turn the trim wheel because STAB TRIM CUTOUT unpowers it.
- msbarnett 8y ago
  No, the wheel has an extendable crank handle and is meant for manual use precisely when the electronic stabilizer trim is cutout. But aerodynamic forces can conspire to make it very hard or physically impossible for human force to rotate it if the elevator is being used to combat the stabilizer at high speeds. See here for more discussion of the physics behind it: https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-range.html https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-ra...
  - cjbprime 8y ago
    Thanks, that makes sense. When the report says that the pilots found manual trim impossible after STAB TRIM CUTOUT, do you think it's referring to an attempt at manual electric trim (expected to fail) or both pilots manually moving the trim wheels with the handle?
    - msbarnett 8y ago
      My (non-expert) reading is that the co-pilot couldn't move the wheel, while the pilot was engaged in fighting to to keep the stick pulled back (a Swedish pilot tried this scenario in a simulator last week and literally had to keep both arms locked around the stick towards the end). Counter-intuitively, letting go of the stick in brief increments might have been the correct move. Letting the nose pitch down would take force off the jackscrew and let both pilots crank hard on the stabilizer. Boeing manuals once covered this, but apparently they haven't since the 1980s, and their directive after the Lion Air accident made no mention of the necessity of such a procedure. Also they were only 7,000 feet above the ground so whether or not they'd have recovered in time is hard to say. Quite possibly the MCAS had already doomed the flight.
      - cjbprime 8y ago
        > they were only 7,000 feet above the ground No, it's worse: the origin airport is at 7000ft elevation. They only had around 1000ft height AGL for all of the flight. So I agree that releasing the elevator at all seems surely suicidal.
        msbarnett 8y ago
        They're at 12,000-14,000 feet altitude at 05:43ish, when the co-pilot reports the non-functional manual trim. So ~6,000-7,000 height. Still damn close to the ground to be comfortable letting the plane dive.
        cjbprime 8y ago
        Oh! I think this is new information in the report -- the FlightRadar24 ADS-B data never shows them above 8500ft altitude, ~1000ft AGL. FlightRadar24 must have missed the final few minutes? https://www.flightradar24.com/blog/flightradar24-data-regarding-the-crash-of-ethiopian-airlines-flight-302/ https://www.flightradar24.com/blog/flightradar24-data-regard...
        treis 8y ago
        But high enough to re-enable a system you know is malfunctioning? I guess we will have to wait for the final report but the pilot's actions are perplexing here. Even if we accept that they can't trim with the wheel, why enable electric trim and then not use it? Why not enable it, trim to where you want, and then use the cut out switch again?
        msbarnett 8y ago
        > But high enough to re-enable a system you know is malfunctioning? They couldn't land with the stabilizers mis-trimmed. If the wheel wouldn't budge, re-enabling it and trimming electronically was the only option likely to occur to them, since the only other possible option (release the column, let the plane nose down so that the forces on the screw ease and they can trim manually) is completely counter-intuitive and was removed from Boeing's documentation and simulator training 30-40 years ago and was not covered in the FAA/Boeing MCAS directive. > Even if we accept that they can't trim with the wheel, why enable electric trim and then not use it? They did use it after re-enabling it. They just didn't re-disable it within 5 seconds of their last input manual electronic trim command, so the MCAS ran again a final time. Whether that's because they just didn't get to the switches in time, weren't aware they had such a short window to do so (the FAA/Boeing bulletin does a piss poor job of communicating this), or were just so thoroughly overwhelemed by everything that was going on is hard to say at this point.
    - phire 8y ago
      The flight data graph shows no manual electric trim inputs at that time. So must be manual trim wheel movements.
- Rooster61 8y ago
  It cuts out any electronic input, namely autopilot influence and the electric trim control present on the yoke. The trim wheel is not electric, and is the intended control when electric stabilizer control is either unavailable or intentionally disabled.
- deleted 8y ago
  [deleted]
- WalterBright 8y ago
  The trim wheel is mechanically connected via cables to turn the jackscrew.
VBprogrammer 8y ago
> I believe that's correct given that they had an Unreliable Airspeed Indicator warning (due to the malfunctioning AoA sensor), per the memory items on the UAI checklist. The idea that a pilot would knowingly leave thrust at climb power while in modestly level flight at low altitude and expect not to overspeed is very strange to me, checklist or no checklist. I think they were too busy fighting the controls to worry about that.
- wazoox 8y ago
  Part of the procedure is to keep thrust or augment it, after disengaging auto-throttle. Remember that they were in stall warning, too (stick shaker), which certainly shouldn't incite anyone to throttle down.
- acqq 8y ago
  They followed Boeing instructions (provided in the report, Appendix 2, and also here: https://theaircurrent.com/wp-content/uploads/2018/11/B737-MAX-AD-1107.pdf https://theaircurrent.com/wp-content/uploads/2018/11/B737-MA... ). Note IAS DISAGREE alert: "In the event of an uncommanded horizontal stabilizer trim movement, combined with any of the following potential effects or indications resulting from an erroneous Angle of Attack (AOA) input, the flight crew must comply with the Runaway Stabilizer procedure in the Operating Procedures chapter of this manual: - Continuous or intermittent stick shaker on the affected side only. - Minimum speed bar (red and black) on the affected side only. - Increasing nose down control forces. - IAS DISAGREE alert. - ALT DISAGREE alert. - AOA DISAGREE alert (if the option is installed)" About which was written here: https://leehamnews.com/2019/04/03/et302-used-the-cut-out-switches-to-stop-mcas/ https://leehamnews.com/2019/04/03/et302-used-the-cut-out-swi... "the high speeds observed" ... "were logical. It’s a consequence of following the Emergency checklist for “IAS disagree” (IAS is Indicated Airspeed, i.e. the dynamic air pressure experienced by the aircraft) after takeoff." In short, seeing that Boeing indeed wrote about the IAS alert being activated when the sensor fails, and MCAS gets activated, I can only conclude that Boeing indeed knew that the pilots would make the plane uncontrollable if they followed Boeing's instructions. It seems they just gambled on the chance that the second crash won't happen so soon after the first. Either that, or we'd have to believe that what a single pilot manage to do for his Youtube video a company which is to deliver the planes in worth of hundreds of billions (!) of USD wasn't able to do.

cjbprime 8y ago

I think they couldn't turn the trim wheel because STAB TRIM CUTOUT unpowers it.

msbarnett 8y ago
No, the wheel has an extendable crank handle and is meant for manual use precisely when the electronic stabilizer trim is cutout. But aerodynamic forces can conspire to make it very hard or physically impossible for human force to rotate it if the elevator is being used to combat the stabilizer at high speeds. See here for more discussion of the physics behind it: https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-range.html https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-ra...
- cjbprime 8y ago
  Thanks, that makes sense. When the report says that the pilots found manual trim impossible after STAB TRIM CUTOUT, do you think it's referring to an attempt at manual electric trim (expected to fail) or both pilots manually moving the trim wheels with the handle?
  - msbarnett 8y ago
    My (non-expert) reading is that the co-pilot couldn't move the wheel, while the pilot was engaged in fighting to to keep the stick pulled back (a Swedish pilot tried this scenario in a simulator last week and literally had to keep both arms locked around the stick towards the end). Counter-intuitively, letting go of the stick in brief increments might have been the correct move. Letting the nose pitch down would take force off the jackscrew and let both pilots crank hard on the stabilizer. Boeing manuals once covered this, but apparently they haven't since the 1980s, and their directive after the Lion Air accident made no mention of the necessity of such a procedure. Also they were only 7,000 feet above the ground so whether or not they'd have recovered in time is hard to say. Quite possibly the MCAS had already doomed the flight.
    - cjbprime 8y ago
      > they were only 7,000 feet above the ground No, it's worse: the origin airport is at 7000ft elevation. They only had around 1000ft height AGL for all of the flight. So I agree that releasing the elevator at all seems surely suicidal.
      - msbarnett 8y ago
        They're at 12,000-14,000 feet altitude at 05:43ish, when the co-pilot reports the non-functional manual trim. So ~6,000-7,000 height. Still damn close to the ground to be comfortable letting the plane dive.
        cjbprime 8y ago
        Oh! I think this is new information in the report -- the FlightRadar24 ADS-B data never shows them above 8500ft altitude, ~1000ft AGL. FlightRadar24 must have missed the final few minutes? https://www.flightradar24.com/blog/flightradar24-data-regarding-the-crash-of-ethiopian-airlines-flight-302/ https://www.flightradar24.com/blog/flightradar24-data-regard...
        treis 8y ago
        But high enough to re-enable a system you know is malfunctioning? I guess we will have to wait for the final report but the pilot's actions are perplexing here. Even if we accept that they can't trim with the wheel, why enable electric trim and then not use it? Why not enable it, trim to where you want, and then use the cut out switch again?
        msbarnett 8y ago
        > But high enough to re-enable a system you know is malfunctioning? They couldn't land with the stabilizers mis-trimmed. If the wheel wouldn't budge, re-enabling it and trimming electronically was the only option likely to occur to them, since the only other possible option (release the column, let the plane nose down so that the forces on the screw ease and they can trim manually) is completely counter-intuitive and was removed from Boeing's documentation and simulator training 30-40 years ago and was not covered in the FAA/Boeing MCAS directive. > Even if we accept that they can't trim with the wheel, why enable electric trim and then not use it? They did use it after re-enabling it. They just didn't re-disable it within 5 seconds of their last input manual electronic trim command, so the MCAS ran again a final time. Whether that's because they just didn't get to the switches in time, weren't aware they had such a short window to do so (the FAA/Boeing bulletin does a piss poor job of communicating this), or were just so thoroughly overwhelemed by everything that was going on is hard to say at this point.
  - phire 8y ago
    The flight data graph shows no manual electric trim inputs at that time. So must be manual trim wheel movements.
Rooster61 8y ago
It cuts out any electronic input, namely autopilot influence and the electric trim control present on the yoke. The trim wheel is not electric, and is the intended control when electric stabilizer control is either unavailable or intentionally disabled.
deleted 8y ago
[deleted]
WalterBright 8y ago
The trim wheel is mechanically connected via cables to turn the jackscrew.

VBprogrammer 8y ago

> I believe that's correct given that they had an Unreliable Airspeed Indicator warning (due to the malfunctioning AoA sensor), per the memory items on the UAI checklist. The idea that a pilot would knowingly leave thrust at climb power while in modestly level flight at low altitude and expect not to overspeed is very strange to me, checklist or no checklist. I think they were too busy fighting the controls to worry about that.

wazoox 8y ago
Part of the procedure is to keep thrust or augment it, after disengaging auto-throttle. Remember that they were in stall warning, too (stick shaker), which certainly shouldn't incite anyone to throttle down.
acqq 8y ago
They followed Boeing instructions (provided in the report, Appendix 2, and also here: https://theaircurrent.com/wp-content/uploads/2018/11/B737-MAX-AD-1107.pdf https://theaircurrent.com/wp-content/uploads/2018/11/B737-MA... ). Note IAS DISAGREE alert: "In the event of an uncommanded horizontal stabilizer trim movement, combined with any of the following potential effects or indications resulting from an erroneous Angle of Attack (AOA) input, the flight crew must comply with the Runaway Stabilizer procedure in the Operating Procedures chapter of this manual: - Continuous or intermittent stick shaker on the affected side only. - Minimum speed bar (red and black) on the affected side only. - Increasing nose down control forces. - IAS DISAGREE alert. - ALT DISAGREE alert. - AOA DISAGREE alert (if the option is installed)" About which was written here: https://leehamnews.com/2019/04/03/et302-used-the-cut-out-switches-to-stop-mcas/ https://leehamnews.com/2019/04/03/et302-used-the-cut-out-swi... "the high speeds observed" ... "were logical. It’s a consequence of following the Emergency checklist for “IAS disagree” (IAS is Indicated Airspeed, i.e. the dynamic air pressure experienced by the aircraft) after takeoff." In short, seeing that Boeing indeed wrote about the IAS alert being activated when the sensor fails, and MCAS gets activated, I can only conclude that Boeing indeed knew that the pilots would make the plane uncontrollable if they followed Boeing's instructions. It seems they just gambled on the chance that the second crash won't happen so soon after the first. Either that, or we'd have to believe that what a single pilot manage to do for his Youtube video a company which is to deliver the planes in worth of hundreds of billions (!) of USD wasn't able to do.