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> sufficient nose up trim was not selected by the yoke manual trim switch prior to pulling the trim cutouts The EAD/service bulletin doesn't talk about suffici
by cjbprime 8y ago
> sufficient nose up trim was not selected by the yoke manual trim switch prior to pulling the trim cutouts
The EAD/service bulletin doesn't talk about sufficient nose up trim or caution against performing the cutout with even a slight mistrim.
> And then they did not work the mechanical trim wheel hard enough to reset the residual nose down trim.
It's possible that aerodynamic load (of the stabilizer opposing the elevator) made it physically impossible to manually trim given any mistrim at the time the cutout happened, given the airspeed they had. And, the captain is pulling back on the yoke as hard as he can, so he's unavailable to let go of it and grab a trim wheel instead without immediately losing altitude.
> And, when they (against procedure) (apparently) turned the electric trim system back on
It seems sensible to assume -- and the report states -- that they did this because they found that manual trim was impossible in these circumstances.
- kayfox 8y ago> It's possible that aerodynamic load (of the stabilizer opposing the elevator) made it physically impossible to manually trim given any mistrim at the time the cutout happened, given the airspeed they had. Its been noted in previous discussions that while it would be difficult, the load can be overcome if both pilots were to operate the manual trim wheel at the same time. Not that its a good situation to be in.
- 3JPLW 8y agoIs it still possible at or near VMO? And when the trim is maxed out? I don't think I've seen an authoritative answer on that, and that's the situation they faced.
- cjbprime 8y agoAnother commenter pointed out that the captain is pulling back as hard as he possibly can on the yoke; releasing it to turn a trim wheel instead may have been just as deadly.
- VBprogrammer 8y agoIf my reading of the graphs is correct they are above Vmo for most of the episode. I'm surprised the flight surfaces weren't torn from their hinges.
- na85 8y agoVmo incorporates a factor of safety in its specification, so exceeding it is not a "parts fall off the aircraft" event. Vne is where things start to get scary because it's usually based on flutter, which can catastrophically damage/destroy the aircraft in very short order.
- VBprogrammer 8y agoYes. But it doesn't include full control deflections and certainly not control deflections in opposition to one another. Even at Va rapid control movements and control movements in multiple axes can permently damage the aircraft.
- na85 8y agoCorrect
- cjbprime 8y ago> Its been noted in previous discussions that while it would be difficult, the load can be overcome if both pilots were to operate the manual trim wheel at the same time. I haven't seen that noted anywhere, and have seen it theorized as impossible without motor assist, given their airspeed and with the elevator opposing. Got a link?
- deleted 8y ago[deleted]
- swhitt 8y agoManual Stabilizer Trim Excessive airloads on the stabilizer may require effort by both pilots to correct the mis-trim. In extreme cases it may be necessary to aerodynamically relieve the airloads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually. https://www.pilot18.com/wp-content/uploads/2017/01/B737NG_FCTM_31-10-05.pdf https://www.pilot18.com/wp-content/uploads/2017/01/B737NG_FC... page 312 (8.16)
- cjbprime 8y agoTo be clear, that's a link that agrees with me that it can be impossible, not a link that says they just needed to work harder. It's also from a 20 year-old manual, and isn't present on newer manuals, and pilots haven't trained on it in decades. Not appropriate to expect pilots to know, and possibly not helpful here: the pilots were low altitude, so allowing the nose to drop even further (to relieve aerodynamic load) may have been deadly too.
- swhitt 8y agohttps://www.satcom.guru/2019/04/stabilizer-trim-loads-and-range.html https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-ra... does a pretty good job of summarizing it I think. It sounds incredibly difficult to do.
- davepage 8y agoThe Boeing/Ethiopian ETH-2 bulletin quoted in the above linked AAIB preliminary report on page 33 says "Initially, higher control forces may be needed to overcome any stabilizer nose down trim already applied. Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT ..."
- msbarnett 8y ago> Electric stabilizer trim can be used to neutralize control column pitch forces before moving the STAB TRIM CUTOUT switches to CUTOUT .. Which really under explains the situation. In reality the STAB TRIM CUTOUT must be switched to CUTOUT within 5 seconds of using the electronic stabilizer trim, or the MCAS will re-establish the forces.
- verytrivial 8y agoWow. This sub-thread is making the images of the grounded MAXs look a whole lot less like an overabundance of caution. If accurate this failure dynamic sounds more like some nasty boss at the end of an escape room game than standard fault diagnostics.
- cjbprime 8y agoYeah, I think that's fairly put. The MCAS system was pretty straightforwardly trying to kill them, and the only way to disable it also disabled their method of recovering from what it was doing.
- calecon323 8y agoYou're right that the service bulletin doesn't talk about selecting selecting a sufficient nose up trim because control of the aircraft using manual trim is a continuous action performed in combination with the pitch up command of the control yoke. Just pulling up on the control column and activating momentary manual trim isn't sufficient or intended. I understand why people think the checklist implies that performing a STAB TRIM cutout occurs instantly after the autopilot is disengaged because of the "if runaway continues" language.