Originally Posted by
yoko1
3. Trimming forces. Speaking as someone who has actually used the manual trim wheel on the Boeing, I can assure you that the trim resistance increases in both directions. It is not like rolling a boulder uphill where it is very difficult in one direction and easy in the other. The increased resistance is due to binding forces at the jackscrew thread/nut interface. Not sure if it is strictly frictional force or some type of deformation or misalignment. Keep in mind that while the aerodynamic forces are acting roughly parallel to the jackscrew body, those forces are mostly perpendicular at the thread/nut interface. There will be some directional bias, but that is a relatively small component. BTW, this is one of the reasons why a jackscrew mechanism is used in the first place - to minimize those directional forces and make it easier for the motor. However, one point needs to be emphasized again: You completely avoid these issues by not letting the stab get out of trim in the first place. There is no, none, not one iota of evidence that the Main Electric Trim did not move the stab when it was used. And yes, by not being attentive to such matters, it is absolutely possible to place the aircraft in a state where things don't work as they should.
4. There is a lot of reference of what are perceived as anomalies in the trim system in the final moments of both accidents. Personally, that's not what I see, however I'm not going to claim I have a better microscope than anyone else here. That being said, I must again point out that in those final moments, the aircraft had exceeded the certified flight envelope. Engineering safety factors aside, there is not any expressed or implied guarantee that any system will work as it is supposed to once you exceed the limits of that system. The lesson here is that you keep that aircraft within its design envelope - just like the crew of the penultimate Lion Air 610 flight did.
Yet again it must be pointed out
the only reason the crew of the penultimate Lion Air 610 managed to keep the aircraft within its design envelope was because they were lucky enough to have a jumpseater who not only observed the operation of the trim wheel but also persuaded the crew to disable automatic trim.
Frankly the lesson here is that Boeing should never, ever, ever again ship an aircraft with functionality that can programmatically take the aircraft out of its design envelope due to an easily foreseeable fault. Boeing made that mistake all by itself and should cease trying to muddy the waters by pointing the finger at overwhelmed crew trying to deal with multiple alarms caused by that fault.