Clearly these incidents have raised many questions, especially for the trained professionals that operate the 737. The consequences of these accidents could maybe also demonstrate issues relating to trim on other 737 variants including the 737 NG. We must remain open to learning the lessons and exploring the wider implications.
If the Ethiopian crew did follow the Stabiliser Runaway memory actions as per QRH NNC, then clearly the very next action that would have followed would have been an attempt to use the manual trim handles. Should the excessive control loading have either blocked or restricted movement of that control due to control loading, the crew would find themselves in a perilous situation and dive. At that point out of desperation perhaps they tried to use the electric trim again and reactivated the electric trim cutout switches. I don't think that is too extraordinary to have attempted out of desperation if the trained procedures were failing were failing.
Manual control of trim on the 737 NG is usually straightforward, but it not impossible to find yourself possibly in a nasty situation. I have once experienced in a level D 738 sim following double engine flameout, high power nose high prior to failure, to a quick ensuing nose low dive. It shocked me the difficulty to raise the nose again and high loading on the manual trim handles. We recovered, but recall unloading the control column back pressure for a short time to help having read about such possible scenarios.
It is too early to say, but there could be lessons here not only just applicable to the Max but perhaps even the NG with regards to control loading whilst using manual trim. If we consider accidents including the 738 Fly Dubai nose dive, control loading implications to manual trim operation are important. After any stab trim runaway, it is essential that we have confidence in the effectiveness of the manual trim throughout the full range of movement and control loadings.
More attention is needed imo to why repeated AOA failures are happening on both 737 NG & Max variants.
For the crew operating these aircraft, there is little doubt that they were suddenly faced with multiple simultaneous failures and warnings. They had seconds to react to complex failures that could easily overload competent crews. If faced with Airspeed Unreliable, possibly altitude disagree, stick shaker, possible other master caution warnings, Stabiliser Runaway, increasing pitch down descent rate, followed by possible manual trim control being restricted by control loading (not proven yet) then they really did have a lot to deal with and the odds were stacked against them.
I think we should all be respectful to the families of the professionals that died trying in vain to regain control. It is too easy for armchair pilots to be judgmental without understanding the human factors and technical issues involved on the flight deck in these incidents. There is no evidence that crew training or that the airlines themselves had much if any involvement in outcome pf these accidents.