Ok, so for aircraft with trimmable horizontal stabilisers (THS) you must never hold the trim switch engaged for extended periods, but must release the switch frequently, so as to allow the column to return to neutral - and allow the forces to diminish.
So, that process would be:
1. "Blip" the thumb-switch for a second or two, then release;
2. Relax grip on the wheel, allowing it to move toward neutral;
3. Assess the remaining forces;
Then repeat. Blip, relax, assess. Blip, relax, assess.
But this is NOT how pilots are taught to trim an aircraft. When pilots first learn to fly, one of the first things they learn is to hold the attitude, then trim until the control forces go to zero.
But it seems that trimming a large, THS aircraft that way can be fatal.
Do pilots undergo retraining on how to apply trim when they transition onto these larger aircraft? It appears not.
Of course, when the trim change is small, pilots will instinctively employ and repeat the "blip, relax, assess". This is something they have learnt, by themselves, to do - as it is perfectly appropriate. But things are much more interesting when the trim change is large, it would appear.
Drawing on
gonebutnotforgotten's excellent trimming article once again, I quote this passage from Boeing, relating to nose-high upsets:
... the use of some nose-down stabilizer trim should provide sufficient elevator control to produce a nose-down pitch rate. It may be difficult to know how much stabilizer trim to use, and care must be taken to avoid using too much trim.
Are pilots trained to know how much "some" nose-down trim is? And how to "avoid using too much trim"?