Maninthebar, #1832 not known; the background is in the EASA reference below.
“Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope,”
“The trim wheel can be used to trim the airplane throughout the entire flight envelope. In addition, the autopilot has the authority to trim the airplane in these conditions.
The reference regulation and policy do not specify the method of trim, nor do they state that when multiple pilot trim control paths exist that they must each independently be able to trim the airplane throughout the flight envelope.”
“The main issue being that longitudinal trim cannot be achieved throughout the flight envelope using thumb switch trim only.
…
Boeing set the thumb switch limits in order to increase the level of safety for out-of-trim dive characteristics (CS 25.255(a)(1)). The resulting thumb switch limits require an alternative trim method to meet CS 25.161 trim requirements in certain corners of the operational envelope.
The need to use the trim wheel is considered unusual, as it is only required for manual flight in those corners of the envelope.”
The inference is that the stick trim switches are electrically inhibited at a particular tail position, preventing either ANU or AND movement (otherwise it would invalidate the purpose of inhibition for safety reasons - trim runaway).
Points to note: the ‘discrepancy’ was noted in simulation. ‘Authority’ or inhibited?
Apparently (significant speculation) the use of trim wheel, in simulation (as the alternative to elect trim) was restricted to the certification overspeed requirements, and not the full range of physical trim movement, or ability to manually trim over this trim range. Thus did not identify any inability to move the trim manually.
Apparently the autopilot / auto trim has the ability to trim over full range. This is ambiguous; either as required for autopilot control - stay in trim, but not manual trim cert requirements. Also, this could invalidate the safety inhibition as above - trim is still electrically enabled, and ‘if so’, and ‘if’ MCAS was routed via the ‘auto’ FCC, then it could electrically signal the tail to move in conditions which the stick trim could not.
Reference to ‘aisle stand trim switches’ is confusing, perhaps trim wheel.
https://www.easa.europa.eu/sites/def...20ISS%2010.pdf Page 15