Originally Posted by
DaveReidUK
ICAO Annex 6 lists mandatory FDR parameters, and is well worth a read. One of those parameters is "Cockpit pitch trim control input position" (my emphasis, not ICAO's).
That's why FDAUs came along in the 1970s.
The FDR trace show only one cockpit pitch trim control input position, it does now show BOTH thumn switches, that might lead to assume is an elaborated signal that is taken at the servo motor controller. Of course the traces might have been elebaorated before been published, however the information published does not validate a theory of the trace representing the thumb switch at the source