Would it be fair to say that the previous crew worked an issue whilst not understanding the nature of the issue? In other words, they thought they had a trim runaway and flicked the stab trim switches. Luck rather than judgement?
A trim runaway is not taught to come in intervals. I guess you a right, probably pure luck.
You see the tech log from the previous flight, it says:
"IAS and ALT Disagree shown after take off","Performed flushing Left Pitot Air Data Module (ADM) and static ADM. Operation test on ground found satisfied."
"feel diff press light illuminate","performed cleaned electrical connector plug of elevator feel computer carried out. test on ground found OK"
There is nothing said about a trim or stab runaway or STS issue (the comment about STS trimming wrong was made to an IT system that you did not have access to before the flight)
You are mentally prepared for an IAS and ALT disagree, but somehow you expect it had been fixed.
You are not mentally prepared for a trim system to push the nose down on its own.
You pull, stick shaker goes off on rotation.
You get IAS disagree.
You remember the tech log saying ALT DISAGREE, your co asks ATC. They tell you you are at 900 feet. This seems to be in line to what you see.
You are somehow comfortable with the speed, you retract the flaps at 2000 feet.
You see the trim wheels spinning nose down in increments. This is the unexpected. You are surprised.
You pull and from your NG experience you expect the trim to stop, it does not, it keeps trimming.
You think trim down makes sense somehow to fight the stick shaker. But you have not heard of a 737 system that does that.
You think it is better to set flaps again.
Stick shaker remains. Does not make sense to you. But trim stops.
You are fixed on speed. You are uncertain which is the correct speed, left or right or stby. (The report suggests from the ATC tape that all three speed indications were different).
You are confused, you order flaps up again, you are concerned about speed.
Trim spins down again in intervals, you never heard of a 737 doing that before.
You think it is not a runaway stab, it is not STS, it does not stop when I pull, it is something different. But what?
You are fixed on not becoming too slow and stall and not too fast and break the plane.
You trim up, the plane trims down ...
You do not trust any indication in front of you.
You ask PNF to ask ATC about ground speed.
ATC tells you are at 322 kts, you are at 5000 feet. You think that's damned fast and not far away from 320 KIAS, damned fast,
You are fixed it is an UAS issue.
You have never heard of an automatic trim down function in intervals.