Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Training / Qualification; Troubleshooting; Confusion
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Overcame Equipment Problem; Flight Crew Landed in Emergency Condition; Flight Crew Diverted
Human Factors; Aircraft; Company Policy; Procedure
On departure passing 500-1000AGL; I began to notice abnormally more aft nose down tendency with increased back pressure to the yoke. I attempted to trim nose up; but trim switch would only trim nose down. The auto trim was also trimming nose down as well while accelerating. After a few attempts to trim; both the CA (Captain) and FO (First Officer) trim switches ceased to function. We continued to clean up on schedule and advise ATC that we were having problems controlling the aircraft pitch. I requested a block altitude 4000-6000 and a turn to the west with intent to land back at SeaTac. My FO began trimming nose up to help offset the back pressure. We [advised ATC]; and began running the jammed flight control QRC which led us to the Stab Trim inoperative checklist. My intent was to maintain aircraft control and return to SeaTac. We briefed FAs (Flight Attendants); passengers; advised dispatch; and planned a flaps 15. Landing was uneventful. I chose not to follow the checklist when it directs to engage the autopilot. Because of the yoke back pressure; difficulty maintaining control hand flying; and not knowing for sure what the problem was; I elected to not introduce another variable (autopilot engaging and then abruptly letting go). My intent was to maintain aircraft control hand flying and return to SeaTac. In addition; I did not hand over the controls to my FO to make calls to dispatch; PAs; checklists etc. I determined aircraft control the highest priority.The partial functionality of the trim created Initial confusion with which checklist to start with. The sim profile a few years ago gave an immediate indication with no trim wheel movement. Recommend a possible scenario with partial trim inoperative.
Air carrier Captain reported issues with their elevator trim causing a nose down pitching motion and a heavy yoke.