A CRJ-200 CAPT RPTS OF A UNCOMMANDED MOVEMENT OF THE HORIZ STAB TRIM TO THE NOSE DOWN POSITION ON CLBOUT AND AGAIN AFTER LEVELING OFF AT 4000 FT.

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ON INITIAL CLB OUT; THE STAB TRIM AURAL WARNING SOUNDED BRIEFLY. AFTER FLAP RETRACTION AND COMPLETING THE AFTER TAKEOFF CHECKLIST; WE WERE LEVELING OFF AT 4000 FT. THE TRIM WARNING SOUNDED AGAIN FOR A SHORT TIME THEN STOPPED; AND THE FO (PF) SAID HE HAD A PROBLEM. THE STAB TRIM APPEARED TO BE CLOSE TO OR AT THE FULL NOSE DOWN POSITION. IT WAS NOT CLR TO ME AT THIS POINT WHETHER WE WERE DEALING WITH A TRIM RUNAWAY OR A STAB TRIM FAILURE. I THOUGHT THAT THE FO WAS UNABLE TO MAINTAIN A LEVEL ATTITUDE SO I TOOK HOLD OF THE YOKE ALSO; TO HELP PULL BACK AND KEEP THE NOSE UP. AS I DID THAT; I HIT THE STAB DISC SWITCH AND DISCONNECTED BOTH STAB CHANNELS. I ATTEMPTED TO REENGAGE ONE OF THE CHANNELS. THINKING THAT ONE OF THE CHANNELS WAS CAUSING THE PROBLEM; I WANTED TO GET TRIM CAPABILITY BACK WITH THE GOOD CHANNEL. I ENGAGED STAB CH 2 AND THE TRIM DID NOT RESPOND. I DISCONNECTED AGAIN AND ENGAGED STAB CH 1. WITH STAB CH 1 ONLY ENGAGED WE GOT TRIM CONTROL BACK AND RETURNED THE ACFT TO A NORMAL TRIM CONDITION. I ELECTED TO CONTINUE TO ZZZ1. WE REVIEWED THE QRH PROC FOR STAB TRIM RUNAWAY AND STAB CH 2 INOP; CLOSELY MONITORING THE STAB TRIM INDICATIONS AS WE CONTINUED THE FLT TO ZZZ1. THERE WERE NO FURTHER PROBLEMS. DURING THE TIME FROM WHEN THE FO SAID THERE WAS A PROBLEM TO WHEN WE GOT BACK STAB TRIM CAPABILITY; I HAD BEEN COMMUNICATING WITH THE DEP CTLR. I HAD LET HIM KNOW THAT WE HAD A TRIM PROBLEM; WE WERE WORKING ON IT; AND WOULD TAKE VECTORS BACK TO THE FIELD WHEN WE COULD. HE CLRED US FOR ALTITUDE AT OUR DISCRETION. INITIALLY WE HAD BEEN CLRED TO 4000 FT ON DEP; BUT WHILE STABILIZING THE ACFT WE HAD CLBED TO 4500 FT; AND AT ONE POINT ACCELERATED TO 270 KTS. FO (PF) RECOGNIZED ACFT HANDLING PROBLEM. CONTROL INPUTS TO COUNTERACT THE UNCOMMANDED NOSE DOWN TRIM; THEN ATTEMPTS TO RESTORE THE STAB TRIM FUNCTION. THERE WAS UNCOMMANDED MOVEMENT OF THE STABILIZER TRIM TO THE NOSE DOWN POSITION.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THE FO WAS FLYING AT THAT TIME AND USING THE PITCH TRIM LEVER SWITCH ON THE RIGHT THUMB POSITION OF THE CO-PILOT'S CONTROL WHEEL. AFTER LEVELING OFF AT 4000 FT AND THE STAB TRIM WARNING 'CLACKER' GOING OFF; THEY WERE NOT SURE IF THEY WERE DEALING WITH A HORIZ STAB TRIM RUNAWAY OR A STAB TRIM FAILURE. DUE TO THE STAB'S UNCOMMANDED LEADING EDGE POSITION; HE RECOGNIZED THE FO NEEDED ASSISTANCE IN HOLDING BACK THE YOKE TO MAINTAIN LEVEL CONTROL WITH THE ELEVATORS. RPTR REALIZED THE STAB WARNING 'CLACKER' WAS INCONSISTENT; NOT A STEADY SOUND; BUT; ON AND THEN OFF; THEN ON AND OFF AGAIN. NORMALLY THE 'CLACKER' WOULD SOUND AFTER 3 SECONDS OF STAB TRIM OPERATION. HOWEVER; THIS WAS ONLY MOMENTARY AND NOT CONSISTENT WITH A RWY TRIM. NO MANUAL HORIZ STAB TRIM WHEEL EXISTS ON THIS ACFT. ALL FLT CONTROL POSITION INDICATIONS AND MESSAGES; INCLUDING THE STAB TRIM POSITION; ARE ON THE EICAS SECONDARY DISPLAY; CENTER INSTRUMENT PANEL. THE EICAS STAB POSITION INDICATION DOESN'T ALWAYS REFLECT REAL TIME POSITION OF THE STAB TRIM. THE STAB PITCH TRIM ARE SPLIT ROCKER TYPE SWITCHES; SO BOTH MUST 'GO' (ENGAGE) BEFORE ANY COMMAND IS INITIATED TO THE STAB. THIS DESIGN IS THE SAFETY FEATURE OF THE PITCH TRIM. THE STAB TRIM CHANNEL 1; CHANNEL 2 AND THE MACH TRIM ARE PUSH BUTTON SWITCHES ON THE CENTER PEDESTAL. RPTR ALSO STATES HE HAS EXPERIENCED UNCOMMANDED DISENGAGING OF THE STAB TRIM CHANNELS 1 AND 2 WITH A MASTER CAUTION COMING ON ON MORE THAN A FEW OCCASIONS. MAINT INFORMED HIM (RPTR); THEY FOUND THE CO-PILOT'S PITCH TRIM SPLIT ROCKER SWITCHES ON THE CONTROL WHEEL HAD SHORTED AND APPEARED CORRODED AND THE SAFETY FUNCTION WASN'T EVEN WORKING. RPTR CONCLUDED WITH HIS CONCERNS WITH THE SAFETY FEATURE DESIGN IF THIS SPLIT ROCKER SWITCH CAN STILL SUBJECT THE ACFT TO UNCOMMANDED PITCH TRIM INPUTS.