A CRJ100 IN CRUISE AT FL340 DECLARED AN EMER AND DIVERTED DUE TO A RUNAWAY HORIZONTAL STABILIZER NOSE DOWN. NO EICAS WARNING PRIOR TO FAILURE.

Narrative

WE WERE IN CRUISE FLT AT FL340; .77 MACH; WHEN WE HEARD THE AUTOPLT DISENGAGE. THERE WERE NO MESSAGES ON THE EICAS BESIDES THE SMOKING STATUS MESSAGE. THE AIRPLANE STARTED DSNDING QUICKLY AND WE RECOVERED AFTER A 500 FT ALT LOSS. THE YOKE WAS APPLYING A CONSIDERABLE NOSE DOWN PRESSURE AT WHICH TIME WE NOTICED THE STAB TRIM INDICATION WAS 0.0. I TRIED APPLYING NOSE UP TRIM WITHOUT SUCCESS. WE NOTICED THE TRIM WOULD RISE APPROX 1.0; BUT IMMEDIATELY RETURN TO 0.0. THE FO ALREADY HAD THE SEATBELT SIGN ON BY NOW AND WE CONTACTED THE FLT ATTENDANT. WE DETERMINED WE HAD A STAB TRIM RUNAWAY AND FOLLOWED THE APPROPRIATE QRH EMER PROC. WE CONTACTED ATC INFORMING THEM OF OUR SIT AND ASKED TO LEVEL OFF AT A LOWER ALT. WE WERE APPROX 40 MILES NW OF ZZZ AT THE TIME SO WE DECLARED AN EMER AND DIVERTED TO ZZZ. WE NOTIFIED THE FLT ATTENDANT AND THE PAX OF OUR PROB AND THAT WE WERE DIVERTING. THE APCH AND LNDG IN ZZZ WERE SAFE AND UNEVENTFUL.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE HORIZONTAL TRIM WENT NOSE DOWN AND CAUSED AN ALT DEV OF 500 FT. MANUAL HORIZONTAL TRIM WOULD NOT CORRECT THE TRIM. ON THE GND; MAINT TESTED THE HORIZONTAL STABILIZER TRIM CTL UNIT AND ALL THE TESTS FAILED. THE UNIT WAS REPLACED AND THE AIRPLANE WAS TEST FLOWN WITH NO PROBS.