CRJ200 EXPERIENCES RWY AILERON TRIM AT FL240.

Narrative

AT FL240; WHEN I NOTICED THAT THE AILERON TRIM WAS FULLY DEFLECTED TO THE R; I TRIED TO GET THE AILERON BACK IN TO THE CTR POS BUT THERE WAS NO RESPONSE FROM THE TRIM. THE TRIM DID NOT MOVE AND THERE WERE NO EICAS MESSAGES DISPLAYED. I CONTACTED MAINT AND THEY ASKED ME TO CHK THE AILERON TRIM CIRCUIT BREAKER ON CIRCUIT BREAKER PANEL #2. I CHKED THE CIRCUIT BREAKER AND IT WAS IN PLACE AND NORMAL. THEN THEY ASKED ME TO DISENGAGE THE AUTOPLT AND SEE WHAT WOULD THE AIRPLANE DO WHEN IT WAS DISENGAGED. THE FO DISENGAGED THE AUTOPLT AND TOLD ME THAT THE AIRPLANE WAS TURNING TO THE L. HE IMMEDIATELY ASKED ME TO ENGAGE THE AUTOPLT FOR HIM. I RESPONDED BACK TO MAINT THAT THE AIRPLANE WAS TURNING TO THE L AFTER THE AUTOPLT WAS DISENGAGED AT A PRETTY GOOD RATE AND YOKE CTL HAD TO BE APPLIED TO GET THE AIRPLANE BACK ON A STRAIGHT AND LEVEL FLT. MAINT DISCONTINUED THE CONVERSATION AND PATCHED ME TO OUR DISPATCHER. AT THAT TIME CTR WAS BRINGING US DOWN FOR THE DSCNT AND CONTACT WITH DISPATCH WAS LOST THROUGH RADIO. I ACARS DISPATCH THAT WE WERE DECLARING AN EMER AND ZZZ ARPT WAS THE BEST OPTION AT THAT TIME. WX WAS BETTER AND THE ARPT WAS CLOSER. EMER WAS DECLARED WITH CTR AND PAX WERE BRIEFED AND PREPARED. ATC VECTORED TO ZZZ AND LANDED ON RWY X WITH NO FURTHER PROBS.