A CL65 IN CLB AT FL180 DECLARED AN EMER AND DIVERTED DUE TO ELEVATOR SPLIT CAUTION WARNING WITH AUTOPLT ENGAGED. NORMAL OP WITH AUTOPLT DISENGAGED.

Narrative

THE FLT PROCEEDED NORMALLY UNTIL PASSING THROUGH FL180. THE FO ENGAGED THE AUTOPLT AS HE HAD BRIEFED PASSING THROUGH FL180. THE ACFT OSCILLATED SLIGHTLY ABOUT 4 TIMES IN THE PITCH AXIS AFTER AUTOPLT ENGAGEMENT. VERY SHORTLY THEREAFTER; THE ELEVATOR SPLIT CAUTION MESSAGE POSTED. I DECIDED TO ACCESS THE FLT CTL SYNOPTICS PAGE. THEN WE RECEIVED AN AUTOPLT NAV DISPLAY CAUTION MESSAGE. I NOTICED ON THE FLT CTL PAGE THAT THE STABILIZER TRIM WAS RUNNING NAV DISPLAY AND THERE WAS A VERY SLIGHT ELEVATOR SPLIT. THE L ONE WAS SLIGHTLY LOWER THAN THE R. I DIRECTED THE FO TO DISENGAGE THE AUTOPLT. THE AUTOPLT NAV DISPLAY CAUTION IMMEDIATELY EXTINGUISHED AND THE TRIM STOPPED MOVING. THE ACFT WAS COMPLETELY CONTROLLABLE FOR THE REST OF THE FLT. THIS ALL HAPPENED WITHIN 30-60 SECONDS. WE CONSULTED THE AOM FOR THE ELEVATOR SPLIT AND SUBSEQUENTLY RETURNED UNEVENTFULLY TO ZZZ. WE INFORMED THE FLT ATTENDANTS; OPS; AND DISPATCH OF OUR SIT. WE DECLARED AN EMER WITH ZZZ CTR. WE DID NOT HAVE TO CONSULT WITH MAINT CTR BECAUSE OF THE HIGH WORLOAD AS WE WERE W OF ZZZ VOR WHEN WE BEGAN OUR RETURN. WE COMPLIED WITH THE AOM PROC FOR ELEVATOR SPLIT.