Incident Description
The captain reported engaging the autopilot as the airplane passed through 10,000 feet mean sea level. Approximately 10 minutes later, the crew observed illumination of a 'PITCH MISTRIM, TRIM NOSE DOWN' annunciator light. Attempts to adjust the trim using the manual elevator trim resulted in only limited movement. The crew subsequently declared an emergency, returned to Rapid City, and landed the airplane without further incident.
Postincident Inspection
A postincident examination of the aft fuselage compartment revealed that one of the two master links in the electric elevator trim system had separated. The master link, which held the chain and cable assembly, had come apart, allowing the chain to become entangled in the elevator trim cables. Further inspection showed that the 'C' clip, which snaps onto the two posts of the master link base plate and secures the master link cover plate, was missing.