What happened
While passing through 10,000 feet mean sea level (msl), the captain engaged the autopilot of the aircraft. Approximately 10 minutes later, the crew observed the illumination of a 'PITCH MISTRIM, TRIM NOSE DOWN' annunciator light. In an attempt to correct the condition, the captain tried to use the manual elevator trim; however, the system provided only limited movement.
Following the malfunction, the crew declared an emergency and returned to Rapid City. The aircraft landed without further incident, and there were no injuries reported.
The investigation
A post-incident inspection of the aft fuselage compartment was conducted to determine the source of the trim failure. Investigators discovered that one of the two master links, which connects the chain and cable assembly to the electric elevator trim system, had disconnected. This separation caused the chain to become entangled within the elevator trim cables.
Further examination of the components revealed that a 'C' clip, which is designed to snap onto the two posts of the master link base plate to secure the cover plate in place, was missing.