What happened
On January 12, 2014, at approximately 1615 CST, a Cessna 680 Sovereign, registration N531RC, was performing a flight test under Part 91 regulations. The flight originated from Chicago Executive Airport (KPWK) in Wheeling, Illinois, under visual meteorological conditions with a visibility of 10 statute miles and winds from 190 degrees at 13 knots.
While climbing to 15,000 feet mean sea level, the flight crew experienced an autopilot disengagement accompanied by an autopilot disconnect chime and an "AP fail A-B" message. The pilot reported that extra force was required to push the aircraft's nose down while attempting to slow the climb. Although the pilot attempted to apply nose-down trim, there was no response, and the elevator trim tab indicator showed the system was in a full nose-down position. The crew decided to return to the departure airport and landed the aircraft without incident. There were no injuries to the two crewmembers.
The investigation
Post-flight examinations conducted by the operator included testing the stabilizer trim through its full range of travel, which revealed no discrepancies in the system or the elevator trim tab indicator. Upon contacting a Cessna Aircraft Company Service Center for troubleshooting, investigators determined that the connecting pin linking the driveshaft to the left-hand elevator trim tab actuator had become dislodged.
Findings
Investigation revealed that the failure of the elevator trim tab drive shaft pin had occurred on two other occasions in similar aircraft. The manufacturer is proposing a change to the dimensions of the drive pin and the method used to secure it to prevent the pin from dislodging in the future.