Incident Overview
On August 15, 2012, about 0835 central daylight time, a Cessna 560XL, registration N108EK, diverted to Des Moines International Airport (DSM), Des Moines, Iowa, following a loss of control during cruise at Flight Level (FL) 400. The aircraft, registered to New Heights Aviation LLC and operated by Elliott Aviation under 14 CFR Part 91, was on an instrument flight rules flight plan from Flying Cloud Airport (FCM), Eden Prairie, Minnesota, to Arlington Municipal Airport (GKY), Arlington, Texas. Visual meteorological conditions prevailed. The two pilots and six passengers were not injured, and the airplane sustained minor damage.
Flight Crew Actions
After approximately 10 minutes of cruise at 40,000 feet pressure altitude, the flight crew observed an autopilot pitch trim mis-trim (AP Pitch Mis-trim) annunciation along with the associated master caution. As they began to reference the applicable checklist, the autopilot automatically disengaged, and the airplane suddenly pitched down to about 20 degrees nose-low. Airspeed increased to about 30 knots over the redline. The flight crew recovered and leveled the airplane at 33,000 feet pressure altitude, then elected to divert to DSM. The autopilot was not used for the remainder of the flight, but the electric pitch trim functioned normally when used passing 10,000 feet mean sea level.
Cockpit Voice Recorder
The cockpit voice recorder (CVR) captured the sequence of events. As the airplane leveled at FL 400, the co-pilot noted "it's not keeping up with it" and commented about referencing the checklist, adding "R-five." About 30 seconds later, a high-pitched tone was recorded simultaneously with a "thunk" and rustling sounds. Two additional high-pitched tones followed, then a beeping tone consistent with the overspeed warning lasting 40 seconds. The flight crew discussed deploying the speed brake and declared an emergency. After regaining control, they diverted to DSM. During the descent, they decided not to use the autopilot. They executed a full flap approach and landed at 0844 without further incident.
Postflight Inspection and Examination
A postincident examination of the autopilot, pitch trim systems, and two-position horizontal stabilizer system revealed no anomalies. The left and right elevator electric trim actuators and the autopilot elevator drive servo were subsequently replaced. The airplane was returned to service without further difficulties. The flight crew noted that an abnormally high amount of water was observed draining from the tailcone during the postflight inspection. The required pilot actions for an AP Pitch Mis-trim annunciation were contained on page R-5 of the Pilots' Abbreviated Checklist – Emergency/Abnormal Procedures, which included pressing and releasing the AP Trim Disc Button, adjusting pitch trim as required, and engaging the autopilot as desired.