Date: 201212
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: Icing; Rain; 0
Light: Night
Aircraft: Golden Eagle 421
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Autopilot
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: Pilot Flying; Single Pilot
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Distraction; Situational Awareness; Time Pressure; Training / Qualification; Troubleshooting; Workload
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Loss Of Aircraft Control
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Regained Aircraft Control; Flight Crew Landed As Precaution; Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action; Flight Crew Diverted
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While I was waiting for the passengers; I refueled the aircraft; obtained a computer generated weather briefing; and filed an IFR flight plan. I also completed a weight and balance check; a center of gravity check and reviewed performance numbers for departure. The manifest; weight and balance information and flight following information were phoned into our communication center via telephone. All in accordance with company procedures. We started the engines; taxied out to run-up area and performed a run-up in accordance with company checklist procedures. No abnormalities were noted. Weather condition [were] as follows: Moderate rain with a reported ceiling at 4;400 FT broken and 5;000 FT overcast [and] visibility of 7 SM. I wanted to receive my IFR clearance on the ground prior to departure. After several varied attempts; I was able to receive my clearance via Flight Service as follow. We departed via the obstacle departure procedure. I was unable to acquire initial contact with Center due to poor radio reception. I flew the departure procedure to within about a mile of the VOR and began to reverse course to pick up the Victor Airway to the north. After turning back to the north; I was able to confirm communication with Center. During this time I received a clarification of my IFR clearance. Center further amended my clearance to direct my destination which I was able to accept. He subsequently cleared us to climb to 16;000 MSL. We entered the clouds at about 4;500 FT MSL. I checked for ice accumulation once with the de-ice lights somewhere around 6;000-7;000 FT MSL. There was barely a trace present; not enough to exercise the boots and I had all other de-ice equipment on. Up to this point; in the departure; I was hand flying the aircraft. I intended to top the weather en route to my destination. Once through approximately 7;500 FT MSL I engaged the autopilot for the continued climb to 16;000 FT MSL. The autopilot was on for only about a minute when passing through about 8;000 FT MSL; the autopilot went into an uncommanded nose pitch over rapid descent. I immediately went to the autopilot disconnect switch on the yoke while trying to arrest the descent with elevator back pressure. The switch did not disengage the autopilot. I tried the switch again but it would not disconnect. I then tried the go-around switch and disabled the electric trim; again to no avail. I then went to the autopilot on/off switch on the autopilot controller head and selected the off position. It again failed to disconnect the autopilot. During this sequence and while in the descent; I reduced the engine power via the throttles. I told the crew I was having trouble disconnecting the autopilot and they asked if they could help. I asked our passenger to come up front and if he could bring a flashlight. He moved to the co-pilot seat and initially attempted to help me pull the yoke back to recover from the dive. We could not over come the autopilot servos with both of us pulling. I asked him to use the flashlight to look over my left shoulder for the autopilot circuit breaker. He scanned for a few seconds and then I told him to look on the right side circuit breaker panel as I believed it was on the right. It was at this point that we came out of the clouds and could see the extreme pitch down attitude we were in due to the visual reference with the ground. Right after this; [a frequent passenger] found and pulled the autopilot circuit breakers and the yoke pressure I was fighting against was released. I then began to pull the aircraft out of the steep dive. I was concerned that the aircraft might come apart in the recovery and I tried to balance the amount of back pressure I was applying with the approaching terrain. I was able to affect recovery back to level flight somewhere in the vicinity of 2;000 FT MSL and then began a slow climb at which time I made sure my passengers were uninjured. The airplane was responding normally after the recovery. I had dropped out of radio contact with Center and called the nearby airport Approach. I informed them who we were and what had just occurred. I asked them for vectors to land at the nearby airport. I did not declare an emergency at this point as I felt the aircraft was responding as it should. I did not communicate with ATC in the uncontrolled descent as I was too busy trying to resolve the emergent flight condition. While being vectored toward the airport; we contacted our Communications Center; and informed them of what happened and that we were diverting. Approach vectored us for localizer approach and we picked up the field just outside of the final approach fix. We landed on and taxied to the FBO and shutdown the aircraft normally using the checklist.

Callback: The reporter stated that the autopilot controller was removed and found to have corrosion which started the pitch over. Apparently the pitch trim servo clutch also failed which locked the trim full forward and prevented the elevator trim movement after it ran away full down. When the circuit breaker was finally pulled he was able to manually trim the aircraft. The reporter estimated that the pitch down was between 70 and 80 degrees. As ground contact was reestablished; he was intent on a smooth pull up so as not to cause structural failure or damage.



Synopsis: A Cessna 421 entered an uncommanded pitch over at 8;000 FT with the autopilot engaged. The pilot recovered at about 2;000 FT after the autopilot circuit breaker was pulled.