Weather and Flight Experience
A few years back I was flying over Evansville, Indiana at FL180 headed for Dallas. Conditions were IMC and I would have liked to go higher, but we had a 100kt head wind at FL200 and traffic. Cruising along in IMC with autopilot engaged we began to pick up some ice, but nothing that the ice gear couldn't handle. However, about 15 minutes of this we started to lose true airspeed at about 2-3kts per minute as icing was getting worse. I didn't want to go higher due to winds and traffic, so requested lower due to icing conditions.
Controller advised that he would have lower, but it would be in 5 miles due to opposite direction traffic. All of the sudden the airplane went what seemed to be straight up with airspeed decreasing rapidly. I disengaged the autopilot and hand flew it at the edge of a stall in IMC. The trim wheel was fully nose up which added to the excitement of getting out of an unusual attitude. I had gained about 1000 feet in that event and immediately told the controller that I had a problem and was now descending. He cleared me to whatever altitude I wanted. I kept going down about 1000fpm until 9000ft and the ice started to melt.
The problem was that the autopilot was engaged and ice was forming between the elevator and the edges of the horizontal stabilizer and the only thing keeping the airplane in straight and level was the trim working its way nose up. When it finally came to the stops it broke loose and started it's high pitch attitude climb. To prevent this, always fly manually when in ice and keep the controls moving to keep ice from forming bridges where you don't want them. Second, keep sharp by practicing getting out of unusual attitudes.