Fifteen years ago, I was asked to fly and old Queen Air from Florida to Puerto Rico for a friend. When I arrived in Florida the day before the trip I thought that I should give the old bird a short hop around the patch to check it out. I had limited experience with autopilots. A friend of mine asked to ride along, which was okay with me. As we taxied out and went through the runup, things were fine. I ignored the autopilot as always. The takeoff went well and the old bird performed as expected. We climbed out to about 5,000 ft and played with props and power settings, etc. I gave the Queen Air a good checkout, and the old bird checked out as I was told it would. So, after about an hour I headed for home port. For some reason as we were flying back I had the urge to turn the autopilot on. Which I did (first mistake). At first the autopilot worked perfectly. First a left bank, then right then I set the heading bug for dead ahead to the airport and me and my passenger settled into a little idle conversation. Yeah, right. With no warning the Queen Air nose over. I had not touched anything. So I took hold of the yoke and applied back pressure. The more I applied, the more noses - over we became. By now my passenger (who is a pilot also) was pulling on the yoke too, trying to help, to no avail. I had no idea about how the autopilot system worked. All I knew was that I was in trouble and needed to do something fast. My airspeed was climbing and I was looking at a part of Florida that I would rather be looking at out the side window. How and just why I＇ll never know, but I caught a glimpse of the trim running wild nose - down. I told my now - copilot to disconnect the autopilot as I reached for the trim to try and stop it. The autopilot was turned off but the trim was still trying to run. Not until my friend unplugged the monster and I re - trimmed by hand did we gain control of the air plane. When we finally leveled off, we were only about 500 ft above terra firma. I＇m sure I don＇t need to tell you just what had been scared out of me and my friend. The rest of trip back was uneventful. When we were back on the ground I inquired about the autopilot. Not a soul knew how it worked or anything about it，But I didn＇t give up. I found an old salt who knew autopilot systems inside and out and got a very well- need ed ground school. I learned that the pitch trim is the most powerful force an autopilot can muster and that if you pull or push the control wheel against the movements of the autopilot the trim will turn in the opposite direction. The autopilot itself can＇t overpower the weakest pilot, but if allowed to run away, the pitch trim can＇t be overcome by the strongest pilot. I was very lucky, had my friend not been with me that day I don＇t think that I would be here now. I bet there are a lot of pilots who don＇t have a clue about the real workings of autopilots, let alone how to preflight one or to identify a trim runaway. Now I make sure that I understand everything there is to know about the autopilot I am flying.4. The trouble was resulted from （ ） .