An M20J pilot encounters icing conditions while IFR in a busy terminal area and is unable to obtain a clearance to exit the condition. Must divert to a nearby airport and temporarily loses control when the airplane pitched down aggressively when the autopilot was disconnected.

Narrative

This was my first time flying into the ZZZ area. The ceiling was scattered to broken and it appeared from PIREPS that I would be on top at 11;000 MSL. The temperature was above freezing at the surface. There were large areas of blue sky mixed with areas of cloud. I was flying a Standard Instrument Departure but started to receive vectors which would put me into the clouds. I requested a deviation to stay clear of the clouds because I had noticed that the OAT had dropped below freezing. ATC was unable to approve any immediate deviation due to landing traffic at ZZZ; but said that they would have higher in 5 miles. At this point; I was in and out of the broken layer of cloud and began to accumulate rime ice. I notified ATC; and they authorized a climb from 8;000 feet to 9;000 feet. This did not allow me to get clear of clouds; and the accumulation intensified in the climb. Also; the rate of climb was significantly lower than usual; even at that altitude. At this point; I elected to divert to a nearby airport. Only a very short time elapsed from the time I noticed the first trace of ice until I elected to divert. I began to receive vectors for the ILS. I then put the plane on automatic pilot while I searched for the approach plate. After I intercepted to localizer; I was cleared for the approach. When I turned off the autopilot the VSI showed a steep descent momentarily until I pulled back on the yoke. At this point; I received a low altitude alert. After that; the ice began to melt and I made an uneventful no flap approach and landing. I learned a lot from this experience. First; I learned that I need to improve my weather briefing review and need to anticipate icing any time I am flying in visible moisture. There were some pilot reports of icing; but they weren't in the immediate area of my flight and I was expecting to stay clear of clouds. Second; I cannot count on ATC permitting a climb that will permit me to remain clear of clouds; particularly in a busy terminal area. Third; I saw firsthand the impact of icing on the tail and how its effects are masked by the autopilot. While I was thinking of the effects of the high altitude at ZZZ; I probably underestimated somewhat its effect on the airplane's performance; particularly in icing conditions. This made the prospect of a successful climb on top unlikely. Fourth; I need to be more precise and maintain focus at all times; particularly on headings and altitudes in the terminal area. The icing had become a distraction to precise flying. Finally; I learned that I have to improve my cockpit organization; and should have all conceivably necessary approach plates in arms reach in case I need to divert. While I had read a lot about icing and its effects; I did not anticipate how quickly performance would degrade from such a brief encounter; particularly regarding the tail. There was never more than perhaps 1/4 inch on the wing; but likely more on the tail and that was enough for it to lose some of its effectiveness. I was glad that I diverted immediately and made a relatively uneventful landing; but I will be extra vigilant next time icing is forecast.