What happened
The pilot was provided with a standard preflight weather briefing as well as an inflight weather update prior to the flight. These briefings indicated that icing conditions were expected while flying in clouds or precipitation along the intended route.
During the flight, the aircraft entered cloud formations, resulting in the onset of airframe ice accumulation. The pilot continued toward the destination for at least ten minutes following the initial buildup of ice. Upon deciding to reverse course, the aircraft remained within the cloud formation, where the ice accumulation reached a significant level. During his final two radio transmissions, the pilot reported that the aircraft was inverted and stated that he was in trouble and "...going down."