I remember seeing a head on photo of an 18 Sqn Wessex which was covered in a very thick layer of rain ice (three or four inches thick) It was being used for USL training at Junkers farm, RAF Gutersloh, which is within the airfield boundary, when the heavy rain began to fall. The ice built up so quickly that the aircraft didn't make it back across the airfield to dispersal; it was too heavy!
The photo appeared on the front cover of "Air Clues" in the early 1980s if I recall correctly (might actually have been the cover of the incident report).
The photos of icing trials I've seen were nowhere near as exciting!
I was later based at Gut and experienced rain ice there myself, thankfully not in the air. Again, it was so severe that the weight of it on the local trees caused large boughs to snap off and some paths around the base had to be closed for safety.
Very scary stuff, that image has stayed in my mind ever since and I have in the past cancelled all flying on the thankfully few days rain ice is forecast to occur and in one case successfully persuaded another chief pilot to do the same.
Unfortunately, I don't have the relevant photo of the Wessex but this might jog the memory of someone who has access to it.