Thanks for the great responses...
The address a few of the very valid points raised in the responses:
Fortunately neither of us were wet. We had departed 45 minutes earlier for pilot (mine) familiarization, and aside from flying in rain part of the time, had otherwise not opened a door or window, or otherwise got wet. There were a few drops of water dripping in from somewhere around the fuel cutoff lever, but no volume of consequence.
The fog was on the inside of the windows, were it to have been on the outside, the delightful rain would have certainly washed it off.
This helicopter is very well maintained, and the director of maintenance had been flying with us the previous evening, as he is a valuable part of the effort. We did get a little for the night before, but the heat cleared it immediately. He has been notified of our difficulty, and I will presume that he will file an SDR, but I will ask him today...
Opening sliding windows and doors was an option that we did not get to, but knew it was there. Being the recipient of the training, I just kep flying on the instruments while my very experienced mentor pilot dealt with the the problem. He had al the heat, and fan going, and I was certinly pulling enough power to keep the bleed air flowing well.
This helicopter is not AC equipped, though I bet the owned will be thinking about that now! My new diesrl VW Jetta uses AC to clear the windows, even on a cold morning (I kept looking ad the light on the AC switch, thinking "I selected heat, did I bump the AC button by accident?"). AC systems are not popular because of the weight, but if it clears the windows right away, I'd be thinking it's minumum safety equipment for an EC120.
For those who fly 120's, who have not experienced this alarming event, I can tell you that it did not appear to be caused by any particular change in conditions. We were over the airport setting up for a second practice auto, and as I was climing up between 400 and 600 AGL, the fog started, and obscured about 50% of the lower outboard portions of both windsields. I asked my mentor to get me some defog going, and he was right on it. Within a second or so, complete IMC inside the cabin, which remained for close to ten minutes with only minor improvement. If this had happend to a pilot on low approach, it could have been very bad. As it was, I would not have attempted to land next to the hangar, I would have just landed on the runway, where there was lots of peripheral contrast with which to judge.
The saving element of this would have been the fact that we also had a FLIR camera working on board, and landing with reference to that image would have made things much safer. My opinion is that FLIR is a major safety tool for helicopter operations.
Thanks for your thoughts fellow pilots....
Pilot DAR