By the way, contrary to the popular but incorrect myth, the ATR when properly operated can cope with the ice wery efficently and safely. The two-chamber de-icing boots are really good!
After the mods you mean?
Friend of mine flew the route of the first ATR that crashed in the alps about 10 minutes in front of them with a KingAir 90 and had his airplane stall on autopilot. He made it into Bergamo and stayed there for the night. His warning about severe ice was never relayed to the ATR crew.
I had ice on a KingAir 200 about 2 inches thick in FL180 and had to descend over the irish sea. Not a pleasant experience. Stay above the minimum speed in icing to prevent under wing icing, as you cant see it and cant get rid of it.
Be aware of the possible Tailplane stall and what you have to do to unstall it.
There is several excellent NASA videos on the subject: