To answer your question directly: TAI (as per the books) prior to entering clouds within the magic icing window between 10°C TAT and -40°C SAT, WAI usually only after accumulating a bit of ice on the leading edge, or if its just a short icing encounter shortly after exiting the icing conditions to clear the wing... Thats on the 737NG.
Last edited by STBYRUD; 21st Feb 2011 at 09:19.