Originally Posted by
H.I. McDunnough
2 possibilities:
Ice accretion on inbound flight. Not enough time/temp to melt it off during the turn. Could be CAVU and 44F but still need a Type1 spritz.
Or super-cooled fuel in wings + temp/dewpoint spread which would allow for dew formation that would freeze due to wing temp.
I'd go with option two here. The aircraft had come in from RDU that morning, so any fuel in the wing tanks was going to be quite cold after five and a half hours inflight. Imagine the wing being like a beer glass kept in a freezer, and when removed from the freezer and brought into warmer, more humid air, frost forms on the outside of the glass. If the fuel is cold enough and the air is humid enough, an hour on the ground won't be enough to warm everything up enough to melt any ice that has formed on the wings. There are some interesting videos out there that explain the decision making process of when de-icing is necessary, even in above-freezing and clear conditions.