The Phoenix-based flight crew was on its first flight of the morning, leaving Calgary to return to Arizona, and the weather was frigid — just about 4 degrees Fahrenheit.
METAR CYYC 241400Z 00000KT 15SM BKN120 OVC240 M21/M25 A3012 RMK AC6CI2 SLP308
Boeing recommendations:
Takeoff with light coatings of cold-soaked fuel frost on upper wing surfaces is permissible, provided the following are met:
The outside air temperature is above freezing