MINNEAPOLIS (AP) — Delta Air Lines has learned that summer is a good time to prepare for winter — and how to de-ice planes so they can keep flying safely in freezing temperatures. Every summer, Delta brings about 400 workers to Minneapolis to a three-day summer de-ice "boot camp.” They go through computer-based training, watch demonstrations by instructors, and then practice spraying down a plane — using water instead of the chemicals found in de-icing fluid. The boot campers, who rotate through in groups of 10 or so, return to their home bases and train 6,000 co-workers before October, says Jeannine Ashworth, vice president of airport operations for the Atlanta-based airline.