The pilot ____ the supply plane over the mountains twice a week that winter.

توضیح

Travelling through the air is fly — flew — flown, and a repeated finished action in that winter takes the past simple flew. Fled is the past of flee — fled — fled, meaning to run away from danger, flowed belongs to the regular verb flow used of liquids, and flown is a participle, which cannot stand alone as a past tense.