BILLINGS, Mont. — Frontier Airlines is returning to Billings Logan International Airport in Montana with a nonstop flight, three days a week to Denver. Frontier route planner Jonathon Nield says the airline is expected to begin service in Billings on May 30. Nield tells The Billings Gazette that fights will be on Tuesdays, Thursdays and Sundays and may be booked now from Frontier’s website. Generally, he says flights departing Denver for Billings will take off at 9:55 a.m.