Over the past two years, rising fuel prices and fare wars in parts of the U.S. have combined to put pressure on many airlines' profitability. Alaska Air (NYSE: ALK) has been one of the hardest-hit companies.In 2015 and 2016, Alaska's adjusted pretax margin reached 24%, a phenomenal achievement. However, its pretax margin fell to 17% last year and is on track to plunge into single-digit territory in 2018.