It was my understanding as well that airlines locked in fuel prices but as this article explains, apparently they are able to take advantage of lower fuel prices.
Airlines fly lower fuel costs to lofty profits
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Three more airlines – United, Southwest and Alaska – reported Thursday big profits for the fourth quarter, largely thanks to lower fuel prices.
Cheap fuel has allowed the airlines to reward shareholders with stock buybacks and dividends, and to thank workers with profit sharing. They’re also buying planes to update their fleets.
“This was by far our best year for earnings in the company’s history,” Southwest Airlines CEO Gary Kelly told reporters during a conference call, crediting low fuel prices, full planes and more seats to fill with fliers. “The convergence of all of these things made 2015 a very successful year.”
http://www.usatoday.com/story/money/2016/01/21/airlines-fly-lower-fuel-costs-lofty-profits/79119356/