U.S. airlines gave bullish guidance this week for their first-quarter results, buoyed largely by plummeting fuel prices. Four airlines said they will save hundreds of millions of dollars in fuel costs starting this year, with global oil prices down more than 57 percent since June. Fuel is the biggest variable cost for airlines, often representing a third or more of total operating expenses. Fourth-quarter results did not fully reflect the windfall, because many airlines failed to anticipate the steep oil price slide, and made fuel hedges months ago that ended up costing the carriers hundreds of millions of dollars. On Thursday, Alaska Airlines ( ALK.N ) projected a group low of $1.85 per gallon for the first quarter, down from $2.64 in the fourth quarter of 2014. United Airlines ( UAL.N ) said it will pay between $1.96 and $2.01 per gallon for the quarter, down from […]