Buffett's Berkshire Hathaway invests in major airlines

The billionaire investor has famously avoided the volatile sector in the past, although his Berkshire Hathaway does own NetJets, which sells partial ownership interests in business jets.

"For U.S. airlines, a combination of slightly higher fuel costs and higher labor costs at American Airlines, Delta Air Lines, Southwest Airlines and United Continental Holdings will contribute to a 20 percent contraction in operating profits over Moody's outlook horizon," the ratings service said.