Alaska Air Group (ALK) shares slipped in extended trading Tuesday after the carrier reported mixed second-quarter results. The airline beat on earnings but missed revenue estimates, as fuel costs spiked 85% from a year ago.

Alaska Air reported a quarterly loss of 92 cents per share, better than the 99-cent loss analysts had expected, according to market data. Revenue came in at $4.07 billion, just shy of the $4.09 billion consensus.

The big story was fuel. The carrier paid $4.43 per gallon in the quarter, up 85% year-over-year. That kind of cost jump is hard to ignore, but CEO Ben Minicucci tried to put it in perspective.

"Our second quarter results were defined by a fuel spike outside our control — but underneath it, this company is executing better than ever," Minicucci said. He pointed to industry-leading on-time performance for the first half of the year, the completion of the Hawaiian Airlines integration, the launch of European service, and a return to profitability in June.

Looking ahead, Alaska Air expects third-quarter earnings per share between zero cents and $1, well below the $1.38 analyst estimate. That guidance likely contributed to the stock's 2.35% drop to $44.39 in after-hours trading.

For investors, the takeaway is mixed: the airline is running well operationally, but fuel costs and cautious guidance are keeping a lid on enthusiasm.