Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of United Continental Holdings (NASDAQ:UAL) ended the trading day with a 12.7% pop after the company reported strong second-quarter passenger revenue per available seat-mile (PRASM) growth for its second quarter.

So what: United Continental recently projected that PRASM would grow in the 1% to 3% range for the second quarter, but it announced today that the metric had grown 3.5%, beyond expectations. The company's non-fuel costs also remained flat for the quarter, which is certainly better than cost growth in the 1.25% to 2.25% range, which had been forecast earlier this year. United's available seat-miles also grew slightly, by 0.8%.

Now what: In the more than eight years since United Airlines exited bankruptcy (United offered its stock to effect the merger with Continental, making the combination technically an acquisition), its shares have only gained more than today's 12.7% 30 times, and all but one of those trading days occurred during the tumult of the global financial crisis. By that measure, today is clearly a momentous day for United Continental; but does a slight uptick in the airline's core top-line metric really justify double-digit gains today?

Before the pop, United Continental's shares had nearly returned to even for the year after slipping from year-to-date gains of 25%. It's been an erratic year for airline stocks, in general, and many of United Continental's peers reacted favorably to the news on what was otherwise a lousy day on the markets. But this is still just a short-term reaction to a long-term story, which has played out amid a historically strong economic environment for airlines. With United Continental shareholders already holding a double for the past two years despite flat revenue  and weaker earnings per share than what was reported two years ago, it may be time to consider taking some profit before valuation pressures take this high-flying stock back down to the runway.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.