What happened

It didn't take long for investors to shrug off United Continental's (NYSE:UAL) infamous passenger-assault incident. During May, all eyes were on new signs that the domestic unit revenue environment for airlines was improving.

As a result, while United shares languished in the immediate aftermath of the April incident, they got back on track last month, soaring to a new all-time high. In total, United Continental stock rose 13.5% in May, according to data from S&P Global Market Intelligence.

UAL Chart

United Continental May Stock Performance, data by YCharts.

So what

On May 2, Delta Air Lines revealed that its unit revenue grew slightly more than expected during April. A week later, American Airlines raised its second-quarter unit revenue guidance by 0.5 percentage points. These favorable updates caused airline stocks broadly to rally.

United didn't change its own unit revenue forecast when it reported its April traffic results. Nevertheless, it participated in the sector rally. Nearly all of the stock's gains during May came in its first week and a half.

There wasn't much else to explain United Continental stock's surge last month. The carrier did highlight that its operational performance continues to improve, with fewer cancellations and more on-time departures than ever before. That should help it keep unit costs down. But it could take years to derive any meaningful unit revenue benefits from improved reliability.

Now what

Earlier this week, United Continental reiterated its original forecast that unit revenue would rise by 1% to 3% in the second quarter. However, President Scott Kirby noted that "the Pacific region is experiencing incremental weakness."

A United Airlines plane

United is facing new unit revenue headwinds in a key region. Image source: United Airlines.

This comment may have been meant as an informal warning that unit revenue is trending toward the lower half of the guidance range. That would put United near the bottom of the industry in terms of unit revenue growth. Meanwhile, it has forecast that adjusted non-fuel unit costs will rise 4% to 5% this quarter.

With United Continental stock trading for more than 11 times projected 2017 earnings -- a premium compared to its two larger mainline rivals -- this might be a good time for shareholders to lock in some gains.

Adam Levine-Weinberg owns shares of Delta Air Lines. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.