* Flight cancellations aid key revenue measure
* Analyst cuts profit estimates for some carriers
March 10 (Reuters) - Major U.S. airlines on Monday said cancellations of thousands of flights would hurt first-quarter profit, but some added that revenue would still show improvement.
Flight cancellations tend to add to airline costs during the first quarter, when winter storms hit the hardest. Five U.S. carriers - Delta Air Lines Inc, American Airlines Group Inc, JetBlue Airways Corp, Southwest Airlines Co and United Continental Holdings Inc - said that winter storms had led to more than 78,000 canceled flights among them.
Some airlines cited a noticeable increase in their canceled flights tied to bad weather this year, with American Airlines saying its 28,000 cancellations for January and February more than doubled from the year earlier.
Delta said that as of the end of January, its cancellations from bad weather had topped all of those for 2013.
Delta told a J.P. Morgan investor conference disruption from winter storms hurt revenue for the current quarter by $90 million as it canceled 17,000 flights. The storms dented pretax profit for the quarter by about $55 million, Delta added.
The carrier said it expected operating margin, a measure of income to costs, of 6.5 percent to 7.5 percent for the first quarter, compared with a prior view of 6 percent to 8 percent.
Cancellations can help unit revenue, an important measure of pricing power and how full planes are, as more travelers are put on remaining flights.
Delta said unit revenue would rise 3 percent to 4 percent in the current quarter, compared with its prior view of a 2 percent to 4 percent increase. American added it still expected unit revenue to rise about 2 percent to 4 percent in the period.
United said it halted 23,000 flights tied to weather in the first two months of 2014. The company had disclosed last month that disruption from the storms would hurt first-quarter unit revenue.
JetBlue told the same conference it canceled nearly 4,000 flights because of the winter storms, and Southwest said it halted about 6,500 flights in January and February.
JetBlue forecast a 1 percent to 2 percent rise in first quarter unit revenue, while Southwest outlined expected growth of 3 percent.
Cowen & Co airline analyst Helane Becker cut her first quarter profit estimate for Delta to 30 cents a share from 32 cents in a note to clients, citing the updated outlook. She raised her quarterly estimate for Southwest to 17 cents a share from 14 cents, but also cut her estimate for JetBlue for the period to 6 cents a share from 13 cents.
Shares of the major U.S. airlines ended mixed, with Delta down 0.5 percent to $35.17 and United off 2.3 percent to $46.66. American was off 0.2 percent to $38.96 and JetBlue fell 1.8 percent to $8.87. Southwest shares rose 2.2 percent to $23.60.