NEW YORK (CNN/Money) - Southwest Airlines posted a plunge in second-quarter earnings Thursday that apparently was slightly steeper than expected and the company warned it won't meet forecasts for the current quarter.

The nation's No. 6 airline, and by far the most profitable U.S. air carrier, earned $84.5 million, or 10 cents a diluted share, excluding a gain from reduction in estimated refunds and exchanges. Including that gain it earned net income of $102.3 million, or 13 cents a diluted share. Both figures were well below net income of $175.6 million, or 22 cents a diluted share, in the year-earlier period. Analysts surveyed by earnings tracker First Call had a consensus forecast of 11 cents, but while the data service said it believed the 10 cent EPS is the proper comparison to estimates, it couldn't be sure until it heard from the analysts.

Southwest also warned that it expects its third-quarter profit to be below second-quarter results. First Call's consensus third-quarter EPS forecast is for 15 cents, with estimates ranging from 11 to 23 cents. Still, the discount carrier is likely to be one of the few airlines to report a profit in the current period.

Revenue fell 5.2 percent to $1.47 billion from $1.55 billion a year earlier. Unlike its competitors who have cut schedules in the face of lower traffic, miles traveled by paying passengers on Southwest increased 1.8 percent to just under 12 billion in the period. But even the discount carrier saw average fares fall as the amount paid by passengers per mile traveled was off 9.3 percent to 11.6 cents.

In other airline financial news, Delta Air Lines and Northwest Airlines reported slightly lower-than-expected losses in the second quarter.

Atlanta-based Delta, the nation's No. 3 airline, lost $162 million, or $1.34 a share, excluding special items, wider than the loss of $123 million, or $1.03 a share, a year earlier. First Call's consensus forecast a loss of $1.39 a share.

Including special items, Delta posted a net loss of $186 million, or $1.54 a share, up from a net loss of $90 million, or 76 cents, a year earlier.

Revenue fell to $3.47 billion from $3.78 billion. Miles traveled by paying Delta customers fell 5.4 percent to 26.3 billion, while the average amount paid per mile traveled was off 3.8 percent to 12.2 cents.

Northwest Airlines, the nation's No. 4 air carrier, also saw wider losses but landed better-than-expected results for the period.

The Minnesota-based carrier lost $93 million, or $1.08 a share, wider than the $55 million, or 65 cents a share, loss a year earlier, but better than the First Call consensus forecast of a loss of $1.21 a share. Estimates ranged from a loss per share of 90 cents to $1.60.

Revenue fell 11 percent to $2.4 billion, roughly in line with First Call's forecast. Miles traveled by paying passengers fell 9.1 percent to 18.6 billion, while amount paid per mile traveled was off 4.2 percent to 10.7 cents.

Shares of Southwest (LUV: Research, Estimates) gained 18 cents, or 1.3 percent, to close Wednesday at $13.93, while shares of Delta (DAL: Research, Estimates) lost 81 cents, or 4.7 percent, to $16.45 and shares of Northwest (NWAC: Research, Estimates) lost 4 cents to $9.46.