DALLAS — Planes are full and airlines are making huge profits, but airline stocks tumbled for a second straight day on Thursday because investors fear that the carriers are growing too rapidly.

They worry that airlines are adding so many flights and seats that they will spark fare wars that cut into profits.

In two days, the stock-market value of the six largest U.S. airlines dropped by more than $14 billion.

The stock slump overshadowed more evidence of strong demand for travel that is making the airlines so profitable.

American Airlines Group Inc. predicted Thursday that higher revenue will produce 2018 profit far exceeding what Wall Street was expecting.

The airlines also are getting a boost from the recently enacted tax bill. It is already showing up on the bottom line of Southwest Airlines Co., where fourth-quarter profit more than tripled largely because of the way the new law affects future tax liabilities.

Southwest earned a record $1.89 billion in the fourth quarter, up from $522 million a year earlier. It got a $1.15 billion benefit from a reduction in liability for deferred taxes. Chairman and CEO Gary Kelly said the new tax law, with its 21 percent rate on corporate taxes, down from 35 percent, will save Southwest hundreds of millions of dollars and “significantly boost our earnings in 2018.”

Dallas-based Southwest’s revenue rose 3.9 percent to $5.27 billion, topping Wall Street expectations. Its planes were on average 85 percent full — a record for any fourth quarter — helping overcome a 2.7 percent dip in the average fare.

American, the world’s largest airline, earned $258 million, down 11 percent from $289 million a year earlier, but revenue rose 8.3 percent.