Discount airlines reshaped the U.S. aviation industry by offering cheap fares and charging extra for pretty much every service imaginable. And they made lots of money doing it.

Now, however, those companies are under intense pressure.

Aviation experts say the largest of the discount airlines, a diverse group of businesses known as ultra low-cost carriers, have become victims of their own success. They expanded rapidly, but may have grown too much. Today, they are struggling to manage rising costs and to compete with one another and giants like Delta Air Lines and United Airlines, which co-opted the strategies that made them so successful.

Those and other unique challenges pushed Spirit Airlines, a pioneer of the business model, to seek bankruptcy protection this summer, for the second

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