In attempts to prevent a bankruptcy filing, Spirit announced in October that staff layoffs would take place as part of $80 million in cost-saving measures. It also said it had reached an agreement to sell 23 jets to GA Telesis, an aviation services company, for $519 million.

The disparity in size between the low-priced airlines and the top firms underscores a fissure in the industry separating budget carriers in select markets from the major airlines that dominate flight offerings nationwide, Mike Boyd, president of aviation consulting firm Boyd Group International, told ABC News.

The company has reassured passengers that its Chapter 11 bankruptcy will not affect their bookings and that staff will still be paid.

Tucked in the far corner of Terminal 3 at Chicago’s O’Hare airport, mostly unseen by business travelers, are the counters for Spirit Airlines, an ultra-low-cost airline that in 2010 became notorious as the first U.