Spirit’s continued indebtedness, while not an aberration for an airline, is nonetheless representative of a larger problem: the business model of ultra-low-cost airlines is inherently fragile, Daniel Y. Gielchinsky, a partner at DGIM Law, told Law360.

Spirit advertises fares starting at $20, and Frontier at $19, for one-way flights, similar to the lowest advertised prices on bus transit company Greyhound. Running an airline on such low prices is at odds with the high costs for labor, fuel, airplanes, fees owed to airports and regulatory requirements, and the only way to make such a business work is to artificially sustain it on copious debt, Gielchinsky said.

“There’s no reason that taking a regional jet from Chicago to Dallas should cost the same as a bus,” Gielchinsky said. “The entire notion that you’re selling airplane seats for the cost of a bus is a pipe dream, and it’s a dream that’s financed by debt.

Both consolidation and more bankruptcies are on the horizon for the airline industry, Gielchinsky said. While the recent string of highly publicized airplane accidents across the country are not driving the trend, the crashes and near-misses “underscore how expensive it is to have this industry,” he added.