This article first appeared on GuruFocus. United Airlines Holdings Inc. ( UAL, Financials ) and Spirit Airlines Inc. traded public barbs Tuesday over the future of the ultra-low-cost carrier model, underscoring growing pressure on the bankrupt discounter. Speaking at the U.S. Chamber of Commerce's Global Aerospace Summit in Washington, United CEO Scott Kirby said Spirit's approach was an interesting experiment that had failed, adding that he doubts the airline can remain in the industry. Spirit, which filed for Chapter 11 last month for the second time in a year, quickly responded on social media, saying its customers love low fares and premium product offerings.