DALLAS (AP) – The CEO of Frontier Airlines resigned and was replaced by the carrier’s chairman and president, who vowed to improve a record of frequent flight delays and customer complaints.

Frontier said Wednesday that Dave Siegel stepped down for personal reasons.

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Siegel’s departure came the same day that consultants J.D. Power ranked Frontier last in a survey of customer satisfaction. On Monday, the federal government said that Frontier had the highest complaint rate and the worst on-time performance among the nation’s 14 leading airlines in March.

Frontier, based in Denver, said that Chairman Bill Franke will manage strategy and finances and President Barry Biffle will handle daily operations. Siegel had led Frontier since January 2012.

After Republic Airways sold Frontier in 2013 to Franke’s investment firm, Indigo Partners, the airline switched to a so-called ultra-low-cost carrier. It offers low base fares but adds fees to stow a carry-on bag, get an assigned seat and other extras.

Customer complaints have soared. In March, passengers were 34 times more likely to complain about Frontier than about Southwest Airlines, according to the U.S. Department of Transportation. Frontier cited a new reservations system that left callers on hold for an hour or more. The airline said it has hired additional workers since then.

More than one-third of Frontier’s March flights arrived at least 15 minutes late.

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Biffle, the president, said the airline has taken steps to cut delays and complaints, from changing the schedule to starting the first flight on time. To cut costs, Frontier is flying its planes longer and installing lightweight seats to save fuel.

The airline is on track to meet cost-cutting targets, he said, but it is disappointed with the complaint rate, which he said has come down in April and May.

“We are 100 percent behind having the lowest costs in our space,” Biffle said in an interview. “But low cost does not mean a bad experience. We want to offer low fares and reliable service.”

Frontier operates a hub at Denver International Airport and flies throughout the U.S. and to Mexico, Costa Rica and the Caribbean.

– By DAVID KOENIG, AP Airlines Writer

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