MIAMI — Frontier Airlines has appointed president Barry Biffle as CEO, thus filling a void left by former chief executive David Siegel when he resigned abruptly in May 2015. Biffle, who joined Frontier in July 2014, will also be part of the airline’s board of directors, according to a statement from the carrier.

Last year, Frontier’s poor operational performance took the airline to the second-to-worst record among major U.S. carriers in terms of customer complaints, at a rate of 5.09 per 100,000 passenger enplanements, nearly three times the average rate for the 13 largest U.S. carriers. However, data released this week from the U.S. DOT indicates that Frontier is now ranking as N° 4 among U.S. carriers in January, with over 85 percent of flights on time. The airline also received fewer customer complaints when compared to December.

“Under Barry’s executive leadership, Frontier has made significant progress in its operational, financial and customer service performance over the last year” said Bill Franke, Co-Founder and Managing Partner at Indigo Partners LLC, owner of Frontier Airlines.

Indigo acquired Frontier from Republic Airways Holdings in 2013 and directed the airline to an ultra-low-cost carrier business model. Both Franke and Biffle were involved with Spirit Airlines, pioneer in adopting the ULCC model.

“I’m excited about this opportunity provided by the board of directors and the confidence it places in our management team,” Biffle said in a statement. “Our entire Frontier family has done a tremendous job over the past two years in restructuring and refocusing Frontier to the success it is today.”

Frontier is expanding its service this spring with dozens of new nonstop routes. The airline launched service from Denver to Philadelphia and Charlotte, and next May will launch daily non-stop service from Denver to San Antonio, Columbus and Pittsburgh, besides bringing back flights to Colorado Springs from Las Vegas and Phoenix.

Interestingly, Frontier has also filed an application to the U.S. Department of Transportation for regular services from Denver to Cuba, however the U.S. DOT is still deciding which airline will get the routes.