Frontier spokesman Jim Faulkner declined to comment, while a representative for Indigo didn't immediately comment. Volaris, Virgin, Spirit A U.S.-listed airline IPO is a rare occurrence. Only three companies have completed initial share sales in the past five years: Mexican low-cost carrier Controladora Vuela Cia. de Aviacion SAB, known as Volaris, Virgin America Inc. and Spirit Airlines Inc. Indigo bought a stake in Spirit in 2006 before the deal, and Franke was chairman of the board until 2013. Frontier has changed hands twice since filing for bankruptcy protection in April 2008. Republic bought the carrier in October 2009 for $108.8 million, and agreed at the time not to pursue a $150 million unsecured bankruptcy claim. After the Indigo buyout, Frontier's new owner transformed it into a so- called ultra-low cost carrier — an airline that offers inexpensive base fares and adds fees for everything else.