Spirit Airlines has filed for bankruptcy protection and will attempt to reboot as it struggles to recover from the pandemic-caused swoon in travel and a failed attempt to sell the airline to JetBlue. Spirit is the biggest U.S. budget airline. It has lost more than $2.5 billion since the start of 2020 and faces looming debt payments totaling more than $1 billion over the next year. Spirit says it expects to operate as normal as it works its way through a prearranged Chapter 11 bankruptcy process and that customers can continue to book and fly without interruption.