If Spirit Airlines, the second-largest carrier at Detroit Metropolitan Airport, closes due to financial difficulties, it could leave travelers stranded or forced to pay the cost of airfare. The U.S. Department of Transportation has advised consumers to contact their credit card and travel insurance companies if Spirit Airlines closes to see if they might be eligible for a refund. United Airlines is preparing to assist Spirit Airlines customers and employees in the event of a closure, but did not provide details about how it would help. The loss of Spirit Airlines could significantly impact the bottom line of consumers. As of 5:30 p.m. Friday, the airline's website still allowed travelers to book flights to over 70 destinations from Metro Airport.