Frontier Airlines Draws Most Complaints in U.S. for May.

Frontier Airlines is the worst among major U.S. carriers for the month of May when it comes to complaints.  The U.S. Department of Transportation shows a rate of more than three grievances filed per 100,000 boardings for Denver's hometown airline during that time period.  That number is slightly higher than Spirit Airlines.  May is the second month in a row Frontier has come in as the worst in the complaint category.