United Airlines' chairman and chief executive, Jeff Smisek, has resigned over a corruption investigation.
Federal authorities are investigating whether or not Mr Smisek authorised a flight that would lose money for the benefit of the head of the Port Authority of New York and New Jersey.
Two other, so far unnamed, United Airlines executives have also resigned.
According to US media reports, the route in question is a twice-weekly flight between Newark airport and Columbia, South Carolina. At the time, United Airlines was lobbying for major improvements at Newark airport - owned by the Port Authority. It is understood that former Port Authority chairman David Samson owned a vacation home in Columbia at the time.
United Airlines started the route shortly after Mr Samson joined the Port Authority, and the route was cancelled after he left the position.
United Airlines said it was co-operating with the federal investigation. Oscar Munoz, chief operating officer at CSX Corp, will replace Mr Smisek.