United Airlines Inc. (UAL) and Orbitz Worldwide LLC sued to prevent the travel website Skiplagged.com from helping consumers buy what the companies call improper “hidden city” plane tickets that undercut their sales.

Skiplagged helps travelers find cheap airfares by enabling them to book multistop flights and deplane before the flights reach their as-booked final destination. Sometimes a fare that travels through a hub city to another location can be cheaper than a ticket to the hub city alone.

“In its simplest form, a passenger purchases a ticket from city A to city B to city C but does not travel beyond city B,” according to the companies’ complaint. “‘Hidden City’ ticketing is strictly prohibited by most commercial airlines because of logistical and public-safety concerns.”