United Airlines on Monday doubled its fee for a second checked bag to $50, citing volatile fuel prices. United, a unit of UAL Corp., said the $50 one-way fee would apply for tickets bought beginning Tuesday for travel beginning Nov. 10. in the U.S. or to or from Canada, Puerto Rico and the U.S. Virgin Islands.

Travelers in first or business classes, “Premier” frequent fliers, and active-duty military personnel traveling on orders will be exempt. United said the fee would apply to about one in seven passengers.