It keeps getting more expensive to fly -- one small fee at a time. The latest in what analysts are calling "stealth" price increases is a checked-bag fee for passengers that check their luggage.The first major U.S. airline to institute a checked-bag fee is American Airlines, which yesterday began charging travelers $15 in order to check their first piece of luggage. It started charging them $25 to check a second bag last month."While we understand that these fees affect customers, we also believe that our pricing for the services we provide remains extremely competitive in the industry and continues to offer our customers ample choice and value," American Airlines Chairman and CEO Gerard Arpey said in a statement last month, upon announcing the $15 checked-bag fee. "The bottom line is that our revenues, which include ticket sales and fees, must keep pace with our increasing costs."Although American is the first airline to institute a checked-bag fee, other airlines are expected to follow suit. In fact, United Airlines and US Airways have already announced that they plan to enact similar luggage fees."With record-breaking fuel prices, we must pursue new revenue opportunities, while continuing to offer competitive fares, by tailoring our products and services around what our customers value most and are willing to pay for," John Tague, United's chief operating officer, said in a statement. United estimates that its checked-bag fee will apply to one of every three customers, generating approximately $275 million in extra revenue per year.