Apr. 1--Many Delta Air Lines Inc. passengers flying out of Boston's Logan International Airport will face fare increases of $9 or more, after the carrier yesterday began passing through airport charges it had been absorbing.

Delta's move came as United Airlines confirmed it is raising walk-up business class fares by as much as $100 round-trip, and adding $4 to $10 to coach and leisure tickets in many markets.

The moves reflect a growing aggressiveness by US airlines to raise fares -- and make the higher prices stick -- to dig out from $40 billion in industry-wide losses since 2001 and a 79 percent jump in jet fuel prices over the past two years. Even discount carriers like JetBlue Airways Corp. and Southwest Airlines Co. have moved to raise ticket prices by $10 in recent weeks.

Technically, Delta is making more passengers pay for "passenger facility charges" of up to $4.50 each time you take off from an airport. Airports have been assessing carriers these fees to fund terminal upgrades, but now airlines are passing those costs on to consumers. Delta had been making passengers pay the fees on direct flights from Boston, and now will begin collecting them from many passengers connecting to other destinations through its Atlanta, Cincinnati, and Salt Lake City hubs, where it previously absorbed the fees, spokeswoman Chris Kelly said.

But Terry Trippler, who runs the CheapSeats.com website in Minneapolis, said, "Is it a fare increase? Absolutely. It was a fee that Delta was eating. Now they're unmasking the fee."

American Airlines had quietly begun following a similar policy a week ago on tickets for flights through its Chicago, Miami, and St. Louis hubs, but had not made a public disclosure until Delta's yesterday. Northwest Airlines tried twice in the past year to begin passing through the facilities charges, but retreated after other airlines refused to follow suit.

However, Roz Garber, president of Garber Travel in Chestnut Hill, said that while Delta and American have disclosed general guidelines for fee increases, there appeared to be a number of exceptions.

"It really depends on the cities you're flying between," Garber said. "There is no consistency, and no guidelines."

In recent years, airport improvement taxes and fees and security charges have grown. Between 1993 and 2004, the percentage of total US domestic ticket prices that represented taxes and fees jumped to 16.1 percent from 10.9 percent, according to a Massachusetts Institute of Technology/Daniel Webster College Ticket Tax Project study.

Delta has been operating under bankruptcy protection since September, as has Northwest. Delta racked up $500 million in losses in January and February. Delta and affiliates most months rank as the second biggest airline network serving Logan, measured by passenger volume, after American Airlines and its American Eagle regional-jet affiliate. Besides its New York shuttles and service through the three hubs, Delta flies nonstop to 23 other destinations from Boston.

United is the fourth-biggest carrier at Logan, from which the airline's major destinations are Dulles International Airport and Chicago.

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