The $10 and $20 surcharges that air travelers are paying this Thanksgiving, Christmas and New Year's are generating so much extra money that three airlines are extending them — and increasing them up to $50 — all the way to Memorial Day.

A fourth airline, US Airways, is imposing a different new surcharge equal to 5 percent of the fare price on all domestic flights starting May 8.

United began the new round of surcharges, and Delta and its subsidiary, Northwest, have matched them.

Most surcharges extending into the spring range from $10 to $30. Many are in March or April, which would coincide with Easter or spring breaks.

The $50 surcharge applies to tickets only on one day: Monday, Feb. 8, the day after the Super Bowl.

Major airlines began imposing $10 surcharges on heavy travel days around fall and winter holidays in September. Since then, many have doubled them. Last week, the new ones began to be extended to a total of 41 days between Thanksgiving and Memorial Day.

Fare tracker Tom Parsons at www.BestFares.com noticed them yesterday and began ringing the alarm for consumers. "This is a shocker," Parsons says. "In 2008, at the height of fuel price surge, I remember Southwest putting on a $30 fuel surcharge that all the others (airlines) matched. I don't ever remember there being a $50 surcharge on a domestic ticket, even in the worst of times."

United spokeswoman Robin Urbanski says her airline introduced the new surcharges to match a conventional fare increase imposed by Southwest.

So far, only Delta and Northwest have matched United — leaving open the possibility that the surcharges could be rescinded if other airlines don't rush in.

How can airlines charge more when fewer people are traveling?

Carlos Bonilla, a consultant at www.AirlineForecasts.com, says carriers have reduced the number of seats and flights available to match fewer fliers. "It's basic economics," Bonilla says.

Despite the rising fares, average prices remain relatively low historically and below the break-even point of some airlines. U.S. airlines are expected to lose about $4 billion this year.