Passengers at Dulles International Airport are hearing a new excuse for flight delays this year. It's not weather. It's not hangups at the security checkpoint. It's just luggage.

A surge in air travel combined with a busy schedule of flights, all trying to depart at the same time, has overwhelmed the Transportation Security Administration's ability to screen checked luggage on time. The launch of budget carrier Independence Air and expanded service by foreign carriers boosted traffic by 39 percent in April, the most recent figure available, compared with the same month a year ago, the Metropolitan Washington Airports Authority said.

As a result, a few airlines have encountered repeated delays because there aren't enough luggage screening machines for their use in the basement of Dulles's main terminal. After passengers board, the planes must wait for the checked luggage to be screened and loaded in the hold. All checked luggage must be screened through a minivan-sized explosive-detection machine, according to federal requirements put in place after the terrorist attacks in 2001.

Tom Tripp, spokesman for Lufthansa, said afternoon flights to Munich and Frankfurt are often delayed as much as an hour because of the limited number of luggage screening machines. "Unfortunately, it's not uncommon" to have a 45-minute to one hour delay, Tripp said. "We are deeply concerned. The worst thing is we experienced a lot of this last year, and it's unfortunate we didn't get some lessons learned from last summer."

Independence Air, which rapidly expanded at Dulles last year, said it regularly is hit by luggage delays, but usually lasting no more than 10 minutes.

Other airlines suffering delays include JetBlue, Air France, Virgin, Korean, Austrian, Scandinavian, Taca and Continental. All of these carriers use the same four machines in the terminal's basement, which can each scan about 100 bags per hour.

United, which operates a hub at Dulles, has six luggage screening machines in its basement and several upstairs in the ticket counter area. Delta, Northwest, American, British Airways and KLM share four screening machines in the basement.

Yolanda Clark, a TSA spokeswoman, said the agency is evaluating the airport's request for four additional machines, but it does not have the money to buy the machines, install them and hire more screeners to operate them. Also, she said, conditions may be too tight for more machines in the southeast basement.

The screening problem exists because many of the airlines thatshare the same luggage belt system have scheduled large international flights to depart from 2 p.m. to 5 p.m, according to several carriers. Morning and late evening flights and other airlines do not seem to have been affected. During the afternoon, TSA screeners have 1,000 to 1,200 more bags than the machines can possibly screen, said Richard P. DeiTos Jr., who represents airlines at Dulles.

The slowdown is sometimes made worse because passengers on international flights tend to carry large pieces of luggage -- and more of it. Sometimes the bags don't fit inside the screening machines. Other times, they contain food items that set off alarms on the machines and require screeners to dig inside the bags, DeiTos said.

"You're seeing traffic increase to meet and exceed pre-9/11 levels," DeiTos said. "TSA needs to expand equipment and personnel to meet that demand."

Some carriers, such as Austrian Airlines, have requested to move to another ticket counter so they can use a different screening station. Others, such as JetBlue and Lufthansa, are meeting with the TSA directly to argue for installation of more screening machines.

TSA screeners at Dulles, who declined to be identified for fear of being reprimanded by superiors, said the machines break down nearly every day because they are outside and cannot perform well in the summer heat. Repairs can last at least an hour and in some cases take machines out for the entire day.

Airline personnel have been asked to help out by carting luggage to and from other screening stations.

James E. Bennett, chief executive of the airport authority, recommended that passengers on international flights arrive at the airport 21/2 hours before their scheduled departures. He said the airport has asked the TSA to help resolve the problem, but he could not offer a timetable. "It's a challenge for all parties involved to get the resources to match the demand," Bennett said. "Dulles airport is experiencing record growth."