More travelers are expected to fly this Thanksgiving weekend than at any other time since the Sept. 11 attacks, challenging a freshly trained force of federal security screeners who will be scanning more luggage for explosives and enforcing new rules about what passengers may bring on planes.

Airline, airport and federal security officials say they are working to ensure that this holiday flying season doesn't repeat the chaos of last year, when lines stretched so far through terminals that some passengers missed their flights.

This Wednesday and next Sunday, traditionally the busiest flying days of the year, will be the first holiday stress test of the nation's air travel system under the new Transportation Security Administration.

"It's a test of customer service," said Dick DeiTos, who represents the airlines that use Dulles International and Reagan National airports. "It's not a test of security because the [TSA] does a good job of securing the airports on a daily basis. It's a matter of managing the additional people. That'll be the biggest challenge."

Officials at Baltimore-Washington International, where passengers fumed in long, confusing and intersecting lines during last year's holiday season, said they are ready for the crowds this week even though the airport looks like a construction zone.

Drivers dropping off or picking up passengers may face snags on the airport's main road, which has been narrowed by construction crews renovating the roads and part of the terminal.

But inside, BWI officials said they plan to manage the crowds more efficiently. The TSA knocked down walls and relocated stores to widen security checkpoints with eight additional security lanes. The airport is recruiting 100 volunteers to help direct passengers, and several airlines, such as Continental and US Airways, have added check-in kiosks to help cut waiting times.

"We've been working on improving the airport experience after the congestion we experienced at airports last year, particularly at BWI," said Jim Parker, chief executive of Southwest Airlines, BWI's largest carrier. "It's heartening to see the improvement."

Airports will be more crowded this year. Officials said lines could be even longer than last year at times, as the nervousness about air travel after the terrorist attacks begins to wear off and low airfares draw travelers back to the skies.

"You've got a new screening force on duty, and you may have people who haven't traveled in a long time and don't know the rules," said Tom Sullivan, a spokesman for the Metropolitan Washington Airports Authority, which operates Dulles and National. "Just that combination could cause some delays at security screening."

About 5.1 million people are expected to fly nationwide this Thanksgiving weekend, up 6 percent from the 4.8 million who flew during last year's holiday, according to AAA.

BWI said it expects about 650,000 passengers during the Thanksgiving period, which stretches from last Friday to Dec. 2. That would be a 12 percent increase over last year's passenger total. Dulles and National expect almost 1 million. The airports authority does not have statistics for last year's Thanksgiving travel, a spokesman said.

Thanksgiving swamps airports more than Christmas because the crunch is concentrated on the Wednesday before and the Sunday after the holiday, while Christmas and New Year's travel is spread out across more than a week.

DeiTos said lines also could grow because federal screeners are doing longer, more thorough checks than the contract screeners were required to do last holiday season. That includes more hand-wanding of passengers, more shoe X-rays and more thorough searches of coats and laptop computers. He said airlines are hoping the increased number of federal screeners will offset the effect of the more thorough inspections.

Federal employees, wearing a signature yellow emblem on their uniforms, took over all security checkpoints in May at BWI, the first airport in the nation to get the federal screeners. They arrived at National in September and at Dulles in October. The screeners will be scanning more luggage with explosives-detection machines, although not every bag is required to be scanned until Dec. 31, the deadline required by a post-Sept. 11, 2001, airport security law.

TSA's goal is to keep passenger waits at the security checkpoint to 10 minutes or less. The agency said travelers should expect to see more uniform procedures. But the real key to shorter lines will be passengers who know the rules and think carefully about how they pack, said TSA spokeswoman Heather Rosenker.

TSA launched a Web site this year, www.tsatraveltips.us, to give advice. It suggests passengers forgo jewelry and metal clothing items, such as belt buckles and underwire bras. Signs also will remind people to remove their laptop computers from their bags and take off their coats before reaching the checkpoint.

Airlines advise domestic travelers to arrive at their airports about 1 1/2 hours before the scheduled departure.

"If the travelers help us, they can foresee a smooth transition through that line," Rosenker said. "They have a role in this, just as we do."

David S. Stempler, president of the Air Travelers Association, which represents frequent travelers, said he's optimistic lines will move more quickly this year because the federal screeners are better trained and passengers are more familiar with what they can and cannot bring on board. Most passengers now know to have their tickets and identification ready, he said.

Last year, "passengers who hadn't flown since 9/11 didn't know a lot of the rules and requirements," Stempler said. "I think there were a lot of delays from people not getting their tickets out and not knowing what to put on the [X-ray machine conveyor] belt and what not to."

If Thanksgiving is the first holiday test of the federal air security system, Christmas threatens to be an even greater strain.

By Dec. 31, TSA is required to screen passenger luggage with machines that detect explosives, though not every airport will meet that deadline.

The new machines will be up and running as they're installed. That means newly trained security workers will be screening tens of thousands more bags at the busiest time of the year, when passengers carry the most luggage.

BWI expects to get about a dozen machines, each the size of a minivan and weighing several tons. National will install a half-dozen machines, but passengers most likely won't notice a big difference because the airport already uses a large number of machines regularly.

Dulles plans to install many of its machines in the baggage sorting area downstairs, out of passengers' way. It also will use trace-detection machines, which require swabbing luggage with cotton that is then tested for explosive residue.

Todd Hauptli, an airports lobbyist who urged Congress to delay the deadline, said the larger bomb-detection machines have high false-positive readings. That requires screeners to open luggage and inspect it, resulting in extra waits.

Holiday flying is typically a hassle, he said.

"Any time you introduce new elements of uncertainty to that -- and you necessarily will this travel season -- people ought to expect it will take longer," Hauptli said.

Continuing construction at Baltimore-Washington International Airport crowds the lanes approaching the terminal, but airport officials say they expect to move passengers more efficiently once they're inside.Travelers use makeshift corridors to pass through construction at Baltimore-Washington International, the region's busiest airport. Tips on holiday travel are on Page C4.