Frequent flier Samir Bhatnager books for the bump.

Bhatnager intentionally tries to get onto flights that have an increased chance of being oversold. If he picks right and gets bumped, he receives either a free round-trip ticket or a voucher for about $200 off his next trip, depending on the airline.

Now that the summer travel season has begun -- a season that is expected to be the busiest in four years -- Bhatnager says he's ready to start racking up the free trips and cash vouchers as airlines routinely overbook their flights.

"Some people take mileage runs -- I take bump runs," said the Arlington- based hospitality manager. On a mileage run, a frequent flier takes a cheap flight to a place such as London or Amsterdam and returns on the same day just to bulk up the mileage account.

Many carriers are predicting that their planes will be nearly 80 percent filled this summer -- and that means more passengers can expect to be bumped. Even before the summer, bumping was on the rise. According to the Transportation Department's latest statistics, 235,052 airline passengers were bumped from their flights between January and March, up nearly 27 percent from the same period in 2003.

Since January, Bhatnager's bump runs have netted him two free round-trip tickets and five cash vouchers worth $1,500, most of which were on his favorite airline, Northwest.

Bhatnager takes personal flights on the weekends or arranges his business return flights with a possible bump in mind. He often chooses late-night flights into Baltimore-Washington International Airport, especially on Sundays. Bhatnager has found that late-night flights are often the fullest, especially if the airline backed up earlier in the day, forcing passengers onto the later flights.

Bhatnager also targets cities that have major weekend sports events or festivals. His strategy: Buy a cheap ticket in advance and schedule a return trip on the Sunday after the event, when travelers are rushing to get home.

To make sure he's one of the first people asked to give up his seat, Bhatnager visits the airline's Web site and prints out his boarding pass and seat assignment at least a day before the flight. Then he arrives at the gate at least an hour before the flight and alerts the agent that he's available to give up his seat if necessary.

Frequent US Airways traveler Alysia Brown said that though she has racked up six free round-trip flights since 2002, she prefers cash vouchers to free tickets. A voucher can be put toward a flight that will earn her frequent flier miles and help her boost her elite status with US Airways. Also, the airlines often don't allow a traveler flying on a free ticket to upgrade to first or business class.

Brown, an Alexandria-based Web developer, said the free seats are best for last-minute trips such as visiting an ailing relative or attending a funeral.

Brown knows which flights to target: departing flights on Thursday and Friday evenings out of Reagan National Airport to destinations in Florida. She said return flights on Sunday evenings are also packed.

With bumping on the increase, several airlines said they are taking steps to help reduce the number of passengers denied boarding. Continental Airlines, for example, added 200 planes and 100 flights to handle the increased passenger demand, said spokesman Rahsaan Johnson.

Alaska Airlines had the largest percentage of bumped passengers among large carriers in the first quarter. JetBlue Airways and Arlington-based US Airways had the fewest of the nation's top 10 airlines.

Alaska Airlines now limits the bookings on some late-afternoon flights to keep space available for any passengers displaced because of flight problems earlier in the day. The airline has also implemented a system that scans its passenger lists for possible duplicate bookings.

"If we do it right, those who want to get bumped get bumped. And those who don't won't get bumped," said Donald Garvett, Alaska Airlines' vice president of planning and revenue management.

Garvett said many airlines overbook between 20 and 40 percent on average, mainly to make up for no-shows. The industry practice doesn't violate government rules, but the government does require compensation for some passengers who are involuntarily bumped because of overbooking.

If a passenger who is involuntarily bumped arrives at his destination within an hour of the originally scheduled time, the airline is not required to offer compensation. But if bumped passengers get to their destinations more than an hour and less than two hours late, the airline is required to give 100 percent of its one-way fare, up to a maximum of $200. Passengers are entitled to 200 percent of the one-way fare, up to a maximum of $400, if their arrival at the destination is more than two hours late domestically or four hours late on international flights.

Travelers interested in learning more about their rights when bumped can visit the DOT's Web site at http://airconsumer.ost.dot.gov/publications/flyrights.htm#overbooking.