Airline passengers face a Thanksgiving holiday travel season in which they’ll pay more to squeeze onto planes flying at their fullest since World War II.

“It’s like a war zone already,” said Brad Phillips, an Atlanta-based executive who will take a Delta Air Lines jet to Dallas when he and his wife visit their daughter. “I don’t even want to think about what the holidays are going to be like. It’s going to be mayhem.”

Sold-out first-class cabins will force Phillips to squeeze his 6-foot-3-inch frame into a coach seat. He will have plenty of company, because the U.S. industry’s trade group projects airlines will carry 24 million people, 3.5 percent more than in 2009, in the 12-day holiday period starting Nov. 19.

The crowding reflects cuts in airlines’ capacity and a rebound in travel after the recession spurred businesses to pull back and vacationers to stay home. Airport hassles may rise as well, as the Nov. 25 Thanksgiving holiday draws leisure travelers unfamiliar with guidelines for carry-on items.

Passengers trying to avoid fees on most big airlines of as much as $25 each way for a first checked bag will jockey for overhead bin space, raising the prospect of delays at gates while excess luggage is stowed in the hold.

“Delays and chokepoints are going to be at security, where people don’t know what they can carry on and what they can’t,” said Mike Boyd, president of consultant Boyd Group International Inc. in Evergreen, Colo. “The situation with carry-on luggage is terrible now; it can’t get any worse.”

Slowdowns also may loom with the spread of full-body scanners to more airports, because passengers who decline the checks will be subjected to physical searches. There were 300 scanners in use as of last month, up more than sixfold from early in 2010, and that total will reach 500 by year’s end, according to the Department of Homeland Security.

“When it’s a combination of very full flights, plus extra bag charges, plus crowding in overhead bins, plus additional pat-downs in security, it can lead to a challenging environment, especially when you are talking about a peak travel period like Thanksgiving,” said Genevieve Shaw Brown, senior editor for the Southlake, Texas-based travel website Travelocity.com.

Round-trip domestic holiday fares average $378, up 7.4 percent from 2009, including taxes, fees and surcharges, according to Travelocity.com. Average international tickets are $689, 14 percent higher than a year earlier. The 2010 prices are still less than 2008 levels.

An average 90 percent of aircraft seats will be filled on the busiest days of Nov. 19, 24, 28 and 29, according to the Washington-based Air Transport Association, which represents the nation’s largest airlines.

Load factor — the percentage of seats filled — is poised to finish the year at its highest since 1944.