Most of the nation's biggest airlines operated their planes fuller last month than in any previous March, virtually snuffing out hope among passengers for an empty middle seat to sprawl across.

American, Delta, Continental and United last week each reported their highest-ever percentages of filled seats for March.

US Airways, the product of a late-2005 merger, filled a higher percentage of seats last month than in March 2006, the only comparable month. Northwest was close to a record March. Each airline ran more than 80 percent full on average, meaning that many flights on the most popular routes at the most popular times ran full.

As the big airlines have clawed their way out of the deep financial losses that followed terrorism and recession earlier in the decade, they've aggressively moved to fill every seat possible with a paying passenger. In March, spring break vacationers and bad weather jammed even more people than normal into planes.

"The weather impact has been unusual in early 2007," says Tim Wagner of American Airlines.

Because of cancellations, airlines had to consolidate passengers on fewer planes than initially planned. Planes were so full that passengers waited days to be rebooked on other flights.

Chris Romano, a high-mileage business traveler from South Riding, Va., waited 16 hours after a storm knocked his planned Friday, March 16, trip from Charleston, S.C., to Washington Dulles off track. Fellow passengers heading for New York were told they faced the possibility of waiting until Monday to get out, Romano said.

No. 2 United reported the highest percentage of filled seats among big carriers: 85 percent. The timing of the Easter holiday may have affected the number, said United spokeswoman Robin Urbanski. Some early Easter-related travel appeared to fall into late March, she said.

Packed planes in March may signal a continuation of uncomfortably full planes during the summer, when they're typically most crowded. American, for instance, reached its highest system-wide load factor for any month (87 percent) last July, Wagner says.

Not all airlines saw fuller planes in March. Discounters Southwest and JetBlue filled a smaller percentage of seats in March than a year earlier.

Last month, Southwest filled 73.3 percent of seats, versus the March record of 75.5 percent in 2006, says Marilee McInnis, an airline spokeswoman. She attributes the slip to an increase in the number of available seats on Southwest and to softer travel demand than in 2006.

Travelers have taken note of the increasingly crowded planes and have tried to cover themselves by booking earlier, says Amy Ziff of online travel seller Travelocity.

Domestic travelers, for instance, bought their tickets on Travelocity an average of 34 days in advance of departure this year, versus 33 days last year, she says.