CHICAGO — For three days United Airlines has operated like a carrier under siege from the weather, canceling more than 600 flights and scrambling to rebook thousands of passengers during a peak travel period.

The skies have been clear since Monday, but the turbulence has continued.

United said yesterday it is still recovering from the ice and wind that lashed Chicago's O'Hare International Airport last weekend, leaving pilots and planes stuck out of position when operations resumed Monday.

According to its pilots union and other sources close to the airline, United has run low on crews to fly its planes. That's a result of lean staffing, scheduling practices and freakishly bad weather that have caused large numbers of pilots to hit the maximum number of monthly duty hours allowed by federal regulators well before December's end.

"The weather wouldn't have mattered if they had enough people," said Herb Hunter, a Boeing 747 captain at United who is a spokesman for the Air Line Pilots Association, which has been harshly critical of some of United's management practices.

Either way, the cancellations continued to pile up: another 173, as of last night, or 10 percent of the flights that United was scheduled to operate. The Chicago-based carrier also canceled 324 flights on Christmas Day and 148 on Monday, according to FlightStats.com.

United canceled one flight from Chicago to Honolulu on Christmas Day, and another flight from Denver to Honolulu arrived about 2 1/2 hours late, according to the airline's Web site. Yesterday, United reported only minor delays for flights to Hawai'i.

For some passengers, the disrupted operations have meant long delays, uncertainty and the added stress of potentially missing out on holiday celebrations with friends or families.

Michael Janas, a Los Angeles-based architect, says that three of the four United flights that he was scheduled to take for a trip home to Cleveland over the weekend were canceled, including both of his return flights to California on Christmas Day. Told that it would be two days before United could get him back to Los Angeles, Janas eventually found seats on Continental Airlines.

"It was really an awful experience," Janas said. "I think United must have had a complete operations meltdown."

United plans to operate at close to a full schedule today, said spokeswoman Megan McCarthy, although it may have to cancel flights in Denver because of an approaching snowstorm.

Foul winter weather has proven especially hazardous to smooth airline operations in this era of full airplanes and an airline system under strain. United's pilot workforce, for example, has dropped by about one-third since 2001, when the airline industry went into a nose-dive: from 9,968 to 6,277 at the end of 2006, according to the Bureau of Transportation Statistics.

American Airlines sparked a passenger rights movement last December when it stranded planes on a tarmac in Austin, Texas, for up to eight hours. JetBlue Airways canceled thousands of flights this February after a Valentine's Day ice storm socked its hub at New York's John F. Kennedy International Airport, a snafu that eventually cost the airline's founder, David Neeleman, his CEO title.

United says the succession of winter storms that pounded its hubs this month, including O'Hare, subjected it to a greater number of air traffic control "challenges" and ground delay programs than its competitors.

However, neither Southwest nor American Airlines, its two largest competitors in Chicago, suffered similar disruptions. Both have canceled only a handful of flights.

"The reality is, we can staff the schedule we have," said Tim Smith, an American spokesman.

The final straw for United was Sunday's storm, which pounded O'Hare, its busiest hub, with gale-force winds on one of the busiest travel days of the year. United says it operated flights into the early hours of Monday in an effort to get holiday travelers to their destinations.

"Because of the weather, many of our crews and aircraft were not in position to serve our customers so we proactively canceled flights to get our crews and aircraft in position to operate flights and get our customers to their destinations," McCarthy said.

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