Updated from June 25

Northwest

(NWA)

has cancelled more than 500 flights over the past four days, apparently because of crew shortages.

The cancellations could be a precursor to problems that will plague travelers throughout the summer if airline pilots -- frustrated by reduced compensation and increased work hours -- are unwilling or unable to volunteer to fly beyond their normal scheduled hours.

According to

FlightStats.com, Northwest called off 453 flights from Friday through Sunday. Another 173 flights were taken off the schedule Monday. Minnesota-based Northwest operates about 1,400 daily departures.

"This staffing problem is something we have pointed out for a long time," said Wade Blaufuss, spokesman for the Northwest chapter of the Air Line Pilots Association. "The company wants to operate the airline at the red line. It looks great on paper, but intangibles like fatigue and morale and stress don't show up on a balance sheet."

A Northwest spokesman declined to immediately comment. The carrier is just weeks removed from its emergence from bankruptcy protection.

Federal regulations limit pilots to flying 1,000 hours a year and 100 hours a month. Normally, airlines fly heavier schedules in the summer, when travel surges, and rely at times on volunteers to enable them to operate a full schedule.

"It looks like Northwest got the short straw," said Meara McLaughlin, spokeswoman for FlightStats, which compiles flight statistics for various travel businesses. "At this point, the system is so tightly wound that if you just have reduced staffing or bad weather or airport congestion or a VIP flying through, you

quickly get to the breaking point."

Andy Wisbacher, spokesman for the Northwest chapter of the Association of Flight Attendants, said he has had three A320 flights cancelled in the past two weeks due to a shortage of pilot crews.

"I can only imagine that Northwest knew about this going into the summer and didn't plan properly," he said. "I expect it will go on through the summer."