The pilots and other sources said the airline's lean staffing and scheduling practices combined with bad weather that caused many pilots to prematurely reach their federally mandated maximum number of monthly hours to leave United short on crews to operate its planes.
"The weather wouldn't have mattered if they had enough people," said Herb Hunter, a United Airlines Boeing 747 captain and spokesman for the Air Line Pilots Association.
United Airlines canceled 148 flights Monday, 324 flights on Christmas Day and 173 flights as of 7:30 p.m. Wednesday, FlightStats.com reported.
United spokeswoman Megan McCarthy said the airline planned to operate a full schedule Thursday but some cancellations may result from a snowstorm approaching Denver.
Notable deaths of 2014 [PHOTOS]