The revelation, which came in a memo to staff from the airline’s CEO, Scott Kirby, helps explain why U.S. airlines have had to cancel more than more than 27,000 flights, or about 8% of all scheduled trips, over two weeks starting the day before Christmas, according to FlightAware, a data tracking service. Employees calling in sick and storms that delivered strong winds, rains and in some cases record snowfall at airports nationwide wreaked havoc on United and other companies and stranded many travelers.
