It has been revealed that an estimated 15,000 of the company's flights scheduled to run over the festive period do not have an assigned pilot due to a computer glitch.
The software error gave time off to too many pilots between 17-31 December, the company told employees in an email last week.
Pilots are now being offered more money to cancel their holiday plans and work the shifts.
A US-based aviation union called the incident a “crisis”.