CNBC got an inside look at United Airlines Flight Training Center in Denver. At nearly half a million square feet, this facility is one of the largest airline training hubs in the world. All of United's 12,000 pilots come through here for initial training and return every 9 months to stay up to date on flight procedures and safety protocols. The campus has 31 simulators, each costing $15 million to $20 million. And United plans to have a total of 40 in the next 12 to 18 months. Pilots must complete training in the simulators before flying in real planes. United Airlines operated more than 5,300 flights on the Sunday