Planes go into the hangar for these massive month-long overhauls every four or five years.

"When these guys get it done, it will be in excellent shape," he said.

Though much of United's maintenance is done in Houston and San Francisco, there are crews at 45 other airports around the world to work on the airline's planes.

In some cases they even have contracts to do work on competitors' aircraft.

There are about 8,000 United mechanics worldwide, as well as 2,000 other members of its Tech Ops division, including engineers, maintenance planners and inspectors.

Their goal, they say, is to make sure their planes are safe and they get you there on time and this busy holiday season is no different

"It's the same responsibility as it is every day," Baumgarten told CNN. "Our job doesn't change if a plane is going to go one mile, empty, full -- our job is exactly the same."

"Every plane has to go out safe."