Tulsa's American Airlines Maintenance base is going into the New Year expecting more business than ever.

They're optimistic because mechanics at the base can overhaul 737s, a plane that's a big part of the airline's old and new fleet.

The Tulsa Maintenance base is massive - 3.3 million square feet of buildings, covering 330 acres at the airport. According to the airline, it's the largest aircraft maintenance shop in the world.

Even after the merger, it remains the main overhaul shop for the airline's most common model of aircraft - the 737.

"This is the largest of the four facilities that American has; we also have bases in Charlotte, Pittsburgh, and Dallas,” said American Airlines engineer, Richard Hazlewood.

A big part of what the mechanics do there is modify new planes coming into the fleet.

In a 737 that recently landed in Tulsa, American stripped out the stock interior to put in leather seats throughout, with TV screens in each headrest.

The new planes have advanced lighting inside, a new logo outside and workers are installing the newest WiFi.

The old logo is still on many of the planes that fly into Tulsa for heavy maintenance - the kind of work scheduled every five years or so - where every component is taken out so every surface can be inspected.

“It comes in, it gets opened up and inspected,” American Airlines mechanic, Mike Haught said. “There are normal items we always change every time and there are items that are found after it's been opened up and we do change.”

The process is methodical; each step planned and documented, with a card catalog system of every repair completed on an airplane.

American Airlines Safety Manager, Clay Streater said, "These guys are putting their reputation and quality standards on the line every day when they sign those cards off."

American has 5,800 employees in Tulsa, counting on the airline's continued investment in the 737 - which now makes up almost half of the airline's entire fleet.

American said, besides being one of Tulsa's largest employers, it spends $80 million with vendors in the area.