American Airlines and US Airways moved a step closer to full integration this week after the US Federal Aviation Administration (FAA) gave the green light for the two to legally operate as one airline under a single operating certificate.

“Think of it as the ultimate enabler for integration,” said Ed Bular, a senior vice president in charge of American’s effort to obtain a single operating certificate. “The rest of the airline really cannot integrate until the airline officially becomes one.”

American is “the world’s largest airline and the operation is very large and complex,” said Bular, who has been through four airline mergers. Some 700 employees worked on the 18-month-long single operating certificate process, he said. Now, most of American and US Airways’ flight, maintenance and dispatch procedures will be the same, but some operating policies and procedures will remain separate for a while, officials said.

As of Wednesday, both airlines will use the “American” call sign when pilots communicate with air traffic controllers. US Airways’ last “Cactus” flight departed London Heathrow Airport on Wednesday and landed in Philadelphia the same day. US Airways inherited “Cactus” from America West Airlines in its 2005 merger.