"Now, I'd say its back to normal," he said the passenger mix.

In addition, the Quad-City airport began to see an increase in leakage to Chicago in 2013. As of August, about 31 percent of the bookings originating in the Quad-Cities flew out of Chicago. That percentage was up from 23 percent in 2013 and 19 percent in 2010.

Overall, the Quad-City airport is retaining 66 percent of its bookings for the catchment area, he said, adding that the airport still is reaping benefits from the travelers drawn here during the AirTran days.

He said comparatively, the leakage "is a lot better than other markets."

"If you're in close proximity to a major hub, you have a lot more leakage," Bown said. "Yours, even though it is worse (than it has been), you're still not terrible."

He said some airports with close major hubs have as much as 80 percent leakage.

"Today, you are very reliant on the Quad-Cities (for passengers)," Bown said. The study showed the Quad-Cities only loses 2 percent of its bookings to Cedar Rapids.

Commissioner Molly Foley said the findings show that without AirTran, the Quad-City airport would never had seen the boost in Iowa City-Cedar Rapids passengers that it did.