A proposal to establish an enormous bomber training area over the Northern Plains that advocates say will improve military training and save money got final approval Tuesday despite concerns about loud, low-flying aircraft disrupting civilian flights and damaging rural economies.

The Federal Aviation Administration confirmed that it approved a plan to expand the Powder River Training Complex over the Dakotas, Montana and Wyoming. The move nearly quadruples the training airspace to span across nearly 35,000 square miles, making it the largest over the continental U.S.

"This has been a very long process, but I'm proud we're finally completing this important expansion to provide our servicemen and women with the resources they need to be successful in a combat environment — all done while saving money," South Dakota U.S. Sen. John Thune, who originally announced the plan's approval, said in a statement to The Associated Press.

The airspace would be used by B-1 bombers from Ellsworth Air Force Base in South Dakota and B-52 bombers from Minot Air Force Base in North Dakota. The U.S. Air Force estimates that the expanded training airspace could save Ellsworth up to $23 million a year in fuel costs by reducing the number of training flights to Utah and Nevada.

The Air Force approved the expansion in January, which kicked the proposal over to the FAA for review. Thune has said the expansion could shield Ellsworth from being shut down under Base Realignment and Closure, a federal cost-cutting program. Ellsworth is a significant economic driver for the Rapid City area and was under consideration for closure in 2005.

But elected leaders in Montana and state aviation officials have said the bombers would disrupt rural communities and scare livestock as they roar overhead on maneuvers, dropping flares and chaff, or fiber clusters intended to disturb radar waves.

The Air Force acknowledged in a study released in November that the low-altitude flights and loud sonic booms could startle residents and livestock, including those living on four Native American reservations in the region.

Under the Air Force plan, any given location across the training area could experience up to nine low-altitude overflights annually. Supersonic flights would be limited to 10 days a year during large-scale exercises involving roughly 20 aircraft.

As many as 78 civilian flights a day could be impacted when the large-scale exercises are conducted, the Air Force said. But delays in civilian flights could be avoided if pilots are willing to use "see-and-avoid" rules that would allow some flights when the training area is active.