The Air Force on Friday approved a proposal to expand a bomber training area over the northern Plains, a move that helps keep a South Dakota base open but has raised concerns over loud, low-flying aircraft disrupting civilian flights and damaging rural economies.

The Federal Aviation Administration will now examine the plan to expand the Powder River Training Complex over the Dakotas, Montana and Wyoming. If the FAA approves the proposal it would more than triple the training airspace, making it the largest over the continental United States.

"The expanded training airspace ... is critical to ensuring our airmen and women receive the training they need to protect and defend our nation abroad," U.S. Sen. John Thune, of South Dakota, said in a statement Friday. "The Air Force has made the safety and security of those living within this training airspace its highest priority."

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. Thune 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.

The 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.

An FAA spokeswoman said in an email that the agency didn't have a firm time frame for its evaluation of the expansion.

The Air Force has provided no information on costs associated with the expansion, including potential damages to affected landowners and farmers.

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

The Air Force acknowledged in a study released Nov. 28 that the low-altitude flights and loud sonic booms could startle residents and livestock, including those living on four 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.

U.S. Sen. Steve Daines, of Montana, said Friday that the Air Force plan doesn't address concerns about low-level flights disrupting civilian and commercial flights, including emergency medical flights and others connected to the oil industry.

Daines and Democratic Montana Sen. Jon Tester planned to introduce legislation Friday to restrict Air Force training over the site of a proposed loading terminal for the Keystone XL oil pipeline at Baker, Montana. Thune said during a call with reporters that he hopes concerns over the proposal can be resolved without legislative action.

"Political representatives, that's what they do: They respond to the concerns that are raised by their constituents," Thune said. "We have some of those here in South Dakota, as well, but ... I can't imagine really anything else the Air Force could have done."

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.

Roger Meggers, who manages eastern Montana's Baker Municipal Airport, said even when civilian flights aren't barred, many pilots will avoid the area because of the potential for accidents with the fast, hard-to-spot military aircraft.

"The Air Force has spent millions of dollars on them, developing paint schemes so you can't see them," Meggers said.

"Montana doesn't get anything out of this," he said.

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Brown reported from Billings, Montana.