A federal appeals court in Virginia has reversed a judge's ruling requiring the nation's largest public utility to promptly install upgraded emission controls at four coal-fired power plants.

Three of the Tennessee Valley Authority plants are in Tennessee, and the other is in Alabama.

U.S. District Judge Lacy Thornburg had ordered the accelerated cleanup at the TVA plants, ruling that emissions affecting air quality in North Carolina's scenic western mountains were a "public nuisance."

A three-judge panel of the 4th U.S. Circuit Court of Appeals in Richmond overturned that ruling Monday. Appeals court Judge J. Harvie Wilkinson III wrote that allowing the ruling to stand would undermine the nation's carefully created regulatory scheme.