Air Force Secretary F. Whitten Peters told a Senate committee yesterday that the cost of the C-17 airlifter will remain steady if the service is allowed to use money appropriated last year for a 15-aircraft buy, even though it will only purchase 12 planes in fiscal year 2001. Peters said the Air Force would return to a 15-aircraft buy in 2002 and beyond. "There is one requirement for us to keep the (C-17) costs down and that is the multiyear...