On October 12, 1993, at 1300 hours Pacific daylight time, a Beech C-90A, N487JD, sustained a hard landing on runway 24 at Napa County Airport (APC), Napa, California. The pilots were completing a visual flight rules instructional flight at the time; the pilot had filed and activated an instrument flight rules flight plan, but canceled the flight plan when the flight was 20 miles east of Napa. The airplane, operated by IASCO, Napa, California, sustained substantial damage. Neither the certificated airline transport pilot/certified flight instructor (CFI), the certificated commercial pilot/dual student, nor the passenger was injured. Visual meteorological conditions prevailed. The flight originated at Burbank Airport, Burbank, California, at 1115 hours. The CFI reported in a telephone interview conducted on October 13, 1993, when the airplane was over the runway threshold, at 70 feet above the ground, the student flying the airplane inadvertently placed the power levers in reverse. The CFI immediately pushed both power levers full forward, but without success. The airplane pitched downward and landed hard. Both engines were found displaced downward about 20 degrees with their associated wing spars. The CFI reiterated his telephone statement in the Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1/2. He further said that he had instructed the student to do a short field landing.