On October 26, 1993, at 1525 Pacific daylight time, a Beech BE- 76, N6626X, sustained substantial structural damage to the left main wing spar as well as damage to both blades of the left propeller during a hard landing at Livermore, California. The aircraft was operated by Sierra Academy of Aeronautics of Oakland, California, and was on a local area dual instructional flight. Visual meteorological conditions prevailed at the time and no flight plan was filed for the operation. The certificated commercial pilot/flight instructor and the two students on board were not injured. The flight originated on the day of the mishap at about 1400 from the Oakland International Airport. In his written statement, the flight instructor reported that the student was performing touch and go pattern operations on runway 7L at the Livermore airport. The instructor said that the student flared about 8 to 10 feet above the runway and "abruptly reduced the throttle." The aircraft then made a very hard touchdown. The instructor stated that he took control of the aircraft during the bounce and successfully initiated a go-around. The pilot evaluated the aircraft systems for any possible damage and, concluding there was none, continued the flight. After the aircraft returned to the Oakland airport for refueling, the pilot noted the damage to the left prop. Maintenance personnel discovered damage to the left main wing spar.