According to the certificated flight instructor (CFI), the purpose of the flight was for him to give instruction to the commercial pilot, who was working toward his flight instructor certificate. At the destination airport, the CFI demonstrated a power-off 180-degree approach to landing. The commercial pilot successfully demonstrated four of the same approaches and began the fifth approach. During the landing, the commercial pilot misjudged the landing flare which resulted in a firm touchdown. The CFI attempted to maneuver the airplane in a nose-high configuration, but was unable to apply enough back pressure to prevent a firm touchdown, due to the force being applied by the commercial pilot. Upon touchdown, the CFI executed a go-around maneuver and then flew to the home airport where they landed without incident. Postaccident examination of the airplane revealed substantial damage to the outboard and inboard ribs. The flight instructor stated that there were no mechanical malfunctions with the airplane prior to the hard landing.