The pilot stated that he rounded out too low, touched down too fast, and the aircraft bounced slightly. He attempted to correct with back pressure on the control column, together with increased nose up elevator trim. The aircraft appeared to climb and the pilot eased the back pressure. The nose dropped, the aircraft touched down again in a flat attitude, and porpoising commenced. After three oscillations the nose gear moved rearwards and upwards, shattering the windscreen, and causing the propellers to contact the runway surface. Investigation revealed that there was an intermittent fault in the elevator trim switch, which caused the trim to move only in a forward direction, irrespective of the selection made by the pilot.
