Following the commencement of aircraft vibration in flight at 4,000 feet, the pilot advised Alice Springs that he was returning and expected a normal approach and landing. He attempted to isolate the source of the vibration by checking the magnetoes and by applying carburettor heat, and then he proceeded to cut each engine in turn by moving the appropriate mixture control to the idle cut off position. After cutting the port engine the pilot trimmed out the rudder foot load, but did not increase power on the starboard engine. Some minutes later, realising that he had lost considerable height and was now down to some 500 feet above ground level, he attempted to restore power on the port engine and believes he reselected the mixture control to "rich". He then moved both throttle and pitch levers to climb power settings, but although the pilot still had considerable right rudder trim applied he felt no asymmetric foot load. The aircraft gained 100 feet and then flew level at 90 knots, with the vibration continuing. The pilot lowered the nose and as the aircraft began to lose height again he decided that he must land straight ahead. He transmitted a "Mayday" call and with the wheels and flaps retracted he landed amongst scattered, small trees, damaging the aircraft beyond economical repair. It is possible that the vibration was due to the turbulent airflow induced by the protruding section of the wing walkway. The extra drag arising from this source, however, would not have been sufficient to account for the loss of performance.