1943 the decision was made to convert both aircraft to twin-engine configuration by removing the Lycoming engines and installing a 9-cylinder Pratt & Whitney R-1340-AN1 Wasp engine on each wing. These engines had been imported to Australia in large numbers for use as tank engines. The noses of the two aircraft were to be re-built by installing streamlined structures made of aluminium sheet. By October 1943 VH-UYY had been converted to twin-engine configuration in the Essendon Airport facilities of Australian National Airways which had taken over Airlines of Australia. The increase in total power from improved the takeoff, climb and one-engine-inoperative performance of the aircraft and permitted the maximum weight to be increased to for takeoff. For the next 15 months Tokana was used on the Melbourne-Kerang-Mildura-Broken Hill route. VH-UYY flew for 13,763 hours, including 2,797 hours since its conversion to a twin-engine aeroplane. Investigation Investigators were able to determine the most likely sequence of the in-flight break-up: left outer wing tail assembly right wing right engine Investigators found nothing in the wreckage to indicate there had been an explosion or fire in the aircraft prior to it striking the ground. It was immediately clear that the outer part of the left wing had broken away from the aircraft. The lower boom in the main spar had failed at the outboard edge of the engine nacelle and then the upper boom had also failed as the result of the wing folding upwards under the air loads imposed on it. The rear spar had then failed, allowing the entire outer part of the wing to separate from the aircraft and drift slowly to the ground. The fracture surfaces on the outer part of the left wing were examined by the Council for Scientific and Industrial Research at its Division of Aeronautics in Melbourne. These examinations determined that separation of the left wing was initiated by metal fatigue of the lower main spar boom attachment socket. The primary structure of the Stinson was of welded steel tube construction. A fatigue crack had initiated in weld metal on the inner surface of the socket. After propagating through the weld metal during a large number of flights, the fatigue crack entered the socket's parent metal. This crack eventually affected 45% of the cross-section of the socket before the lower boom failed on the fatal flight. Investigators noted the amount of metal in the socket