1945 Australian National Airways Stinson crash

Investigation

Investigators were able to determine the most likely sequence of the in-flight break-up:

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 that was unaffected by the fatigue crack at the time of the accident and calculated that the wing was capable of supporting loads up to about 2.5 times the weight of the aircraft on its fatal flight. This suggested the gusting winds prevailing at the time, and the turbulence associated with the low altitude at which the aircraft was flying, were partly responsible for the failure.

The matching socket in the spar in the right wing was also examined and found to be affected by a similar fatigue crack in the interior of the weld metal. This crack was detected by magnaflux inspection but could not be seen by visual examination.

The Investigation Panel determined that the conversion from three-engine to two-engine configuration was not a cause of the accident. It found that fatigue failure of the wing was inevitable, and this modification and subsequent operation at a higher weight caused only slight shortening of the time before the failure occurred.

In its report, the Investigation Panel wrote:

"The crash is, as far as is known to the Panel, the first example of a failure inflight of an aircraft structure attributable directly to fatigue. "In the type of construction embodied in these aircraft, where concentrated loads are carried by a small number of heavy members, a single fatigue failure can, and in fact has caused, a complete structural collapse. "The Panel feels impelled to assert that neither the original design nor the original manufacture was at fault... the tragedy has several new lessons to teach all concerned with aviation, and that learning provides some small degree of compensation for the loss of valuable human lives."

The Investigation Panel's report was completed in two weeks and included five recommendations:

Critical joints of all heat treated, welded steel tube aircraft structures should be examined annually by the magnaflux method.

The unique nature of the accident should be drawn to the attention of the Australian Council for Aeronautics. The council should be asked to study the phenomenon of metal fatigue in aircraft structures.

The certificate of airworthiness of VH-UKK Binana, the sole Stinson Model A surviving in Australia, should be cancelled immediately.

Both wings of VH-UKK should be sent to the Division of Aeronautics laboratories for test and examination to advance the knowledge of fatigue of aircraft structures.

The Department of Civil Aviation should obtain a suitable number of V-G recorders and install them in airline aircraft operating in Australia to survey the conditions arising on major air routes.

The Minister for Civil Aviation, Mr Arthur Drakeford, made a detailed statement to Parliament that an undetected fatigue crack in a welded joint in a boom attachment fitting in the left wing caused the crash. Drakeford also said he was satisfied the work of the Investigation Panel was completed competently and thoroughly.