Investigation Summary
The aircraft, named 'Tokana', was flying at approximately 1,000 feet Above Ground Level (AGL) under a low cloud base (estimated at 2,000 feet ASL) in gusty conditions when witnesses heard a sharp crack. The left outer wing separated first, followed by the tail assembly and the right wing as the fuselage entered a violent spiral. The main wreckage struck the ground inverted and was consumed by fire, while the left outer wing drifted to the ground 1.2 kilometers away. Investigation by the Council for Scientific and Industrial Research (CSIR) revealed that a fatigue crack had initiated in the weld metal on the inner surface of the lower main spar boom attachment socket. The crack had propagated through 45% of the socket's cross-section before the final failure. Although the wing could still support 2.5 times the aircraft's weight, the turbulence and gusty winds at low altitude provided the final stress necessary to trigger the failure. The investigation also found a similar, though smaller, fatigue crack in the right wing of the wreckage, which was only detectable via magnaflux inspection.
Final Conclusions
The Investigation Panel concluded that the primary cause was the fatigue failure of the wing structure. While the aircraft had been modified from three engines to two, the panel determined this modification only slightly accelerated an inevitable failure. The initial technical investigation was completed rapidly within two weeks. This was followed by a formal Air Court of Inquiry chaired by Mr. Justice Philp, which commenced on March 27, 1945. The findings led to the immediate grounding and permanent retirement of the only other Stinson Model A in Australia, VH-UKK 'Binana'.
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Systems & Failure Modes
Main Spar Boom Attachment Socket
A welded steel tube joint where the outer wing section connects to the inner wing; the site of the fatigue crack initiation.
Pratt & Whitney R-1340-AN1 Wasp Engines
Two 550 hp engines installed in 1943 to replace the original three Lycoming engines.
Interesting Facts
- 01First fatal aircraft accident in the world officially attributed to metal fatigue.
- 02The fatigue crack was located in the weld metal of a steel tube attachment socket.
- 03The aircraft had been modified in 1943 from a trimotor to a twin-engine configuration.
- 04The left outer wing was found 1.2 kilometers (3/4 mile) from the main crash site.
- 05The crash site was located on the same station where John Robertson Duigan flew the first Australian-made aircraft.
- 06Manufacturer Serial Number (MSN) 9130; Year of Manufacture 1936.
Safety Actions & Advisories
Safe Retirement Life Calculations
The Australian Department of Civil Aviation began the practice of calculating and enforcing safe retirement lives for metal aircraft structures.
Magnaflux Inspection Mandate
The investigation recommended that critical joints of all heat-treated, welded steel tube aircraft structures be examined annually using the magnaflux method.
Fatigue Research Program
The Division of Aeronautics laboratory began a long-term fatigue testing program, eventually testing 200 surplus P-51 Mustang wings to understand structural failure characteristics.