What happened
On October 4, 1966, a Mooney M20A, registration D-ELQU, was conducting a private VFR flight from Nuremberg to Montpellier. While cruising at 7,000 ft MSL several kilometers north of Olten, the aircraft experienced a sudden and violent impact. The force of the event was significant enough to shake the occupants and cause the cabin door to fly open.
Recognizing a critical issue with the propeller, the pilot immediately shut down the engine. Upon the propeller coming to a halt, it was discovered that one blade had broken off. To avoid the risks associated with attempting a landing at the nearby Olten airfield, the pilot opted for an emergency landing in a less populated area. The pilot executed a belly landing with the landing gear retracted in an open field near Niederbuchsiten. During the landing roll, the right wing struck a cornfield, which decelerated the aircraft and caused it to rotate 170 degrees. There were no injuries to the pilot or the two passengers.
The investigation
The investigation involved an on-site examination by the Cantonal Police and a technical analysis of the broken propeller blade by the Swiss Federal Institute of Materials Testing. Investigators examined the maintenance history of the McCauley propeller, noting that the blade in question had been bent during a landing in September 1965 and subsequently straightened by a specialist firm.
Findings
Technical analysis revealed that the propeller blade failure was caused by material fatigue. While the alloy itself showed no inherent material defects, the investigation identified several contributing factors at the fracture site, including intergranular corrosion and a local indentation caused by a foreign object. It is believed that the previous repair to straighten the bent blade may have introduced microscopic cracks that were undetectable by non-destructive testing at the time. The progression of intergranular corrosion significantly facilitated the fatigue failure.