What happened
On August 30, 1961, a Jodel D-9, registration HB-SUN, departed from Dällikon airfield for a private flight toward Lachen. The weather conditions were clear with good visibility. While performing turns at an altitude of approximately 200 to 300 meters near the hamlet of Lieli-Oberwil, the propeller suddenly detached from the engine.
Following the loss of the propeller, the pilot attempted an emergency landing in an open field. During the approach, the aircraft struck a group of trees, causing it to nose over and flip onto its back. Despite the significant damage to the airframe, the pilot escaped the wreckage without injury.
The investigation
Investigators examined the mechanical integrity of the propeller assembly and the engine components. The investigation focused on the mounting bolts and the recently installed propeller cowling. Technical records for the aircraft were noted as being incomplete, and the engine manufacturer, M.V. Agusta, did not respond to all inquiries regarding specific details.
Findings revealed that the propeller mounting bolts had failed at the transition between the head and the shaft. The investigation also looked into the history of the aircraft's modifications, specifically the installation of a custom-made propeller cowling by the aircraft's builder, which had not been submitted to the Federal Office of Civil Aviation for approval.
Findings
- The primary cause of the accident was fatigue failure of the propeller mounting bolts.
- These failures were driven by unexpected bending stresses caused by a loss of preload in the assembly.
- The loss of preload was triggered by the installation of an unapproved and structurally inadequate propeller cowling.
- The cowling installation created a gap in the spiral spline between the hub and the flange, shifting the torque load from the intended spline to the lightweight metal ring of the cowling.
- This shift caused the flange to rotate and wear down the mounting ring, leading to the bolts absorbing alternating bending loads.
- Contributing factors included the use of bolts with sharp transitions and a relatively small 7 mm diameter, as well as engine vibrations.
- Maintenance deficiencies played a role, as a proper inspection during the 100-hour service—which would have required removing the cowling—could have identified the worn components and the loss of bolt tension.