What happened
On May 13, 1967, a Beech A 23 A (registration HB-ENN) was conducting a recreational flight over the eastern Swiss Alps. The flight, which began at Altenrhein with a scheduled stop in Samedan, was en route to Speck-Fehraltorf when a critical mechanical failure occurred. While flying at approximately 1,000 meters above ground level near the Kempten region, one blade of the propeller suddenly fractured.
The sudden loss of the blade caused intense vibrations due to the resulting imbalance in the propeller's center of gravity. To prevent the engine from detaching from the airframe due to these violent oscillations, the pilot immediately cut the ignition. Utilizing the remaining altitude, the pilot successfully identified a suitable landing site and performed an emergency touchdown in a field near the hamlet of Ruetsch tschberg, on the left bank of Lake Pfäffikon. There were no injuries to the two occupants, though the aircraft sustained significant damage to the engine and propeller assembly, estimated at roughly 30% of the aircraft's original value.
The investigation
Investigators examined the remains of the Sensenich M 74 DC-0-60 propeller and the engine assembly. The investigation focused on the history of the propeller's maintenance, specifically regarding a previous service bulletin issued by the manufacturer. This bulletin addressed corrosion issues caused by the water-soluble adhesive used to attach manufacturer decals to the propeller blades.
Technical analysis of the fracture surface revealed a fatigue failure. The crack originated at the base of a microscopic corrosion pit on the upper surface of the blade. Metallographic examination of the propeller remnants also identified intergranular corrosion and a second crack near the primary fracture site. Furthermore, the investigation looked into the maintenance performed on April 6, 1967, during the aircraft's 100-hour inspection.
Findings
- The propeller blade failure was caused by material fatigue triggered by intergranular corrosion.
- The corrosion was localized in the area where manufacturer decals had been applied, as the adhesive used for these decals had compromised the aluminum alloy.
- While a maintenance procedure had been implemented to treat such corrosion by sanding and polishing, the investigation found that the depth of the corrosion pits was never measured during the April 1967 service. The manufacturer's instructions required blades to be scrapped if corrosion exceeded 0.38 mm, but the method for determining this depth was not explicitly defined in the service bulletin.
- The sanding process used during the previous maintenance was insufficient to completely eliminate the underlying corrosive damage, and standard dye penetrant inspections were unable to reliably detect the presence of intergranular cracks.