What happened
On August 4, 1973, a Cessna F 150 K, registration HB-CUZ, departed Beromünster for a private sightseeing flight. The aircraft was carrying one passenger and was intended to fly over the Schwyz region before returning to the departure airfield. During the flight, the pilot encountered deteriorating visibility near Hildisrieden and decided to reroute over Lake Sempach to return to Berom andmünster.
While cruising at approximately 3,000 feet, the engine began to sputter suddenly, followed by a loud bang. The pilot immediately reduced power in an attempt to reach Beromünster. However, as the engine vibrations intensified, the pilot was forced to shut down the engine entirely. Seeking an appropriate site, the pilot identified a landing area and executed a successful emergency landing at approximately 08:55 AM, roughly 3 km south of the Beromünster airfield. There were no fatalities among the crew or passenger, though the aircraft sustained damage.
The investigation
The investigation focused on the unusual nature of the engine failure. The aircraft's maintenance records showed the last 100-hour inspection had been completed in June 1973, and the engine had undergone a partial overhaul in 1971.
Technical examination of the Rolls Royce 0-200-A engine revealed a fracture in cylinder number 2 near the fourth fin close to the cylinder base. To further investigate the failure mechanism, investigators examined another cylinder of the same type that exhibited similar cracks. Both cylinders showed significant scouring marks on the inner cylinder walls caused by the piston pin. Analysis of these cracks revealed that the friction from the piston pin had caused structural changes to the metal, including cold deformation and the formation of untempered martensite.
Findings
- The primary cause of the engine failure was a premature fatigue fracture of the second cylinder.
- This fracture was triggered by the piston pin rubbing against the inner cylinder wall.
- The friction from the piston pin caused surface damage and structural alterations to the cylinder material, which ultimately led to the material fatigue failure.