What happened
On August 25, 1970, a Piaggio P-149 E, registration HB-EBW, was conducting a navigation training flight from Zurich Airport when a sudden engine failure occurred. The flight, operated by the Swiss Air Traffic School, included a flight instructor and two students. Shortly after passing the 'Limmat' checkpoint, a loud bang was heard, followed by irregular engine operation and visible flames and smoke emerging from the engine cowling.
As the engine power rapidly declined, the pilot attempted to return to Zurich Airport but found the loss of power too significant to maintain altitude for a return. Visibility was further compromised by heavy oil spray on the windscreen and smoke entering the cockpit. After executing a turn to head east, the pilot was forced to abandon a planned landing at nearby sports fields due to insufficient altitude, opting instead for an emergency descent into the Altstetten railway station area.
During the final approach, the aircraft struck overhead railway lines, a utility pole, and an insulator bracket. The Piaggio P-149 E impacted platform 6 of the Altstetten station at an angle of approximately 11 degrees. The aircraft slid approximately 50 meters, passing under the Europabrücke before coming to rest between railway tracks 6 and 7. The impact resulted in two serious injuries and one minor injury among the crew and passengers, while the aircraft was destroyed.
The investigation
Investigators examined the wreckage and the engine components to determine the cause of the power loss. The examination of the Lycoming engine revealed that the engine housing had been breached, leading to a total loss of engine oil. Detailed analysis of the internal components identified that the failure originated within the engine's internal mechanics.
Technical scrutiny of the engine parts focused on the piston pins and connecting rods. The investigation also looked into the impact of aftermarket modifications, specifically the installation of two additional exhaust silencers (known as 'Frankfurter Töpfe') which had been added to the aircraft after its delivery to the operator. These silencers were found to increase exhaust gas temperatures by approximately 50°C to 65°C compared to the original configuration.
Findings
- The primary cause of the engine failure was the fatigue failure of piston pin number 6, which subsequently caused the failure of connecting rod number 6.
- The fatigue crack originated from the unhardened inner surface of the pin.
- The emergency landing was conducted in extremely unfavorable terrain, involving collisions with railway infrastructure.
- The use of additional exhaust silencers increased thermal stress on the engine cylinders, though the direct impact of this temperature rise on the component's lifespan could not be precisely determined.
- The crew's injuries were likely exacerbated because only lap belts were utilized; the shoulder harnesses were not in use during the flight.