What happened
On May 15, 1960, a Piper Cub J3C (registration HB-OBX) was performing a flight from Thun to Davos, Switzerland. The pilot was accompanied by his wife as a passenger. During the final approach to runway 05 at Davos airport, the aircraft descended below the height of the final obstacles and struck a building material depot approximately 30 meters past a nearby shed. The impact destroyed the aircraft. The pilot sustained minor injuries, including a broken nose, while the passenger remained uninjured.
The investigation
Investigators examined the flight sequence, the aircraft's technical state, and the cockpit instrumentation. The investigation revealed that the aircraft had recently undergone an instrument change; a new airspeed indicator, calibrated in mph rather than km/h, had been installed the day before the accident. This new instrument featured a minimum speed marker at 45 mph, whereas the previous instrument had a marker at 85 km/h (approximately 53 mph).
While the new instrument was found to be functioning correctly and met the minimum regulatory requirements, the investigation noted that the pilot had not been specifically alerted to the change in units or the different marking. The investigation also reviewed the pilot's flight experience, noting he had relatively low hours in mountain environments, and examined the aircraft's trim configuration, which was found to be tail-heavy at the time of the accident.
Findings
- The primary cause of the accident was a loss of airspeed during the final approach.
- The pilot failed to maintain the necessary speed margin required for the specific environmental conditions.
- The decision to perform a go-around and then approach from the southwest—a more difficult direction due to obstacles like a cable car and buildings—contributed to the unstable approach.
- The recent installation of an airspeed indicator with a different scale (mph) and a lower red-line marker (45 mph) likely influenced the pilot's flying behavior, making the margin for error smaller than previously experienced.
- The aircraft was trimmed in a tail-heavy configuration, which may have contributed to the loss of lift.