What happened
On October 29, 1969, an instructor and a student pilot were conducting a series of landing exercises at various airfields near La Chaux-de-Fonds and Yverdon. After landing at the private airfield in Yens, the instructor took off again to perform a circuit.
During the maneuver, the pilot initially turned right to perform a standard 180-degree turn but diverted the course to the east to avoid another aircraft preparing for departure. The crew climbed to approximately 200 meters and then initiated a wide left turn to return to the airfield. During this sequence, the instructor performed an engine-failure simulation with the engine at idle.
As the aircraft transitioned into the final approach, it was offset to the right of the runway centerline. To correct the alignment, the pilot continued a left turn and then transitioned into a right bank at an altitude of only 5 to 10 meters. Due to the significantly reduced airspeed, the right wing stalled, causing the Piper J3C, registration HB-OXS, to strike the ground in a right-side skid. The impact resulted in no fatalities, though the aircraft was 80% destroyed.
The investigation
The investigation examined the flight maneuvers, the aircraft's mechanical condition, and the environmental factors at the time of the accident. Investigators noted that the aircraft's airworthiness certificate was valid and the engine and airframe had been regularly maintained. The investigation also considered the specific constraints of the Yens airfield, which requires landings in a specific direction due to nearby electrified railway lines, and the meteorological conditions, which included a dry haze and a low sun angle.
Findings
- The primary cause of the accident was a stall resulting from a slow approach performed in a turn.
- The decision to perform an engine-failure simulation during a circuit involving a difficult airfield was deemed inappropriate for a student's first circuit at that location.
- The aircraft's approach was misaligned with the runway axis, necessitating a corrective turn at a very low altitude.
- The airspeed was critically low during the final stage of the maneuver.
- A local tailwind may have contributed to the loss of airspeed.
- The position of the sun, low in the sky and filtered through haze, may have caused glare that prevented the pilot from monitoring the airspeed indicator, though the pilot's attention was likely focused on navigating the narrow approach.