What happened
On February 19, 1972, a training flight involving a Leonhard Stuttgart gas balloon, registration HB-BOR, was underway near Oberbipp, Switzerland. The flight, which included an instructor, a student, and one passenger, departed from Schlieren/ZH for a training and recreational mission.
During the flight, the pilot initiated a landing procedure over an open field approximately 700 meters in length near Oensingen/Oberbipp. As the descent progressed at a rate of roughly 2.5 m/s, the tow rope made contact with the ground. However, shortly after the rope touched the surface, the balloon began to drift laterally. The aircraft drifted toward the right, making contact with high-voltage power lines. The initial contact with the lines via the shroud lines caused a short circuit, and the balloon drifted approximately 100 meters along the wires. During this movement, the balloon's net became entangled in a utility pole. A second short circuit, triggered by the pressure of the balloon envelope against the lines, ignited the balloon, causing it to catch fire at approximately 16:47.
The investigation
The investigation examined the flight conditions, the airworthiness of the HB-BOR, and the meteorological environment. The pilot held a valid instructor certificate and radio telephony rating, and the student held a valid learner's permit. The Leonhard Stuttgart balloon, manufactured in 1969, was found to be airworthy with a valid certificate of airworthiness.
Investigators analyzed the wind conditions at the time of the accident. While the landing site was deemed appropriate, the local wind environment was characterized by moderate turbulence near the ground. The investigation focused on why the balloon drifted toward the power lines despite the landing being correctly planned.
Findings
- The primary cause of the accident was the balloon drifting into high-voltage power lines during the landing phase.
- Unstable wind conditions likely contributed to the drift; specifically, intermittent cold air flowing from the Jura slopes may have introduced a northern component to the prevailing easterly winds.
- The loss of this northern wind component just before landing caused the balloon to drift to the right toward the electrical infrastructure.
- There were no injuries to the three occupants, though the balloon envelope and net were destroyed.
- The aircraft was a 945 m3 capacity balloon filled with lifting gas.