What happened
A flight involving a free balloon (registration HB-BIZ) departed from the Bern gasworks at 10:15 CEST on May 15, 1965. During the flight, the balloon drifted eastward, reaching a maximum altitude of 2,350 meters over the Entlebuch region. Due to frequent transitions between sunlight and cloud cover, the pilot consumed a significant amount of ballast.
As the flight progressed, the pilot decided to land near the Entlebuch area, descending to approximately 300 meters. However, obstacles such as railway lines and electrical wires prevented a safe landing in the Schachen area. Subsequent solar radiation caused the balloon to rise again to 2,000 meters. As the weather turned to rain and thunderstorms, the balloon began to descend rapidly at a rate of 5 meters per second, weighted down by the moisture in the envelope. The flight concluded with a high-speed drag landing on a stony area near the Littau schoolhouse, causing the basket to tip and slide approximately five meters.
The investigation
The investigation examined the flight progression, the weather conditions, and the management of ballast. Investigators noted that while the initial ballast load of 225 kg was sufficient, the consumption rate was unusually high due to the fluctuating weather. The investigation also assessed the impact of the weather change, specifically the onset of rain and lightning, on the aircraft's descent rate and weight.
Findings
- The primary cause of the accident was the insufficient remaining ballast at the time of the final descent. By the time the landing was attempted, only about 50 kg of ballast remained.
- The descent rate reached 5 m/sec, which was too fast to be mitigated by the remaining ballast.
- The weight of the balloon envelope increased because it became wet from the rain.
- The pilot and one passenger sustained injuries, including a laceration to the pilot's lower leg and three broken ribs for a passenger.