What happened
On July 11, 2008, a Kubicek KUB BB2 and registration HB-QJS was conducting tethered flights as part of a private event near Erlenhof/Arnegg, Switzerland. The pilot had completed several successful ascents under favorable conditions, but as the evening progressed, weather conditions began to deteriorate.
At approximately 17:40 local time, the pilot began an ascent with four passengers. While at an altitude of roughly 10 meters, the aircraft was struck by a powerful gust of wind from the northwest. The force of the wind was so great that the ground crew was unable to stabilize or restrain the balloon. The wind caused the balloon envelope to deform, leading to a rapid descent. The basket struck the ground with significant force, briefly lifting again before a second impact occurred. During this second contact, the windward tether snapped, causing the balloon to become completely uncontrollable. The impact resulted in one serious injury to a passenger, while another passenger sustained a minor injury.
The investigation
The investigation examined the meteorological conditions, the aircraft's technical state, and the operational procedures used during the tethered flight. Investigators reviewed weather reports from MeteoSchweiz, which indicated that a pre-frontal thunderstorm cell was moving through the region, bringing significant gusts.
Evidence from eyewitness photographs contradicted the pilot's account regarding the tether configuration. While the pilot stated the ropes were attached at the burner level, the investigation established that the ropes were actually attached to the crown ring. Furthermore, investigators analyzed the wind speeds, noting that gusts of at least 20 knots occurred, significantly exceeding the manufacturer's maximum operational limit of 4 knots.
Findings
- The primary cause of the accident was the loss of control resulting from two violent impacts of the basket with the ground.
- The pilot misjudged the impact of an approaching thunderstorm cell, failing to abort the flight despite visible signs of worsening weather and increasing wind.
- The method of tethering the balloon was improper; attaching the ropes to the crown ring instead of the burner frame increased the vertical force exerted by the wind, pulling the basket toward the ground.
- Wind speeds during the event far exceeded the operational limits specified in the flight manual.