What happened
On the early morning of June 25, 2011, a Wörner NL-1000/STU gas balloon, registration HB-QHJ, departed from Bad Zurzach, Switzerland, for a planned flight across the Alps. The flight, which included the pilot and two passengers, began with a steady ascent. During the initial phase of the flight, the pilot utilized ballast release to manage the rate of climb.
As the balloon reached an altitude of approximately 10,000 feet, the envelope suddenly burst, leading to a massive and instantaneous loss of lifting gas. The aircraft entered a high-speed descent, with vertical speeds averaging between 13 and 15 meters per second. During the emergency, the pilot attempted to use the remaining envelope to create a parachute effect and jettisoned several sandbags to slow the fall. One discarded sandbag struck a barn, causing damage to the structure and a vehicle inside. The descent ended when the basket struck a tree stump near Fisibach. The impact resulted in one fatality and two injuries, with the deceased passenger dying at the scene.
The investigation
SUST examined the flight data from the aircraft's GPS and multifunctional device, radar records, and radio communications. The investigation focused on the state of the balloon's envelope and the pilot's management of the gas filling process. Investigators analyzed the physical condition of the envelope, noting that while most of the material met strength requirements, the tear resistance in one specific panel was slightly below the manufacturer's minimum specification.
Findings
- The primary cause of the accident was the sudden loss of lifting gas following the burst of the balloon envelope.
- The burst was highly likely caused by the balloon exceeding its maximum allowable pressure height while the filling appendix was closed.
- The pilot operated the balloon in a manner that contradicted fundamental physical laws regarding gas expansion and pressure.
- The pilot was distracted during a critical phase of flight due to managing transponder settings and communicating with air traffic control.
- The pilot's lack of clarity regarding the difference between normal flight altitude and maximum pressure altitude contributed to the envelope failure.