What happened
On June 29, 1971, a hydrogen-filled balloon, registration HB-BOA, departed from Stechelberg, Switzerland, carrying a pilot and three passengers. During the flight, the aircraft crossed the Alps before the pilot initiated landing procedures near Rho, Italy, at approximately 14:58 local time.
As the balloon descended to roughly 100 meters above the ground, the pilot attempted to release the vent valve's safety mechanism. However, a Lyra clip had become jammed in the valve, preventing the mechanism from opening. To compensate, the pilot performed the landing using a continuous vent pull.
Upon touchdown, strong surface winds of approximately 60 km/h caused the basket to drag across the terrain for about 75 meters, striking a line of bushes and an oat field. Once the basket came to a halt, a dull explosion occurred, immediately followed by a fire involving the balloon envelope. There were no injuries among the four occupants, though the balloon envelope, netting, and various equipment were destroyed, and property damage to the field was recorded.
The investigation
The investigation examined the mechanical state of the balloon, the weather conditions, and the potential for external ignition sources. The Balloonfabrik Augsburg model, manufactured in 1963, was found to be airworthy with valid certification.
Investigators ruled out ignition from external sources, such as spectators or motor vehicles. Laboratory testing was conducted on remaining fragments of the balloon envelope to analyze their electrical properties. The tests revealed very high half-life periods for electrical charges, indicating that the material was poor at dissipating electrical charge. This property allows for the formation of isolated "charge islands" on the envelope surface.
Findings
- The weather at the time of the accident was warm, sunny, and extremely dry, with a relative humidity of only 23%.
- High solar radiation likely heated the envelope to temperatures between 50°C and 70°C.
- The electrostatic discharge of accumulated charges was the primary cause of the ignition.
- The investigation determined that the buildup of static electricity was facilitated by the low humidity and high heat. As the balloon deflated, the movement and fluttering of the envelope material likely generated significant static buildup. The resulting spark between areas of differing electrical potential ignited the hydrogen-air mixture present during the landing process.