What happened
On July 5, 1972, a hydrogen balloon, registration HB-BIG, departed from Stechelberg as part of an international high-alpine ballooning week. The flight, carrying the pilot and four passengers, was intended to conclude by 14:00. However, as the flight progressed, weather conditions deteriorated rapidly. At 11:30, flight organizers issued an urgent radio instruction to all active pilots to land immediately due to approaching storms.
In response to the directive, the pilot initiated a descent near Meiringen. During the final phase of the landing, the pilot instructed a passenger to hold the vent open. Because the instruction to stop was not given, the vent remained open for approximately 15 to 20 seconds, resulting in a loss of lift of roughly 220 to 300 kg. Consequently, the basket struck the ground with a high rate of descent. While the impact caused one serious injury to a passenger, the pilot and the remaining three passengers escaped without injury. Following the impact, the vent was not fully closed, causing the balloon to deflate slowly. Approximately five minutes after the landing, the deflated envelope ignited and burned.
The investigation
The investigation examined the pilot's actions, the aircraft's technical state, and the meteorological conditions. Investigators noted that the pilot had 240 kg of ballast remaining on board, which would have been sufficient to manage a controlled descent had the vent been handled correctly.
Technical analysis of the remaining envelope material revealed extremely high electrical surface resistance, measuring between 4.2 and 6.0 x 10^11 Ohm. This far exceeded the 10 Ohm threshold identified as a fire hazard for gas balloon envelopes. Investigators also reconstructed the flight path, noting that the balloon had encountered a Föhn wind during its descent.
Findings
- The primary cause of the accident was the improper handling of the balloon during the final landing phase, specifically leaving the vent open without dropping ballast.
- The fire was caused by an electrostatic discharge. The high electrical resistance of the envelope allowed for the buildup of electrostatic charge, likely through friction between envelope sections moving in the wind. When the potential difference between these charged areas became sufficient, sparks ignited the hydrogen.
- Meteorological forecasts and organizers underestimated the speed of the weather deterioration.