What happened
On October 9, 1971, a hydrogen-filled free balloon, registration HB-BOE, departed from Zurzach carrying a pilot and three passengers. The flight proceeded normally until approximately 15:45, when the balloon landed near Bad Dürrheim-Biesingen, Germany. Following the landing, the pilot began deflating the envelope by venting gas. After the passengers had disembarked, an explosion occurred as the vent lines were being handled, which immediately ignited a fire in the balloon envelope. The force of the event caused onlookers to release the basket, which briefly ascended again while burning. The balloon was destroyed in the incident, but the pilot and all three passengers remained uninjured.
The investigation
Investigators examined the construction of the 1965 Ballonfabrik Augsburg balloon and the environmental conditions at the time of the landing. The investigation ruled out external ignition sources, such as engines or cigarettes from bystanders. The inquiry focused on the material composition of the balloon envelope, noting that it was manufactured using two distinct types of fabric. These materials possessed significantly different electrical conductivity and insulation properties. The investigation also considered the state of the envelope during deflation, noting that the balloon was largely empty at the time of the fire.
Findings
The investigation established that the primary cause of the accident was an electrostatic discharge. Because the envelope was composed of two different fabric types with varying electrical properties, the friction caused by the movement of the envelope parts during deflation allowed for the formation of isolated pockets of electrostatic charge. The disparity in conductivity between these fabric sections facilitated a spark as the charges attempted to equalize. This spark ignited the flammable mixture of hydrogen and air present within the envelope.