What happened
On June 15, 1974, a passenger balloon flight involving a Helvetia 1680 m3 balloon, registration HB-BIN, ended in a catastrophic fire near Melchnau, Switzerland. The flight, which began in Schlieren with four passengers, proceeded normally despite unstable air conditions. Upon landing in Melchnau, the pilot attempted to deflate the envelope. During this process, sudden gusty winds caused the envelope to sway violently, leading the pilot to open the vent to accelerate deflation.
As the envelope became slack, it made contact with nearby trees and threatened to fall onto a parked Citroen 2CV. While the driver was attempting to move the vehicle, an explosion occurred, causing the balloon to burn almost entirely. The incident resulted in one injury to a passenger and minor skin irritations to two bystanders. The basket and instruments remained undamaged, but the envelope was 90% destroyed.
The investigation
The investigation focused on identifying the ignition source. Investigators considered three possibilities: ignition from the car's engine, sparks from contact with trees, or static electricity generated by the envelope rubbing against itself. Witnesses reported seeing a red glow within the envelope's filling area just before the explosion, which pointed toward an internal ignition.
Testing revealed that the gas used, a split gas from Schlieren, was extremely dry, containing only about 17% relative humidity. This lack of moisture, combined with the friction of the slack envelope rubbing against itself in the wind, created a high risk of electrostatic discharge. Furthermore, while the balloon had been treated with a conductive coating by the manufacturer in Augsburg, subsequent inspections of similar balloons (HB-BII and HB-BIX) showed that the coating's resistance was several orders of magnitude higher than required by Swiss regulations.
Findings
- The primary cause of the accident was electrostatic discharge occurring within the envelope.
- The friction of the slack envelope rubbing against itself in windy conditions generated static charges.
- The use of very dry gas (17% relative humidity) prevented the natural dissipation of static electricity.
- The internal conductive coating applied by the manufacturer failed to meet the necessary electrical resistance standards.
Safety action
Following the discovery that the manufacturer's conductive coating was ineffective, the investigation led to a mandate for the Federal Office of Civil Aviation to measure all balloons that had received the same Augsburg treatment. Consequently, the flight authorizations for balloons HB-BII and HB-BIX were revoked after they were found to have dangerously high resistance levels.