What happened
On September 11, 2010, a non-commercial hot air balloon flight departed from Künzell, carrying a pilot and three passengers. During the approach to a landing site east of Hünfeld-Großenbach, the pilot extinguished both burner flames in preparation for touchdown.
Upon the initial contact of the basket with the ground, a component of the gas line fractured, leading to an uncontrolled release of liquid propane. The pilot managed to close the main valve on the gas cylinder, but the impact caused the basket to slide and overturn. During the capsize, two passengers were thrown from the basket.
Following the incident, the pilot successfully reignited the burner and performed a second approach. This second descent involved flying over buildings and vegetation, eventually resulting in a collision with a tree approximately 600 meters from the initial landing point. The pilot and one passenger sustained minor injuries.
The investigation
The BFU examined the Balony Kubicek / BB37Z and its gas system components. The investigation focused on the technical failure of the gas line connection. Investigators found that a brass component of the gas line had broken at its threaded interface with a steel T-piece.
Technical analysis revealed that the metal fittings of the gas line connections protruded approximately 10 cm beyond the protective cylinder collars of the propane tanks. While the aircraft had undergone an annual inspection in June 2010, the investigation noted that the specific manufacturer of the gas line was unknown to the inspector, and the installation method allowed for significant protrusion of the hardware.
Findings
- The primary cause of the incident was the overstressing of an improperly installed gas line connection component during landing.
- The gas line fittings were installed such that they extended 10 cm outside the protective cylinder collars, leaving the thin-walled brass material vulnerable to mechanical stress.
- The design of the valve installation on the VA-50 gas cylinders necessitated this protrusion, creating a lever effect that subjected the brass fitting to loads beyond its structural capacity.
- The pilot's decision to extinguish the flames prior to landing prevented the escaping propane from igniting, which avoided a much more serious fire or explosion.