What happened
On the morning of June 17, 2023, a Kubícek BB60Z hot air balloon, registration HB-QZU, was being prepared for a commercial flight near Rainmatt, Switzerland. The pilot was using an external fuel cylinder, known as a filling bottle, to assist in heating the envelope before takeoff. To preserve the main fuel supply, this external bottle was connected to the left burner's fuel system.
During the pre-flight process, the pilot attempted to disconnect the fuel hose from this external cylinder. During this maneuver, liquid gas sprayed onto the corner of the wicker basket. The pilot was unable to reseal the coupling, and the escaping gas ignited upon contact with the burner's pilot light. The basket immediately caught fire.
To prevent the balloon from ascending with a burning basket, the pilot prioritized venting hot air through the parachute valve rather than using a handheld fire extinguisher. As the basket burned, the pilot directed passengers to exit the basket. The balloon briefly ascended to the height of nearby treetops, with the external fuel bottle still dangling from the fuel system, before the pilot successfully landed the craft. Six of the seven occupants sustained injuries, and the balloon was destroyed.
The investigation
SUST examined the wreckage and the fuel system components. The investigation revealed that the fuel hose from the external filling bottle was brittle and damaged. Crucially, the check valve within the hose connection was defective and leaking due to material aging. The investigation also noted that the brass connection thread on the hose was heavily worn.
Because the filling bottle was an external piece of equipment and not a permanent part of the Kubícek BB60Z, it had not been included in the annual aircraft inspection. Furthermore, the investigation found that the valve on the external cylinder had not been closed before the pilot attempted to disconnect the hose.
Findings
- The primary cause of the fire was the improper disconnection of the fuel hose from the balloon's fuel system.
- A defective check valve on the external fuel hose connection allowed liquid gas to escape unexpectedly.
- The pilot did not follow the procedure outlined in the flight manual, which requires closing the fuel cylinder valve and burning off remaining fuel in the line before disconnecting components.
- The external filling bottle was not properly secured to the basket, which caused it to hang dangerously from the fuel system during the unplanned ascent.