What happened
On August 3, 1980, a Colt 105 A hot air balloon, registration HB-BBP, was conducting a training flight near Punt Muragl, Switzerland. Following a period of flight, the instructor attempted to land on a narrow, two-meter-wide embankment of the Flazbach river. During the landing process, the trailing rope was not deployed, and the aircraft settled roughly on the embankment.
In an attempt to stabilize the balloon, the instructor tried to open the vent valve to release hot air. However, a section of the valve control line, which had been improperly extended, detached from the assembly. This caused confusion among the occupants, and the basket was subsequently released into the rising waters of the river. The current swept the basket toward a 60 kV high-voltage power line spanning the river.
As the balloon's envelope made contact with the lowest power cable, an electrical arc occurred near the burner. The student pilot and two passengers managed to exit the basket and reach the shore, though one passenger suffered an electric shock while crossing the water. The instructor remained in the basket and was thrown into the river by the electrical discharge. His body was recovered months later, in April 1981.
The investigation
The investigation by the Swiss Transportation Safety Investigation Board (SUST) focused on the mechanical failures and the landing decision. Investigators examined the condition of the HB-BBP and found that the aircraft was not airworthy at the time of the accident. Specifically, the investigation revealed that the valve control line had been extended using a simple, unsecured knot, which failed under tension.
Furthermore, the investigation uncovered unauthorized repairs to the basket. Following a previous incident where the basket had struck a crane, aluminum plates had been screwed into the floor to cover damage. These plates created sharp edges that had previously damaged a structural steel cable. The investigation also noted that the instructor had only had 4.5 hours of rest prior to the flight.
Findings
- The primary cause of the accident was the selection of an unsuitable landing site.
- The failure of the improperly extended valve line contributed to the loss of control over the basket.
- The contact with high-voltage power lines caused the electrical arc and the subsequent ejection of the instructor.
- The failure to deploy the trailing rope during landing contributed to the unstable landing on the embankment.
- The aircraft was not in an airworthy condition due to improper repairs and uncertified modifications.