What happened
On September 21, 2018, a commercial hot air balloon, a Balóny Kubícek BB85Z with registration HB-QOU, departed from Luzern, Switzerland. The flight, carrying 12 passengers and the pilot, was initially conducted under favorable conditions. However, as the flight progressed toward Biberbrugg, the pilot encountered a shift in wind direction and increased speeds.
As the aircraft descended, it was caught in a localized flow of cold air moving through the valley between Schindellegi and Feusisberg. This accelerated air current pushed the balloon toward the north, making the pilot's original landing targets unavailable. Facing a critical shortage of fuel—with only enough gas remaining to avoid crossing Lake Zurich—the pilot had to choose between attempting a landing near high-voltage power lines or executing an immediate landing in a smaller, more constrained area. To prioritize passenger safety, the pilot opted to land near a forest edge at Birrenstrasse, despite the high risk of damage to the balloon's envelope.
The investigation
The investigation focused on the atmospheric conditions and the pilot's decision-making process. Investigators examined the regional meteorological patterns, specifically the drainage of cold air that accumulates in the valleys of Einsiedeln and Sihlsee overnight and flows toward Schindellegi in the morning. The inquiry also reviewed the aircraft's fuel status and the physical impact of the landing on the balloon's structure and the passengers.
Findings
- The primary cause of the incident was the unforeseen capture of the balloon by accelerated regional cold air drainage, which altered the flight path and limited landing options.
- The pilot's decision to prioritize a safer landing site over the preservation of equipment prevented a potentially more dangerous encounter with power lines.
- The landing was significantly harder than usual, resulting in multiple tears to the balloon envelope.
- While there were no fatalities, three passengers reported physical symptoms, including headaches and muscle pain, following the impact.