What happened
On 23 July 2004, a tethered balloon known as the HiFlyer was conducting an ascent in Lucerne, Switzerland, carrying a group of 24 passengers. During the flight, at an altitude of approximately 40 meters, the pilot identified wind speeds of 18 knots and attempted to initiate a descent. However, the balloon encountered sudden, strong gusts that caused significant lateral movement along the winch cable.
As the balloon drifted, the gondola tilted and struck nearby trees and buildings. This movement caused the winch cable to strike the internal structure of the HiFlyer gondola, severing the octagonal frame and tearing away a floor panel. During the ensuing instability, one passenger fell through the opening in the floor, resulting in one fatality. At least 10 people sustained various levels of injury, with some injuries classified as serious.
The investigation
The investigation examined the operational procedures, weather monitoring, and technical integrity of the HiFlyer system. Investigators reviewed the meteorological data, noting that a wind warning from MeteoSchweiz had been received via fax at the operations center but had not been communicated to the flight crew. Additionally, while a storm warning light was active at a nearby location, the crew failed to recognize it.
Technical inspections of the winch cable and gondola structure were conducted. The investigation also looked into the organization of the operator, noting that some team members had not received training from the manufacturer and that previous ascents had occasionally been performed under overloaded conditions or in prohibited wind speeds.
Findings
- The primary cause of the accident was the crew's decision to proceed with two additional trips despite critically high wind speeds and an overloaded gondola.
- The ascent was conducted with insufficient free lift, which failed to maintain the necessary tension and stability of the cable.
- A failure in internal communication meant the crew was unaware of the official wind warning sent to the operations center.
- The physical layout of the platform near buildings created a high risk of contact during lateral drift.
- The gondola's structural integrity was compromised by the impact of the winch cable, leading to the floor panel failure.