What happened
On August 31, 2003, during an airshow at the La Côte Aerodrome in Prangins, a Colt AS-105 MKII airship, registration HB-QBW, experienced an unexpected ascent during its landing phase. As the pilot approached the landing site, two ground assistants ran toward the aircraft to stabilize it for passenger disembarkation. While the aircraft was on the ground but not yet fully secured, a sudden, strong lateral gust of wind caught the envelope, causing the airship to lift off without the burners being activated.
One assistant lost his grip on the tether, while the second assistant remained attached to a maneuvering line. The airship ascended to an altitude of approximately 20 meters and began drifting westward, moving toward high-voltage power lines and railway tracks. As the aircraft drifted over a farm, the assistant clinging to the rope eventually lost his footing and fell into a walnut tree, which broke his fall before he hit the ground. The pilot was eventually able to regain control and land the aircraft safely.
The investigation
SUST examined the circumstances surrounding the landing and the actions of the crew. The investigation established that the pilot was experienced in both hot air balloon and airship operations. The ground crew consisted of the pilot, his wife, and two young brothers who had assisted in approximately 30 flights.
Investigators noted that the aircraft was properly maintained and that the pilot had followed instructions from the airfield information service (AFIS) regarding the landing location. The investigation also reviewed the technical specifications of the maneuvering lines, noting that the two lines were of different lengths (11 meters and 17 meters) due to previous adjustments by the operator.
Findings
- The primary cause of the accident was that the assistant failed to release the rope during the airship's involuntary maneuver.
- A sudden lateral wind gust caused the airship to lift and drift unexpectedly.
- There was a lack of coordination among the ground crew during the landing, as the assistants were not fully prepared to secure the aircraft at the moment the gust occurred.
- The risks associated with ground crew members attempting to hold onto lines during takeoff or landing maneuvers had not been properly assessed by the crew.
- The assistant's decision to remain attached to the rope was likely influenced by a previous experience where a similar gust had occurred without resulting in a significant ascent.
- The assistant sustained severe head trauma, a fractured pelvis, and fractured wrists, though no permanent disabilities were expected.
Safety action
- The investigation noted that the manufacturer's flight manual does not explicitly address the specific risks faced by ground crew during maneuvers or the use of maneuvering lines.
- The crew had not fully measured the consequences of attempting to restrain the aircraft while being physically attached to its lines.