What happened
On June 6, 2009, at approximately 20:15, a Cameron Z425LW hot air balloon was being prepared for a commercial flight near Amersfoort, Netherlands. The crew and ground team were in the process of inflating the balloon with heated air to upright the basket. To secure the balloon during this phase, an anchor line was attached to a vehicle using a carabiner.
As passengers were boarding the basket, the carabiner securing the anchor line bent open, causing the line to detach abruptly. The sudden release caused the basket to slide several meters and tilt. During the movement, the pilot sustained a head injury from the quick-release system's coupling hook, losing consciousness for several seconds. One passenger fell over the edge of the basket, resulting in a broken arm. No damage was sustained by the balloon itself.
The investigation
The investigation examined the weather conditions, the mechanical integrity of the equipment, and the forces acting on the balloon. While the initial weather forecast suggested manageable winds, actual conditions at the time of the incident involved wind speeds between 8 and 12 knots, with gusts reaching up to 18 knots.
Investigators analyzed the tension loads placed on the anchor system. Based on the balloon's surface area and the wind speed, the calculated tension load on the anchor line was estimated to be between 800 kg and 2150 kg. The investigation also focused on the physical state of the carabiner, which was found to be deformed.
Findings
- The primary cause of the incident was the failure of the carabiner, which bent open under tension.
- The carabiner had been used extensively over many previous flights, which had significantly reduced its structural strength.
- The tension loads generated by the wind (estimated up to 2150 kg) likely exceeded the reduced capacity of the worn component, especially considering that the safe working load for such a component—after applying necessary safety factors—should ideally not exceed 500 kg.
Safety action
Following the incident, the balloon owner implemented new safety measures, including the use of heavier-duty carabiners and new anchor lines. Additionally, a new maintenance protocol was established to replace all carabiners every two years.