What happened
On October 2, 2001, an Eurocopter AS 350 B2 was conducting a flight test at the Münsingen military training area. The mission involved testing a new recovery system designed to safely bring gliders to the ground in emergency situations. For this specific test, a Rolladen-Schneider LS3-a glider was attached to the helicopter via an external sling load system.
The flight began with the helicopter lifting the glider to a target altitude of 800 meters. Shortly after takeoff, the right wing attachment line of the glider detached, followed by the left wing line approximately one minute later. This caused the glider to undergo heavy rolling and yawing motions. While the test director initially instructed the pilot to continue climbing, the glider eventually reached a horizontal position where the forward attachment point (a Tost E72 coupling) unexpectedly released.
As the glider fell, the remaining connection at the tail failed. The sudden release caused the elastic sling load webbing to snap upward and wrap around the helicopter's rotor mast. This entanglement rendered the aircraft uncontrollable, leading to a vertical impact. The helicopter was destroyed, and the pilot was killed.
The investigation
The BFU examined the flight data, telemetry, and video recordings of the accident. The investigation focused on the mechanical failure of the Tost coupling and the structural integrity of the sling load rigging. Investigators also reviewed the procedures used by the test team and the regulatory oversight of the specialized equipment used for the experiment.
Findings
- The primary cause of the accident was the unintentional release of the glider's forward attachment coupling during the climb.
- The subsequent loss of control was caused by the sling load rigging entangling the helicopter's rotor mast.
- The sling load assembly was found to be fundamentally unsuitable for the test: the wing attachments were prone to slipping out of their hooks, and the use of a single webbing strap created a risk of the entire assembly being pulled through the shackle.
- A lack of regulatory oversight contributed to the accident, as the sling load rigging was a custom-made component not subject to aviation certification or mandatory safety inspections.
- The test team and the operator were unable to adequately assess the risks associated with the complex technical setup.