What happened
On June 21, 2008, at approximately 12:38 local time, a single-seat Schleicher ASW 15B glider, registration D-0906, was performing a winch-less aero-tow from the Rubbiano airfield in the province of Parma. The glider was being towed by a Husky aircraft, registration I-BALZ.
While flying near the northern slope of Monte Cassio at an altitude of approximately 3,500 feet, the towing aircraft suddenly entered a steep dive of more and more than 90 degrees. The pilot of the towing aircraft attempted to release the tow cable and recovered the aircraft's attitude. Upon looking back, the pilot observed the glider flying erratically with severe longitudinal oscillations, causing the wingtips to flex upward. The glider subsequently plummeted to the ground, having lost its left wing in flight. The pilot attempted to egress the cockpit and deploy a personal parachute, but the impact occurred before the canopy could fully deploy, resulting in one fatality.
The investigation
The ANSV investigation focused on the structural integrity of the glider and the mechanics of the tow cable. Investigators examined the wreckage, noting that the left wing's main spar had failed. The investigation also reviewed the tow cable, which was identified as a commercial-grade rope used for marine purposes rather than a certified aviation tow cable. This cable lacked a predetermined breaking junction and possessed a breaking strength significantly higher than required.
Findings
- The primary cause of the accident was the structural failure of the left wing due to exceeding the maximum allowable load factor.
- The tow cable snapped because it was subjected to forces exceeding its breaking strength.
- The investigation concluded that the pilot of the glider likely performed uncoordinated control inputs during a period of high stress, which, combined with the sudden tension changes following the cable break, caused the wing spar to fail.
- The use of a non-certified, high-strength marine cable contributed to the severity of the event, as the cable's strength was approximately three times the difference between the glider's mass and the maximum allowable towing force.
Safety action
- The ANSV recommended that the Italian Civil Aviation Authority and the Aero Club d'Italia increase awareness regarding adherence to flight manuals concerning towing procedures and cable specifications.
- A recommendation was made to evaluate the implementation of specific regulations governing the use of breaking junctions on glider tow cables.