What happened
On December 16, 2006, a Grob G1-03C Twin III Acro glider, registration I-IVVP, was performing an aerobatic training flight near Castel Giorgio, Terni. The aircraft had taken off from the Alfina airfield in Castel Viscardo with two occupants on board.
While executing a sequence of maneuvers, including a Cuban eight and a roll, the aircraft experienced a catastrophic structural failure. The pilot reported that during the maneuver, a noise was heard in the cockpit, suggesting an internal object had shifted. Shortly thereafter, the right wing detached from the fuselage, followed immediately by the detachment of the left wing. The fuselage plummeted in a near-vertical trajectory, impacting an agricultural field approximately 1 km south of the airfield. The two occupants successfully deployed their parachutes and landed without injury. The aircraft was destroyed.
The investigation
The ANSV examined the wreckage, which consisted of the fuselage broken into three sections and both wings found approximately 500 meters away from the main fuselage. Investigators analyzed the wing attachment system, which utilizes a combination of steel pins, interlocking spar extensions, and connecting tubes to create a rigid connection between the wings and the fuselage.
Technical analysis of the wreckage revealed that the rear connecting tube had suffered a ductile fracture due to combined tension and bending loads. Examination of the right wing root showed significant damage, specifically at the point where the spar extension's steel pin is anchored. The investigation also reviewed the aircraft's maintenance history, noting two previous heavy landings that had required structural repairs, though no evidence was found to link these past events to the failure.
Findings
- The primary cause of the accident was the exceeding of permitted maneuver limits during flight.
- The structural failure sequence began with the failure of the attachment support for the steel pin at the end of the right wing spar.
- This initial failure weakened the structural integrity of the entire wing-to-fuselage connection, leading to the subsequent detachment of both wings.
- The loss of the wing-to-fuselage connection caused the rear connecting tube to fail under extreme stress, resulting in the total separation of the wings from the fuselage.