What happened
On July 31, 2000, a Nimbus 4DT glider, registration G-9/29, was engaged in a long-distance private flight near Ocaña, Spain. The aircraft, carrying two occupants, was flying at approximately 2,600 meters when it entered a strong thermal. During this maneuver, the aircraft experienced a loss of lift, leading to an uncontrolled state characterized by a steep dive and a developing spin or spiral dive.
As the aircraft accelerated rapidly, the pilot attempted to recover by applying full back pressure and aileron input. This maneuver caused the airspeed to exceed the maximum design speed (VNE). The resulting aerodynamic loads caused the structural failure of both wings. The pilot in the front seat successfully exited the aircraft via parachute and survived. However, the occupant in the rear seat was unable to deploy their parachute effectively because the lines became entangled with the aircraft wreckage, leading to a fatal impact with the ground.
The investigation
The CIAIAC examined the wreckage and the flight sequence to determine the cause of the structural failure. Investigators analyzed the fracture patterns on the wings, noting clean fiber breaks on the lower wing surfaces (tension) and delamination of the sandwich structures on the upper surfaces (compression).
Technical analysis, including finite element methods conducted by NIAR, confirmed that the aircraft's structural integrity was sound and that there were no manufacturing defects or material failures. The investigation focused on the aerodynamic loads generated during the recovery attempt. The investigation also noted that the aircraft was operating at its maximum weight and that the pilot's ability to respond may have been affected by fatigue and slight hypoxia due to the duration and altitude of the flight.
Findings
- The aircraft entered a loss of control due to a combination of low airspeed (near minimum sink speed) and excessive bank angle while maneuvering in a turbulent thermal.
- The crew failed to immediately correct the loss of lift using established procedures, allowing the aircraft to enter a high-speed dive.
- The structural failure of the wings was caused by aerodynamic loads exceeding design limits during the abrupt recovery maneuver.
- The pilot in the rear seat likely faced significant difficulty exiting the cockpit due to the aircraft's orientation and inertial forces, which contributed to the entanglement of the parachute and the subsequent fatality.