Schempp-Hirth Nimbus-4 - Accident History

Accident History

U.S. National Transportation Safety Board (NTSB) investigators queried the German Luftfahrt-Bundesamt (LBA), Germany's equivalent of the U.S. Federal Aviation Administration (FAA) regarding the accident history of the Nimbus-4DM in conjunction with a 1999 accident near Minden, Nevada where both occupants of the aircraft were killed. In this accident, the glider broke up in flight during the recovery phase after a departure from controlled flight while maneuvering in thermal lift conditions. Airborne witnesses in other gliders who saw the beginning of the accident sequence said the glider was in a tight turn, as if climbing in a thermal, when it entered a spiral. With a 45-degree nose-down attitude, the speed quickly built up as the glider completed two full rotations. The rotation then stopped, the flight stabilized on a northeasterly heading, and the nose pitched further down to a near-vertical attitude (this is consistent with the spin recovery technique specified in the Aircraft Flight Manual (AFM)). The glider was observed to level its attitude, with the wings bending upward and the wing tips coning higher, when the outboard wing tip panels departed from the glider, the wings disintegrated, and the fuselage dove into the ground. Several witnesses estimated that the wing deflection reached 45 degrees or more before the wings failed. Examination of the wreckage disclosed that the left and right outboard wing sections failed symmetrically at two locations.

In this case, the NTSB determined "that the probable cause of this accident was the pilot's excessive use of the elevator control during recovery from an inadvertently entered spin and/or spiral dive during which the glider exceeded the maximum permissible speed, which resulted in the overload failure of the wings at loadings beyond the structure's ultimate design loads."

There were then only three previous accidents worldwide on file with the Federal Bureau of Aircraft Accidents Investigation (BFU), Germany's equivalent of the NTSB. The first was a non-injury long landing accident in Fayence, France, on September 4, 1994. The second involved a collision with the ground during takeoff in Fuentemilanos, Spain, on July 27, 1997, which resulted in two fatalities. The improper installation of the horizontal stabilizer led to the third accident in Lüsse, Germany, on June 13, 1999, in which two occupants were injured during an attempted takeoff when the stabilizer separated from the empennage just after liftoff.

Regarding other service/accident experience with the Nimbus-4 family of gliders, the BFU has recorded four incidents/accidents with the single-seat versions. Three events are known of non-injury accidents during off-field landings, and one fatal accident was due to collision with a mountain.

Additionally, during training for the World Gliding Championships in New Zealand in 1995, a Nimbus-4 (owned by the French Air Force) was destroyed in a midair breakup accident. The glider entered a wave cloud, lost control, and broke up at a speed beyond 400 km/h (The never exceed speed, or "Vne" is 285 km/h, and the design dive speed, or "Vd", is 324 km/h). The pilot survived by bailing out.

According to the LBA, "As far as we know, none of the incidents/accidents recorded indicated a technical failure."

NTSB investigators became aware of another accident involving a Nimbus-4DM that occurred in Spain shortly after the Minden, Nevada, accident. According to the Comisión de Investigación de Accidentes e Incidentes de Aviación Civil, Spain's equivalent of the NTSB, the glider broke up in flight following a high-speed excursion beyond Vne. According to preliminary information supplied by the Spanish authorities, the pilot stated they were in a turn when a heavy thermal caused the glider to enter a steep descending spiral. The pilot could not recover the aircraft from the spiral and the airspeed quickly exceeded Vne. The pilot then reported that the right wing failed and he bailed out.