What happened
On May 18, 1999, a Technoflug Piccolo, registration HB-2179, was performing a recreational flight near Uetliberg, Switzerland. The pilot was executing a series of maneuvers in thermal conditions, described by the pilot and witnesses as a "sporty" flight style. While pulling up into a wingover maneuver at approximately 150 km//h, the left wing failed just outside the strut attachment point. The aircraft entered a violent, spin-like descent from an altitude of at least 200 meters, eventually crashing into a wooded area. Despite the destruction of the aircraft, the pilot sustained only minor injuries and was able to exit the wreckage independently.
The investigation
The investigation was conducted by the Swiss Transportation Safety Investigation Board (SUST) in collaboration with the Zurich City Police and experts from EMPA (the Swiss Federal Laboratories for Materials Science and Technology). Investigators examined the wreckage and analyzed the structural integrity of the wing components. The investigation focused on the connection between the strut and the spar, looking for evidence of manufacturing defects, prior damage, or mechanical overload. Material analysis revealed several irregularities in the wing's construction, including deviations from the original design specifications and the presence of a thick, brittle filler layer between the spar and the wing skin.
Findings
Technical analysis determined that the accident was caused by a structural failure of the left wing due to an overload. The primary cause was the failure of the connection between the strut (plywood insert/fabric layer) and the upper spar cap. This failure was triggered by a combination of a momentary mechanical overload during the maneuver and pre-existing structural weaknesses.
Several contributing factors were identified:
- A structural discontinuity at the strut-spar junction created significant stress concentrations in bending, shear, and torsion.
- Evidence of prior structural fatigue, including cracks in the paint on the lower wing surface and existing cracks at the strut-spar connection.
- Potential degradation of the laminate properties due to moisture ingress through the aileron hinges and wing skin cracks.
- Manufacturing irregularities, including air inclusions in the laminate and deviations in fiber volume and dimensions compared to the design blueprints.
- The presence of a brittle microballoon-based filler layer that contributed to structural weakness rather than strength.
Safety action
Following the investigation, a safety recommendation was issued for the Federal Office of Civil Aviation (FOCA) to mandate inspections of the strut attachment points (specifically the glue quality of the filler block) for all aircraft of the Technofflug Piccolo type.