What happened
On March 21, 2009, a UFM 13 Lambáda, registration OK-NUA, was conducting a flight near Nákří, Czech Republic, to verify secondary radar functionality and monitor engine fuel consumption. The flight was conducted under VFR conditions by a two-person crew, both experienced pilots.
While cruising at approximately 371 meters AGL and a ground speed of 194 km/h, the aircraft experienced a sudden and violent oscillation of the elevator control surfaces. This was immediately followed by the aircraft pitching up into an inverted position. The crew activated the pyrotechnic emergency parachute system, but the aircraft's descent became uncontrollable. The aircraft impacted the ground nose-first, causing the fuselage to break apart behind the wings and the tail section to separate from the main body. Both crew members successfully egressed the aircraft following the impact and sustained no injuries.
The investigation
The ÚZPLN investigation focused on the structural integrity of the empennage and the aerodynamic stability of the control surfaces. The commission examined the wreckage and analyzed manufacturing data for the elevator assembly.
Technical analysis involved reviewing vibration test data and aeroelastic modeling provided by the Czech Technical University (ČVUT). The investigation specifically looked at the mass distribution within the composite elevator. Laboratory analysis of the wreckage revealed that the manufacturing process—specifically the manual application of resin—resulted in an uneven distribution of mass. This caused the center of gravity of the elevator to shift toward the trailing edge, creating a significant static imbalance.
Findings
- The primary cause of the accident was aerodynamic flutter of the elevator, which led to the structural failure of the fuselage.
- The elevator assembly was statically unbalanced, with the balance mass providing only 85.2% of the required static moment, leaving a deficiency of 14.8%.
- Manufacturing imperfections in the composite layup caused an excessive concentration of resin near the trailing edge, shifting the center of gravity.
- The aircraft's configuration at the time of the accident involved a specific modification that lacked flaperons, which differed from the standard type certificate.
- The critical flutter speed for the symmetric mode was calculated to be as low as 163 km/h EAS, which the aircraft had exceeded during the flight.