Partial Detachment of the Right Elevator During Descent After a Parachute Drop

Incident Overview and Circumstances On Saturday, July 23, 2011, at 16:30 local time, a Pilatus Porter PC6-B2H2 registered F-GOAG, operated by Centre Ecole Régional de Parachutisme Sportif, departed from Saumur Saint Florent aerodrome (49) for its 13th parachute drop of the day. The aircraft carried six parachutists. After the drop at 1,200 meters, the pilot initiated descent with the elevator trim compensator in the normal nose-up position and reduced power. The pilot detected abnormal elevator behavior, requiring significant control forces to actuate it. During landing, the pilot reported needing very high nose-up force to round out. On the parking area, the pilot observed that the right elevator had partially detached from the horizontal stabilizer. The pilot stated that no anomalies were detected during the pre-flight inspection or during the previous 12 drops. ## Maintenance History and Technical Findings Maintenance records showed the aircraft underwent an annual inspection in March 2011, during which both left and right elevators were removed. Two months later, a 100-hour inspection was performed, including a visual check of the lockwire in accordance with manufacturer maintenance documentation. No anomalies were found. The incident occurred approximately 70 hours after this inspection. All components of the assembly were recovered and examined in a laboratory. The right elevator is connected to the horizontal stabilizer at two points: (A) at its inner end, to the torsion tube transmitting motion, via a plate and two fixing screws; and (B) at two-thirds of its length toward the outer end, via a ball bearing and a fixing screw. The right elevator detached from the horizontal stabilizer at connection B. It remained connected at connection A through the trim control linkage. Damage observed at connection A (sudden fracture and deformation of the plate) was a consequence of the detachment at connection B. ## Root Cause Analysis Observations at connection B showed that the detachment was due to the loss of the fixing screw. This loss resulted from progressive and complete loosening of the screw. Laboratory examinations revealed that the nylon ring of the Nylstop nut was worn and no longer provided its self-locking function. The lockwire anchor tab on the structure was free to rotate. Vibrations allowed the screw to turn and loosen. The screw subjected the lockwire to bending, causing progressive fatigue fracture. The screw completely loosened and was expelled. The Nylstop nut was originally installed on the elevator and was neither accessible nor visible from the outside. This nut requires a self-locking torque measurement after each removal of the axle to determine whether it can be reused or must be replaced. ## Conclusion and Follow-up Actions The incident was caused by the loss of the fixing screw connecting the elevator to the horizontal stabilizer. This loss resulted from progressive and complete loosening of the screw and the fracture of the lockwire. Contributing factors included the Nylstop nut failing to properly perform its braking function and the lockwire attachment tab being able to rotate around the rivet axis. Following the incident, the aircraft manufacturer issued Service Bulletin No. 55-001 dated September 12, 2011, to improve inspection of the assembly during periodic checks and to modify maintenance documentation to reinforce rules and controls related to the use of self-locking nuts.