What happened
On March 16, 2018, a Pilatus PC-6/B2H2, registration F-BTCG, departed from Grenoble Isère airport for a technical test flight following maintenance work. The crew, consisting of a pilot and an aeronautical mechanic, was performing a series of maneuvers in a clear area south of the aerodrome.
During a stall maneuver, the pilot discovered that the rudder pedals had become ineffective, although control of the ailerons and elevators remained intact. The pilot notified the aerodrome controller and initiated a precautionary landing. To mitigate unknown risks, the pilot opted to land with the flaps retracted. While attempting to align with the paved runway 09, the pilot struggled to maintain the centerline. At approximately 300 feet, the pilot decided to land on the adjacent unpaved runway 09 Right.
During the flare, at an altitude of roughly one to two meters, the aircraft experienced a sudden roll and a loss of airspeed. The wingtips and propeller struck the ground, causing the aircraft to enter a ground loop. The aircraft eventually came to rest on the edge of the paved runway, sustaining heavy damage to the airframe, including the fuselage, landing gear, and propeller.
The investigation
The investigation focused on the mechanical failure of the rudder control system. Physical examination of the F-BTCG revealed that the lower collar, which supports the rudder horn and the rudder axis, was improperly secured. Specifically, the assembly was held by only four rivets instead of the required eight. Additionally, the upper collar was secured by only six rivets instead of eight, and evidence of an unauthorized pre-drilling mark was found.
Laboratory analysis of the rivets showed that they had failed due to shear stress caused by an overload. The investigation also identified excessive clearance between the rivets and the holes, which likely allowed vibrations to exacerbate the structural instability over time. The investigation could not definitively trace the origin of the rudder axis, though it was likely installed during the aircraft's reconstruction period between 1984 and 1990.
Findings
- The rudder control system failed due to the shear failure of the rivets connecting the rudder horn to the rudder axis.
- The assembly was non-compliant with design specifications, utilizing significantly fewer rivets than required to withstand aerodynamic loads.
- The pilot's high workload and stress, caused by the loss of directional control, led to a focus on maintaining runway alignment at the expense of monitoring airspeed.
- The aircraft entered a stall during the flare, leading to the loss of control and the subsequent ground impact.