What happened
On May 16, 2017, a Robin DR400-180R (registration F-GLKA) departed from Vinon-sur-Verdon aerodrome, bound for Château-Arnoux Saint-Auban. While climbing to approximately 3,500 feet at a speed of 150 km/h, the pilot experienced intense vibrations accompanied by a loud bang and observed an object flying past the left side of the aircraft.
Reacting to the emergency, the pilot immediately reduced engine power to idle and cut both magnetos. Believing a propeller blade had separated, the pilot elected not to restart the engine and instead focused on an emergency descent. After announcing the engine failure and intentions to land on runway 16, the pilot shut off the battery and fuel valve. The aircraft landed safely at the aerodrome with no injuries and only damage to the airframe.
The investigation
The investigation focused on the DUC H-FLR2_5-D-I_C "FLAIR-2" propeller, which had accumulated 80 hours of use since its installation in March 201SB. Post-accident examination of the aircraft revealed that one blade had separated at the hub level, while a second blade had fractured mid-span, likely due to impact with the first detached blade.
Laboratory analysis of the aluminum alloy hub revealed that the assembly of the two half-hubs had failed. Investigators identified two distinct fatigue cracks. The first crack originated on the inner side of the rear half-hub at a transition radius. A second fatigue crack was found on the front half-hub near the joint between the two halves.
Crucially, during dye penetrant testing of the hub, investigators discovered an additional fatigue crack in the blade retention zone of a different blade on the same propeller. Micrographic examination of this third crack revealed a machining defect: the transition radius contained two radii instead of the single radius specified in the design blueprints. This defect created a stress concentration point.
Findings
- The separation of the propeller blade during climb was caused by the fatigue failure of the hub at the blade foot retention area.
- A machining error resulted in an incorrect radius geometry, which increased mechanical stress in a highly loaded area.
- While the specific fragments of the failed hub were not recovered to confirm the exact origin of the primary failure, the presence of a similar machining defect on another blade suggests it was the likely cause of the fatigue crack propagation that led to the separation.