What happened
On September 28, 2008, at approximately 09:45, a privately operated Jodel DH 251 was conducting a cross-country flight from Meaux to Auxerre. While cruising at an altitude of approximately 1,500 feet, the pilot experienced a sudden, loud noise followed by a significant decrease in airspeed and a loss of altitude. The pilot subsequently performed an emergency landing in a field near Passy.
Upon inspection on the ground, it was discovered that both the propeller spinner and the wooden Evra propeller had detached from the engine. These components were recovered approximately 500 meters away from the aircraft's final position. The aircraft sustained damage during the landing.
The investigation
The investigation focused on the mechanical integrity of the propeller assembly. An examination of the propeller mounting bolts revealed that the fasteners had failed due to a fatigue cracking process. The cracks originated from multi-initiation zones located at the final thread of the bolts.
Investigators determined that the first bolt to crack showed characteristics of multi-initiation typical of insufficient tightening. The remaining five bolts exhibited fatigue cracks that had propagated under high stress levels. Hardness testing of all six bolts confirmed that the material properties were within normal specifications.
Further inquiry revealed that in July 2008, the pilot had noted a significant crack in the original metal spinner and sought to replace it at Toulouse Lasbordes aerodrome. During this maintenance, the propeller was removed to install a composite spinner. To compensate for the fact that the new composite spinner lacked one of the two mounting flanges present on the original metal version, a metal spacer was added to the assembly.
Findings
- The investigation established that the improper installation during the spinner replacement was the primary cause of the failure.
- The added metal spacer was of insufficient thickness, which prevented the bolts from being properly tensioned.
- The bolts were tightened to the manufacturer's recommended torque using a torque wrench, but because the assembly was too thin, the bolts reached the end of their thread without providing adequate clamping force to the components.
- The failure occurred approximately 30 flight hours after the modification was performed.