What happened
On July 7, 2009, a Jodel D 140 C Mousquetaire, registered F-BMBS, was performing a flight from Château-Arnoux to Romans. Approximately twenty minutes after takeoff, while cruising near the Tarsimoure mountains at an altitude of roughly 6,000 feet, the pilot experienced a violent noise. The aircraft immediately entered a right-hand spin, completing one to two rotations.
Upon regaining control, the pilot discovered that the propeller had detached from the engine and that the right wing had sustained damage. The pilot declared an emergency and successfully performed an emergency landing in a field near Valdriment. There were no injuries to the pilot, though the aircraft sustained significant damage.
The investigation
The investigation focused on the mechanical failure of the propeller assembly. The aircraft was equipped with a two-blade wooden propeller, which was not recovered from the crash site. Upon examining the propeller flange, investigators found that one of the six retaining bolts (tirants) was missing entirely. Furthermore, the remaining five bolts were improperly tightened, with only three to four millimeters of thread visible inside the clamping sleeves, whereas the manufacturer's specifications required them to protrude two millimeters outside the sleeves.
Because the missing bolt was not recovered, the investigators could not verify if its specific characteristics or tightening levels met the manufacturer's requirements. The pilot noted that during the pre-flight inspection, he had manually rotated the propeller and performed several compressions but did not specifically inspect the tightness of the propeller mounting.
Maintenance records showed that the propeller mounting had been checked during the 50-hour inspection on March 27, 2009. Since that inspection, the aircraft had flown 31 hours and 55 minutes. The next 100-hour inspection was scheduled for July 26, 2009.
Findings
- The primary cause of the accident was the failure to detect improper tightening of the propeller mounting bolts.
- The propeller flange was missing one retaining bolt, and the remaining five bolts were insufficiently tightened.
- Variations in climate can affect the texture and thickness of wooden propellers, potentially creating play in the mounting bolts; in-flight vibrations can then lead to loosening and subsequent bolt failure.
- There were discrepancies in maintenance manuals regarding the frequency of tightness checks. While some manuals suggested checks every 100 hours or six months, the propeller manufacturer updated their manual in July 2009 to recommend inspections every 50 hours or during significant climatic changes.