What happened
On 25 October 2013, a Fokker 27-500, registered I-MLVT, was performing a postal cargo flight from Paris-Charles de Gaulle Airport to Dole-Jura Airport. During the initial climb, at an altitude of approximately 1,300 ft, the crew heard an explosion originating from the cargo area. Simultaneously, the left engine fire warning light and associated audio alert activated.
Upon inspection, the crew observed that the fire had extinguished but noted that a portion of the left engine was missing. Although the crew attempted to implement engine fire procedures, they found the propeller feathering control was stuck. The aircraft was able to maintain flight, allowing the crew to execute an emergency turn-back and land safely at Paris-Charles de Gaulle without further incident. The aircraft sustained severe damage.
The investigation
The investigation focused on the structural failure of the left engine and propeller. Investigators found that the forward section of the left engine, including the first compressor stage, was missing. The propeller had lost two blades; one blade (No. 1) remained attached via the de-icing cable, while another (No. 2) had separated and was found 600 m away. The separation of blade No. 2 caused it to strike blade No. 1, leading to the subsequent loss of both.
Technical analysis of the pre-load bolt for blade No. 2 revealed a fatigue failure in the fillet area. Scanning electron microscopy identified fatigue striations and a sudden ductile fracture. The bolt material, S99 steel, contained a manganese sulphide inclusion, which may have acted as a stress concentrator.
Investigators also examined maintenance records. The last overhaul of the left propeller occurred in June 2009. However, the maintenance workshop had destroyed the detailed records in 2012 in accordance with existing EASA regulations. Consequently, the investigators could not verify the specific bearing pre-loading values applied during that overhaul.
Findings
- The primary cause of the accident was the fatigue failure of the pre-load bolt on propeller blade No. 2.
- The separation of blade No. 2 caused a secondary impact with blade No. 1, creating a massive imbalance that tore the forward section of the engine from the nacelle.
- Contributing factors included the potential for insufficient bearing pre-loading, which would increase the loads on the bolt, and the presence of a non-metallic manganese sulphide inclusion in the bolt material.
- The loss of maintenance traceability prevented investigators from determining if the bearing pre-loading was within specification at the time of the last overhaul.