What happened
The aircraft sustained minor damage due to an uncontained failure of the left engine. During the event, the engine's fan spinner became completely detached from the assembly.
The investigation
An examination of the aircraft revealed that the left engine's fan spinner was missing and the forward fan blade retainer was missing a 90-degree section of the bolt flange. Several pieces of the fan spinner were found inside the cowl.
Investigators analyzed the fan blade forward retaining ring and identified features indicating an overstress failure. This failure originated on the forward face near a linear impression. The location and length of this impression matched a corner on one of the bolt lugs on the aft side of the spinner, which showed evidence of fretting damage.
While the inside radius between the retaining ring's raised lip and the flange met manufacturer specifications, the spinner's bolt lugs featured a sharp outside corner where they interfaced with the retaining ring's radius. Although the part's forging drawing did not specify a radius, a manufacturer representative noted that engineering drawings required an edge break with a radius between 0.005 and 0.020 inches. This mismatch between the retaining ring radius and the sharp spinner edge caused interference during assembly.
Further examination by the manufacturer identified high cycle fatigue on both the spinner and the fan blade retaining ring. Additionally, three-dimensional finite element analysis of these components predicted a reduced low cycle fatigue life.
Findings
- The combination of the radius on the retaining ring and the sharp edge on the aft side of the spinner caused interference during assembly.
- High cycle fatigue was present on both the spinner and the fan blade retaining ring.