What happened
On 30 September 2017, an Air France Airbus A380-861, registered F-HPJE, was operating a scheduled flight from Paris to Los Angeles. While the crew was transitioning to a new flight level while cruising over Greenland, an explosion occurred in the right outer engine (engine No 4). The crew immediately experienced asymmetric thrust and severe vibrations. The aircraft's electronic monitoring system displayed "ENG 4 STALL" followed by "ENG 4 FAIL" messages. The flight crew subsequently diverted the aircraft to Goose Bay, Canada, where they landed safely.
Post-flight inspections revealed that the engine's fan, the air inlet, and the fan case had separated from the engine during flight. This uncontained failure caused slight damage to the surrounding aircraft structure.
The investigation
The investigation involved a complex search operation to locate debris scattered across the Greenland ice sheet. After initial aerial surveys, investigators utilized synthetic aperture radar and ground-penetkiating radar to locate fragments. In July 2019, a fragment of the fan hub was successfully recovered from the ice.
Laboratory analysis of the recovered hub fragment revealed that a crack had originated in the subsurface of a slot bottom. The investigation focused on the metallurgical properties of the titanium alloy used in the component and the manufacturing processes involved in creating large engine parts.
Findings
- The failure was caused by the progression of a crack originating in a subsurface micro-texture region, also known as a macro-zone, within the titanium alloy (Ti-6-4) fan hub.
- The cold dwell fatigue phenomenon allowed a crack to initiate at stress levels lower than previously understood and at a significantly lower number of cycles.
- The macro-zone in the part was found to be ten times larger and more intense than the average observed by the manufacturer.
- There is currently no reliable non-destructive method to detect these macro-zones during the production or in-service inspection phases.
- The risk of such macro-zones appearing increases with the size of the engine components, as larger engines require larger titanium forgings which may not undergo the same level of deformation-induced reduction of these zones during manufacturing.