French investigators have traced the serious engine failure involving an Air France Airbus A380 over Greenland to a phenomenon known as ‘cold dwell’ fatigue, which had caused a failure in a fan hub slot which houses the root of the fan blade.

The analysis by investigation authority BEA closes a particularly difficult inquiry which had involved a technically-challenging search for fan-hub components from the Engine Alliance GP7200 powerplant, buried in deep snow in remote Greenland, parts which were only recovered 21 months after the 30 September 2017 accident.

Before the retrieval of these crucial components, Engine Alliance had considered that – based on its in-service experience – tool damage from incorrect maintenance was the most likely scenario behind the failure. //

Titanium alloys are vulnerable to creep – a stress deformation – at relatively low temperatures, and the phenomenon has previously been linked to premature fan disk fractures in the 1970s. Dwell fatigue arises from maintained stresses during flight.

Several factors affect the sensitivity of titanium alloys to cold dwell fatigue, says the inquiry, but particular alloys such as Ti-6-4, owing to their chemical composition, were considered as having “little or no sensitivity” to the phenomenon at the time of the GP7200’s certification.

“This was in part due to the significant number of service hours logged by Ti-6-4 components without any incidents,” says the inquiry.

“Although there had been in-depth research into this phenomenon with respect to certain alloys, the mechanisms at the origin of the initiation of a cold dwell fatigue crack were still not completely understood.”