https://www.pprune.org/rumours-news/638797-united-b777-engine-failure.html
Given the similarity between the latest B777 P&W engine failure over Denver and this incident in 2018 I was curious to find out more about the earlier engine failure. The report on the examination of the failed fan blade makes for some interesting reading. Apparently after 5000 cycles (take-off and landing) all fan blades must be examined at P&W by thermal acoustic imaging (TAI) in which ultrasound vibrates the fan blade and if any cracks are present frictional heating at the crack can be thermally imaged. Some sort of paint has to be painted on the fan blade surface to allow TAI and this paint can affect the resultant thermal image. All paint is removed after TAI inspection. Anyway, the failed fan blades were inspected in 2010 and 2015 and this is straight from the report.
“The installed set of fan blades, including the fractured fan blade, had undergone two overhauls at which time the blades underwent a thermal acoustic imaging (TAI) inspection. At the initial TAI in 2010, there was a small indication at the location of the origin of the crack. The review of the records from the 2015 TAI show that there was a larger indication in the same area as where there was an indication in 2010 and from where the crack originated. At the time of each TAI, the inspectors attributed the indication to a defect in the paint that was used during the TAI process and allowed the blade to continue the overhaul process and be returned to service”.
So two inspections, five years apart, showed an indication of a growing fatigue crack in the same place and yet the thermal image was attributed to a paint defect in two different paint coatings.