flightwatch
Doing a bit of digging around, the problem lies in the HPC 'birdmouth' locating points on the casing in some engines they can wear and drop slightly this can cause wear on the blades and as a result the airflow and subsequently the compressor stall margin is compromised.
I believe RR can monitor for this in the trend monitoring thats downloaded during flight.
Our engines are around the 30k mark for both the non Trent and the Trent cored engines.