From the Flight Global article, it appears the GE and PW design recirculates some of the heated fuel back to the upstream face of the FOHE, thus preheating the new cold fuel approaching the upstream FOHE face (thus melting any water ice before reaching the FOHE). The Trent FOHE does not appear to have this warmed fuel recirculation feature, thus the fuel reaching the upstream face of the FOHE is colder than on the GE or PW designs
It appears at least AAIB is onto something here and has done extensive testing to identify the problem with the water ice ahead of the FOHE. So now the solution will have to be some sort of a redesign by RR and of course tested and accepted by the correct authorities.
The question goes back to did GE and PW have previous knowledge of problems with ice, just returned the warmer fuel due to convenience of a place to return the fuel, or dumb luck?
Then again I would also wonder if other RR engines are plumbed in the exact manner of engines in question on the T7 and just not exposed to the long periods cold soaked conditions connected to the T7.
VFD