OK, someone has to ask.....what is the 'coin tap test on the fuel-oil cooler?'?
I think only Trents are specified for this because being 3 spool, it's such an amazingly efficient engine the fuel flows are lower so the danger of stagnant fuel icing up is greater!
I'm not sure this adds up. The "amazingly efficient" Trent with more spools, and more bearings, probably has more heat generated in the oil, and thus any "stagnating" fuel will be exposed to more BTU transfer, leading to LESS probability of icing in the fuel-oil heat exchanger.
Often the most severe design point for the cooler is early in descent, when lube heat rejection is still high, yet fuel flow (the heat sink) is very low.
