Above is a photo of the Rolls-Royce Trent XWB highlighting the aft facing outlet in question.
In a video for the A350 you can see condensation coming out of it. Since I haven't noticed such protrusion on other jet engines before, it got me thinking, can it be a pressure relief outlet to allow higher fan RPM without choking the burner?
In other words, the engine would bite off more than it can burn to gain higher bypass thrust, and yet still be able to vent the extra air when not needed, at idle power for example.
Or is it just an oil drain for the accessory gearbox? That wouldn't explain the condensation though. Educated guesses are welcomed. I couldn't find anything on it.