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Original by Bernd Sieker from Germany (Trent XWB) [CC BY-SA 2.0], via Wikimedia Commons

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.

  • $\begingroup$ google "nacelle drain mast" or "cowling drain mast" $\endgroup$ Jan 26, 2017 at 12:12

1 Answer 1


That is the drain mast and breather outlet combined. There's some discussion about this in airliners.net (not the same engine, but I sure they are similar):

The exhaust from the air/oil separator goes out here. Usually hot air, but when the engine is running at idle you can see smoke coming out. This is engine oil that has got past the seals. The smoke stops as the engine speeds up, the air pressure increases, and the seals work properly.

Also in the main body of the mast are four holes. These are tell tale drains from various parts of the gearbox. They drain out the cavity between the accessories (starter, fuel pump, hyd pump etc.) and the gearbox.

You can see the drain in the cutaway of Trent 500 here. Here's a closeup of the part in question:

Trent XWB drain

Trent XWB drain; image from a380.boards.net; credited to Part of the Trent XWB by JKKW, on Flickr


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