Question for Monarch 757 Engineers
 

We've recently noticed from ATC fumes / smoke emanating from a vent on the underside of port engine, about a third of the way back from the front of the cowling. Probably on starboard engine as well, but can't see it from the tower. Occurs on taxy to stand after landing, and has also been seen on take off run. Only noticeable when backlit by low evening sun, so quantity of fumes is probably very small. Been to inspect the vent with pilot; the vent area seemed slightly wet, but no real clue as to origin of fumes. Pilot's response - 'Dunno, I just sit up front and drive it'. Any ideas to put our minds at rest ?

Most likely generator breather vent,nothing to worry about.

Higher life engines do tend to 'breathe' a bit. Not necessarily a cause for concern unless it results in high oil consumption or cabin air contamination, either of which the airlines are very aware of.

Is there such a thing as a generator breather? Most likely the main engine centrifugal breather. This seperates any air from the engine oil before returning to the oil tank. At low engine speeds the breather (which is driven from the accessory gearbox) does not turn at a high enough speed to completely seperate the oil from the air, hence the air carries a fine oily smoke which is dumped overboard through this vent. The 737-200 engines used to do it alot as does the v2500 on the Airbuses. All in all quite normal and nothing to worry about at all.

It's the breather outlet for the accessory gearbox. Perfectly normal on all RR engines.

As stated above a trademark of RR engines, the Trent 500 as fitted to the A340-600 breathes with the best of them, you can see that smoking away under any lighting conditions.