Hi all, havent been on here in a long time but here's my 2d worth; the APU in Airbuses drains it's oil into an external tank (it has no sump) as part of the shut-down sequence so when you press the Master switch to OFF, you are supposed to wait two minutes and then shut off the batteries, which allowed the APU to start cooling down and get the oil out to the tank, yet leaving a small amount inside the engine to lube it for the next start-up. This shut down sequence also includes venting the case of the engine itself, equivalent to a crankcase breather in a car engine. The start-up sequence, including opening the inlet flap, gets the oil back from the tank to the engine, so that the oil system is pressurised with oil by the time combustion starts. When the internal seals in the engine start to wear out, oil gets misted into the duct from APU to cabin and we, as engs, would get complaints from CCM and pilots. When we would investigate the duct, we sometimes found oil residue immediately inside the duct, at the joint nearest the APU, but not much further in than that.
I have looked into those ducts many times and there would be no evidence of liquid contamination beyond a few inches into the duct. That's not to say that airborne mist did not go further along the duct but that's what I saw. The APUs were generally very reliable and it was often accessories like the oil cooler or the starter motor that gave trouble. The cooler is probably more often a source of oil getting sucked in than the core engine itself. The APU bay in an Airbus is generally very clean as the APU did not shed oil like the APU of a 146. Their APU bay was always filthy. Why? because the APU had to be vented by the eng by pushing a vent button on top of the engine, otherwise when he opened the tiny oil filler cap, the pressurised case would blow a cup full of oil upwards, giving him a coat of oil and a generous coat all over the APU bay. So, SOP was to vent the APU, wait for a few minutes and then check the oil level, which had to be very accurately filled, as any excess would be vented out by the engine when it started. That was on every turnaround with a 146. There were constant complaints about fumes in the 146 and we spent an inordinate amount of time cleaning the APU bay and cleaning the engine itself. The bay was always dirty. Changing over to the A320 was a pleasure as the aircraft told you when it needed APU oil and the bay was always clean, compared to a 146.