Smell In Descent
I have never operated an aircraft where we start the APU in descent - in fact we wait until we're about to turn onto stand...
However, the conclusion I've arrived at for the regular occurence of this smell in descent is as follows:
No.1 (and poss No.2 depending on type) bearings are the most likely culprit as they are upstream in the gas-flow from the air bleeds.
These bearings are normally protected from oil loss by two or more seals, essentially a carbon seal and a labyrinth air seal. The labyrinth seal is a number of sharp-edged discs on the rotor, surrounded by an abradable case. When the seal is installed and the engine starts the discs cut minute grooves in the abradable case lining. The result is a labyrinthine route to the gas flow from the space between the carbon seal and the labyrinth seal - hence the name. The purpose of this seal is simply to provide enough restriction to air flow that when a dedicated bleed air source is fed through holes in the rotor shaft into the space between the two seals there is enough back-pressure for the pressure to exceed oil system (scavenge system) pressure. Because of this air leaks inwards through the carbon seal instead of oil leaking out. This is the reason why air needs to be extracted from scavenged oil.
When the labyrinth seal wears over time the back pressure is reduced. This is not normally a problem because there is a large air supply pressure. However, in descent the bleed air pressure is low because the engine is at idle. Again this is not a probem because the fan is generally acting as an airbrake, meaning that a microscopic end-float movement of the rotorshaft rearwards occurs. This has the effect of moving the discs of the labyrinth seal out of alignment with their grooves, thereby reducing the gap and improving back pressure. However, when the aircraft goes below FL100, or (and THIS is the smell in the middle east folks) a higher speed limit point, and speed reduces, so the AIS-based load on the fan reduces, allowing end-float to reduce, and the labyrinth seal discs to move back towards alignment with their grooves. The back pressure consequently collapses, and we get oil leaking past the seal and into the gas flow.
Next time you smell "the oil rigs" in the gulf think again: those flare-offs should not smell of oil, but of smoke, shouldn't they? What you are smelling is simply oil - from your own engines. If you're one of the 3% you should be worried at that point...
Last edited by Chunky Monkey; 8th Feb 2013 at 09:05.