reply to post by Gmoneycricket
That is a case of the airplane engine and systems themselves being responsible for the air contamination.
It is explained in the article in a way even laypersons can understand it. This is a mechanical defect that can occur occasionally.
Air for the passenger cabin is tapped off from the engine compressor section ('bleed air" it is called). Depending on the set-up (airplane type,
engine type) the point of the bleed air (the "compressor stage") where it is tapped will vary.
The air at this point is generally "clean", as it is simply ambient air that is compressed.....it also gets hot, when compressed. Typically there
are two points in the compression cycle where air is tapped.....a Low Pressure point, ans then farther back (prior to combustion) a High Pressure
point. There are various uses for those two sources of bleed air.....but, for the purposes of pressurization, the Low Pressure bleed is used.
This air goes through an air-to-air heat exchanger, then is routed to a turbine that is part of the Air Cycle Machine (ACM) or commonly called a
"pack". The ACM handles this air through an expansion turbine, where it is chilled down to just above 32° F. Warmer air is routed to modulate
this temperature, and also used to mix into a manifold for distribution into the air conditioning duct work. These two supplies -- very cold, and
moderately warm (about 140° F, typically) are mixed to provide the desired temperature for introduction into the cabin.
Once in the cabin, there are additional fans on mot modern jets that recirculate a portion of this air tat has been introduced, to further "blend"
the temperatures to a comfortable level.
Depending on the jet, the entire volume of cabin air is replenished every several minutes. Since air is constantly being pumped in (again, this is
just ambient outside air), the internal pressure has to be regulated by a valve (or valves) that modulate the outflow....they are called,
thus....Outflow Valves. This is how the cabin air is replenished every few minutes.
Now.....the cases when lubricants or other fluids are "leaked" into the compressor section air supply are due to leaking seals that allow tiny
amounts to bypass normal channels, and enter the air supply. Remember it is hot air, and so these lubricants (and rarely, even hydraulic fluid) will
have an effect, due to the heating...even in small quantities. But again, these contaminants are coming from
the airplane's own
systems, and again are very rare occurrences.
Proper maintenance procedures will result in it not happening at all....bht, machines are machines of course, and can have failures now and then.