Hydraulic fluid in the cabin air?
One of the causes of Fume Events or smell incidents is contamination of the cabin air by hydraulic fluid. The fluid contains several toxic substances, a.o. the highly toxic tributyl phosphate (TBP), used mainly as a flame retardant. When it is strongly heated, pyrolysis will produce many other, often toxic chemical substances.
In 2013, in Germany a research was done commissioned by government agency BG Verkehr. In this study, urine samples were taken from over 300 crew members, within hours after a Fume Event. In these samples, significantly higher levels of a.o. TBP were found, as compared to a control group representative for the whole population. (see p. 7 of this so-called "Schindler study"). One had no clue however, as to how this hydraulic oil could have entered into the cabin air...
Maintenance instructions by Airbus
Manufacturer Airbus apparently have known for years how this can occur. In 2013, Airbus published in their magazine Fast edition 52 an extensive article about measures to prevent air conditioning duct contamination by engine oil and hydraulic fluid. In this article, Airbus explicitly warns to carefully follow their published maintenance procedures. After a hydraulic fluid or engine oil leakage, the leaked fluid should be thoroughly removed first, and the cause of the leakage corrected. After that the complete air conditioning system should be thoroughly cleaned. If this procedure is not carried out properly or not completely, this will give "rise to recurring complaints of bad smells in the cabin", according Airbus.
Main Landing Gear
Photo on the left shows a main gear strut of an A320. Note the visible proof of leakage which has occurred during compression and extension of the gear. During flight, the gear is retracted into the so-called wheel well, which is closed at the bottom with hydraulically operated gear doors. Therefore the hydraulic fluid has leaked further down onto this gear door during flight.
Photo on the right show the corresponding gear door, which is still clearly wet by the hydraulic fluid. But that is not all. As the fluid flowed out, the air flow has taken it along the bottom of the plane, all the way to the back...
APU air inlet
... straight to the APU air inlet, as the photo on the left shows. The inlet was thus strongly polluted. The APU, or Auxiliary Power Unit, is a small turbine which supplies electrical power and.... air for the air conditioning during ground operations. After landing this APU was started as usual, and used to supply air for the air conditioning, A serious Fume Event was the immediate result...
A320 extra vulnerable to oil contamination
The Airbus 320-series is remarkably often involved in Fume Events. One of the causes might be the location of its APU air inlet. On the A320 (as well as other Airbus models, such as A330 and A340), the APU air inlet is located at the bottom of the fuselage, under the tail. That implies it is located directly behind the wheel wells, the space where the gears are stowed during flight. Besides the landing gear however, these wheel wells house several hydraulic components as well, such as pumps and oil reservoirs. The bottom is normally closed by the gear doors. These are opened hydraulically only briefly, to allow retraction or extension of the landing gear. When during flight hydraulic fluid leaks from the system, e.g. due to overfilling or a leaking gear strut, this will end up on the wheel doors, and subsequently flow out of the wheel well. The air flow will then force it along the bottom of the fuselage, all the way to the tail. On an A320 (or A330/340) however, the APU air inlet is directly in the path of the fluid and is therefore prone for contamination. When the APU is then started after landing and used for air supply to the air conditioning, a good gust of toxic vapours will enter the cabin together with the "fresh" air. And so, a Fume Event will be the result, with all its nasty consquences...
(sources: www.p-coc.com, http://avherald.com)