The use of Bleed Air causes air conditioning systems on board of aircraft to get increasingly contaminated over time, in particular by oil-containing particles (Volatile Organic Compounds or VOC's). These particles will easily stick onto the inside of the airconditioning ducts. In a Fume Event, the concentration of these particles increases significantly, and the system contamination quickly increases to unacceptable levels. On the inside of the airconditioning duct a sticky, greasy layer will be deposited. If after a Fume Event this residue is not thoroughly cleaned before the aircraft is returned to service, this will cause even further contamination of the air flowing over it into the cabin. As a result, chances of recurring fume events and physical complaints in passengers and crew will increase drastically.
Manufacturer's maintenance instruction...
This problem of residual contamination has also been known by the aircraft manufacturers for years. And although they are not too keen to admit this in public, they have published maintenance instructions to prevent it. One example is this In-Service Information brochure, where Airbus prescribes the entire air conditioning system to be thoroughly cleaned internally after a Fume Event. This is a time consuming, and therefore costly procedure. For an Airbus A320, this will easily take 2-3 days of work in a hangar, for an A380 probably a multiple of that!
...are not followed up
Unfortunately it appears that in the majority of Fume Event reports (a.o. on Aviation Herald), the aircraft involved is returned to service within a few hours. That implies that, undoubtedly for reasons of cost saving, the airline does not apply the manufacturer's prescribed procedures. Many airlines apply a so-called "Burn Out" procedure instead. The airconditioning system is then run at maximum temperatures for a while to "burn it clean" with hot Bleed Air. This is however far from effective, as the greasy residue which is "burned out" of the first, hottest parts of the airco system, will be redeposited further down the ducts in the cooler parts. Therefore this will only cause the contamination to be relocated and even worsened in the cooler parts. No wonder then that on the internet lists are circulating with the registrations of aircraft that are involved time and again in fume events, with all possible consequences of continued poisoning of passengers and crew!