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During completion of routine fuel sampling, before the first flight of the day, maintenance personnel reported finding a red/ brown liquid present in the fuel sample removed from the Boeing 737 aircraft. An inspection of the aircraft's fuel system was carried out with no blockage of filters or other safety of flight issues related to the contamination found.

Testing of the fuel recovered indicated no fungal (cladisporium resinae) species. However, there was a heavy load of bacteria (pseudomonas) present in the water layer, with a related film between the water and fuel layers. The contamination was not believed to be aircraft created but fuel source related. Operators held meetings with fuel company representatives, CASA, and the ATSB attempting to determine the origin of the contamination. Fuel company representatives contacted their refineries but none reported abnormal water drains. Mapping of the fuel supplied to the aircraft reported as contaminated was completed by the operator with no common fuel upload origin point defined.

Research identified several past similar events in Australia. One such event in 1962 was experienced on Boeing 707 aircraft and was believed related to sulphonates in the fuel combining with trace levels of transition metals (including iron). Similar events, reported in 1996 and 1997, were believed to have been caused by the reaction of a complex of naphthenic/ sulphonic acids with transition metals (including iron).

Despite intensive investigation of this event, the source of the contamination could not be established and sporadic reports of contaminated fuel samples persist. This contamination was not related to the aviation gasoline (AVGAS) contamination reported in December 1999, as analysis confirmed no presence of Ethylene Diamine.

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