Microorganisms Solid Particle Contaminants Surfactants 14-52 Figure 14-95.  Clay filter elements remove surfactants. They are  used in the fuel dispensing system before fuel enters the aircraft. Surfactants are liquid chemical contaminants that naturally  occur in fuels. They can also be introduced during the refining  or handling processes. These surface-active agents usually  appear as tan to dark brown liquid when they are present in  large quantities. They may even have a soapy consistency.  Surfactants in small quantities are unavoidable and pose  little threat to fuel system functioning. Larger quantities of  surfactants do pose problems. In particular, they reduce the  The maximum amount of solid particle contamination  allowable is much less in turbine engine fuel systems than in  reciprocating-engine fuel systems. It is particularly important  to regularly replace filter elements and investigate any unusual  solid particles that collect therein. The discovery of significant  metal particles in a filter could be a sign of a failing component  upstream of the filter. A laboratory analysis is possible to  determine the nature and possible source of solid contaminants. Preventing solid contaminant introduction into the fuel is  critical. Whenever the fuel system is open, care must be  taken to keep out foreign matter. Lines should be capped  immediately. Fuel tank caps should not be left open for  any longer than required to refuel the tanks. Clean the area  adjacent to wherever the system is opened before it is opened.  Coarse sediments are those visible to the naked eye. Should  they pass beyond system filters, they can clog in fuel  metering device orifices, sliding valves, and fuel nozzles.  Fine sediments cannot actually be seen as individual particles.  They may be detected as a haze in the fuel or they may refract  light when examining the fuel. Their presence in fuel controls  and metering devices is indicated by dark shellac-like marks  on sliding surfaces. Solid particles that do not dissolve in the fuel are common  contaminants. Dirt, rust, dust, metal particles, and just  about anything that can find its way into an open fuel tank  is of concern. Filter elements are designed to trap these  contaminants and some fall into the sump to be drained off.  Pieces of debris from the inside of the fuel system may also  accumulate, such as broken-off sealant, or pieces of filter  elements, corrosion, etc. The presence of microorganisms in turbine engine fuels is  a critical problem. There are hundreds of varieties of these  life forms that live in free water at the junction of the water  and fuel in a fuel tank. They form a visible slime that is dark  brown, grey, red, or black in color. This microbial growth can  These filters, if renewed at the proper intervals, remove most  surfactants through adhesion. Surfactants discovered in the  aircraft systems should be traced to the fuel supply source  and the use and condition of these filters.  [Figure 14-95] Surfactants are usually in the fuel when it is introduced into  the aircraft. Discovery of either excessive quantities of dirt  and water making their way through the system or a sudsy  residue in filters and sumps may indicate their presence. The  source of fuel should be investigated and avoided if found to  contain a high level of these chemicals. As mentioned, slow  settling rates of solids and water into sumps is a key indicator  that surfactant levels are high in the fuel. Most quality fuel  providers have clay filter elements on their fuel dispensing  trucks and in their fixed storage and dispensing systems. Strainers and filters are designed with upward flow exits  to have water collect at the bottom of the fuel bowl to be  drained off. This should not be overlooked. Entrained water  in small quantities that makes it to the engine usually poses  no problem. Large amounts of water can disrupt engine  operation. Settled water in tanks can cause corrosion. This can  be magnified by microorganisms that live in the fuel/water  interface. High quantities of water in the fuel can also cause  discrepancies in fuel quantity probe indications. surface tension between water and the fuel and tend to cause  water and even small particles in the fuel to remain suspended  rather than settling into the sumps. Surfactants also tend to  collect in filter elements making them less effective. 