water there is no growth. Remove any free water and growth ceases. Microorganisms in jet fuel can cause severe corro- sion damage to metal aircraft fuel tanks. Organic acids, or other byproducts produced by the growth of fungi or bacteria, react chemically with certain matter contained within the fuel to penetrate tank coatings. Once the coating is penetrated, the metal tank is attacked. Microbiological growth causes fouling of aircraft fuel system filters and erratic operation of fuel-quantity probes. Microbiological contamination is more preva- lent in tropical and semitropical climates because of the more favorable temperature and higher humidity. The presence of microbiological growth in fuel being deliv- ered to an aircraft is a reliable indication of failure of the fuel system cleanup equipment and personnel. The fuel from an aircraft suspected of having microbiological contamination must not be defueled into a clean system. Once a fuel system is contami- nated with microbiological growth, the organisms continue to multiply unless the system is thoroughly cleaned. Emulsions An emulsion is a liquid suspended in other liquids. There are two types of emulsions; water-in-fuel and fuel-in-water (inverse) emulsions. The water-in-fuel emulsion is the most common of emulsions found by fuel handlers. It appears as a light-to-heavy cloud in the fuel. (See the second and third bottles of fuel shown in fig. 3-2.) This type of emulsion may break down and settle to the bottom of the sample container at any time ranging from a few minutes to a week, depending on the nature of the emulsion. Surfactants Surfactant is a contraction of the words surface active agent. A surface active agent is a substance that causes a marked reduction in the interracial tension of liquids. A surfactant in fuel causes the fuel and water to mix more easily and become much harder to separate. Surfactants disperse both water and dirt in fuel and in some cases form very stable emulsions or slimes. The surfactants that appear in jet fuels are usually the sulfonates or naphthenates of sodium. These can be present as naturally occurring materials in the crude oil or as residual refinery treating materials. Refinery proc- essing must be such that it removes all traces of these materials, or poor quality fuel results. Many other materials are also surface active. The list includes common household detergents, clean- ing compounds used to clean fuel storage tanks and earner vehicles, greases used to lubricate valves, and corrosion inhibitors used in petroleum products to re- duce rust in pipelines and tanks. Surfactants in jet fuel can be a major problem. These materials accumulate and concentrate in the coa- lescer elements of filter/separators, destroying the abil- ity of the elements to coalesce and remove water from fuel. Concentrations of less than 1 ppm of a surfactant in jet fuel have been known to cause malfunctioning of coalescer elements. Elements so affected pass free water and suspended particulate matter. Surfactants are also associated with microbiologi- cal slime growths. It is not necessary that surfactants be present for microorganisms to flourish, but they pro- mote luxuriant growth by aiding the mixing and emul- sifying of fuel and water. Microorganisms need free water to multiply and grow. Surfactants help them to get it. The problem with surfactants is that they quite often are not detected in jet fuels until after they have “poisoned” filter/separators, which in turn have al- lowed water and/or slime to be delivered to aircraft. There are laboratory tests for surfactants in fuel, but as yet there are no accurate field tests. However, a surfactant problem can usually be detected by one or more of the following observations: 1. Dark, red-brown, or black water in filter/ sepa- rator sump drains, refueler sump drains, or pipeline low-point drains 2. Excess quantities of dirt and/or free water in the fuel at dispensing points or downstream of filter/sepa- rators 3. Storage tanks not yielding a clear, bright fuel after prescribed settling times 4. Dark or black water and/or slime in drawoffs from storage tank bottoms 5. Triggering of fuel monitors in delivery systems, if installed No two cases of surfactant contamination in fuel systems are exactly alike. However, some general measures can be used to correct and control this type of contamination. Some of these procedures are as follows: 1. Change monitor fuses. 2. Change falter/separator elements and clean out filter/separator cases. 3-10