water is water absorbed in the fuel and is NOT visible. that appropriate surveillance tests and corrective mea- Free water may be in the form of a cloud, emulsion, droplets, or in gross amounts in the bottom of a tank or container. (Entrained water is free water that has not settled to the bottom.) Any form of free water can cause icing in the aircraft fuel system, malfunctioning of fuel quantity probes, and corrosion of fuel system compo- nents. Salt water will promote corrosion more rapidly than fresh water. Ordinarily, a cloud indicates water-contaminated fuel.   Occasionally,   a   cloud   indicates   excessive amounts of fine sediment or finely dispersed stabilized emulsion. Fuel containing a cloud from either cause is not acceptable. When clean and bright fuel cools, a cloud may form, indicating that dissolved water has precipitated out. This precipitation cloud represents a very slight amount of fresh water. This cloud appears when warm fuel is pumped to a cool area where the sample is taken. Remember, even though this is a very slight amount of water, if the fuel is not clear and bright, it does not go into an aircraft. Sediment Sediment appears as dust, powder, fibrous material, grains, flakes, or stain. Specks or granules of sediment indicate particles in the visible size (approximately 40 microns or larger). See figure 3-1. The presence of any appreciable number of such particles indicates a mal- function of the filter/separators, a source of contamina- tion  downstream  of  the  filter/  separators,  or  an improperly cleaned sample container. Even with the most efficient filter/separators and careful fuel han- dling, an occasional particle may be seen. These strays are usually due to particle migration through the filter media and may present no particular problem to the engine or fuel control. The sediment ordinarily encoun- tered is an extremely fine powder, rouge, or silt. The two principal components of this fine sediment are normally sand and rust. Sediment includes both organic and inorganic mat- ter. The presence of large quantities of fibrous materials (close to naked eye visibility) is usually indicative of filter element breakdown, either because of a ruptured element or mechanical disintegration of a component in the system. Usually, high metal content of relatively large particles suggests a mechanical failure somewhere in the system, which is not necessarily limited to a metallic filter failure. In a clean sample of fuel, sediment should not be visible except upon the most meticulous inspection. Persistent presence of sediment is suspect and requires sures be applied to the fuel-handling system. Sediment or solid contamination can be separated into two categories: 1. Coarse sediment 2. Fine sediment Coarse sediment is sediment that can be seen and that easily settles out of fuel or can be removed by adequate filtration. Ordinarily, particles 10 microns and larger are regarded as coarse sediment. See table 3-1 for more information. Coarse particles clog orifices and wedge in sliding valve clearances and shoulders, caus- ing malfunctions and excessive wear of fuel controls and metering equipment. They also can clog nozzle screens and other fine screens throughout the aircraft fuel system. Fine sediment consists of particles smaller than 10 microns in size. Proper settling, filtration, and centri- fuging can remove 98 percent of the fine sediment in fuel. Particles in this range accumulate throughout fuel controls. They appear as a dark shellac like surface on sliding valves. They also may be centrifuged out in rotating chambers as sludgelike matter that causes slug- gish operation of fuel-metering equipment. Fine parti- cles are not visible to the naked eye as distinct or separate particles. However, they scatter light and may appear as point flashes of light or a slight haze in fuel. Maximum settling time should be allowed in fuel tanks after they are filled to allow reasonable settle- ment  of  water  and  sediment.  This  can  be  done  by proper rotation of the fuels. Microbiological  Growth Microbiological growth consists of living organ- isms that grow at a fuel/water interface. These organ- isms include protozoa, fungus, and bacteria. Fungus is the major constituent and the cause of most problems associated with microbiological contamination of jet fuels. Fungus is a vegetable life; it holds rust and water in suspension and is an effective stabilizing agent for fuel-water emulsion. It clings to glass and metal sur- faces and can cause erroneous readings in fuel quantity systems, sluggish fuel control operation, and sticking flow  dividers.  Microbiological  growth  is  generally found wherever pockets of water exist in fuel tanks. It usually has a brown, black, or gray color and a stringy, fibrouslike appearance. For microorganisms to develop in jet fuels, free water must be present. Traces of metallic elements are also necessary, but water is the key ingredient. Without 3-8