multiply rapidly and can cause interference with the proper  functioning of filter elements and fuel quantity indicators.  Moreover, the slimy water/microbe layer in contact with the  fuel tank surface provides a medium for electrolytic corrosion  of the tank.  [Figure 14-96] Since the microbes live in free water and feed on fuel, the  most powerful remedy for their presence is to keep water from  accumulating in the fuel. Fuel 100 percent free of water is not  practicable. By following best practices for sump draining  and filter changes, combined with care of fuel stock tanks  used to refuel aircraft, much of the potential for water to  accumulate in the aircraft fuel tanks can be mitigated. The  addition of biocides to the fuel when refueling also helps by  killing organisms that are present.  reassembled and the tanks have been filled with the correct  fuel, a full engine run-up check should be performed before  releasing the aircraft for flight.  Contaminated fuel caused by the introduction of small  quantities of the wrong type of fuel into an aircraft may  not look any different when visually inspected, making a  dangerous situation more dangerous. Any person recognizing  that this error has occurred must ground the aircraft. The lives  of the aircraft occupants are at stake. Aircraft engines operate effectively only with the proper  fuel. Contamination of an aircraft’s fuel with fuel not  intended for use in that particular aircraft can have disastrous  consequences. It is the responsibility of all aviators to put  forth effort continuously to ensure that only the fuel designed  for the operation of the aircraft’s engine(s) is put into the fuel  tanks. Each fuel tank receptacle or fuel cap area is clearly  marked to indicate which fuel is required.  [Figure 14-97] Visual inspection of fuel should always reveal a clean, bright  looking liquid. Fuel should not be opaque, which could be  a sign of contamination and demands further investigation.  As mentioned, the technician must always be aware of the  fuel’s appearance, as well as when and from what sources  refueling has taken place. Any suspicion of contamination  must be investigated. In addition to the detection methods mentioned for each type  of contamination above, various field and laboratory tests  can be performed on aircraft fuel to expose contamination.  A common field test for water contamination is performed  by adding a dye that dissolves in water but not fuel to a test  sample drawn from the fuel tank. The more water present in  the fuel, the greater the dye disperses and colors the sample.  If the wrong fuel is put into an aircraft, the situation must be  rectified before flight. If discovered before the fuel pump is  operated and an engine is started, drain all improperly filled  tanks. Flush out the tanks and fuel lines with the correct fuel  and then refill the tanks with the proper fuel. However, if  discovered after an engine has been started or attempted to be  started, the procedure is more in depth. The entire fuel system,  including all fuel lines, components, metering device(s) and  tanks, must be drained and flushed. If the engines have been  operated, a compression test should be accomplished, and  the combustion chamber and pistons should be borescope  inspected. Engine oil should be drained, and all screens  and filters examined for any evidence of damage. Once  Another common test kit commercially available contains a  grey chemical powder that changes color to pink or purple  when the contents of a fuel sample contains more than 30  parts per million (ppm) of water. A 15-ppm test is available  for turbine engine fuel.  [Figure 14-98]  These levels of  water are considered generally unacceptable and not safe for  operation of the aircraft. If levels are discovered above these  amounts, time for the water to settle out of the fuel should  be given or the aircraft should be defueled and refueled with  acceptable fuel. Foreign Fuel Contamination Detection of Contaminants 14-53 Figure 14-96.  This fuel-water sample has microbial growth at the interface of the two liquids. Bacterial sludge Biomass Water Air Fuel