In some cases, aircraft must be defueled and then refueled before flight operations can proceed. When a fuel is found to be contaminated, the contaminant must be tracked back to its source and the cause corrected. Until the cause of the contamination is found and corrected, the contaminated system cannot be used. The fuel system may be a mobile refueler, air station hydrant refueling system, or the entire fuels system of an aircraft carrier. Contaminated fuel may affect the operation of one aircraft or the operation of an entire air wing. For these reasons, be careful in every phase of fuel handling to prevent contaminants from entering the fuel. CAUSES OF FUEL CONTAMINATION How can you find out the causes of fuel contamination? How can you find out how much contamination is too much? Before you can determine amounts of contamination, you have to be able to understand the units of measurement used to identify contamination. The two major units for measuring the size of contaminants are microns for solids and parts per million (ppm) for water. There are approximately 25,400 microns in 1 inch. Figure 3-1 gives you 2 microscopic view of a human hair, which is about 100 microns in diameter, and compares it with a 5-micron contaminant. Parts per million is the reference used for water contamination. If you take a 32-oz sample bottle and fill it 3 1/4 inches from the bottom, the amount you will have is about 500 milliliters (ml). Break that 500 ml down into one million little pieces. You now have 1 ppm. As you now realize, the equipment used in the quality surveillance laboratory has to be very accurate to make  measurements  that  small. Operation of the lab equipment will be covered later in this chapter. HUMAN FACTOR Equipment now in use can remove most of the contamination that may be present in a fuel. It cannot separate two mixed or blended fuels. It cannot effectively reduce the contamination below the required limits if the contaminant level is too high. YOU must be careful to prevent the introduction of contamination in all phases of fuel handling. Additionally, all steps of contamination removal MUST be properly performed. Inspection and sampling procedures are the only means to ensure that the equipment is performing properly. Unless the equipment is properly operated and the sampling procedures are carefully followed, the problem will always remain. Thus, the most important factor in preventing and removing contamination in fuels is the awareness of the people who handle the fuel. Figure 3-1.—Enlargement of small particles and comparison to a human hair. The equipment is only a machine. You, the ABF, the educated operator, make a quality surveillance program work. LIMITS OF CONTAMINATION To be acceptable for delivery to aircraft, jet fuels must be clean and bright. They must not contain more than 5 ppm free water or 2 mg/liter particulate contamination. The terms clean and bright have no relation to the natural color of the fuel. Jet fuels are not dyed and they vary from clear, water-white to straw-yellow colored. Clean means the absence of any cloud, emulsion, visible sediment, or free water. Bright, means the fuel has a shiny, sparkling appearance. A cloud, haze, specks of particulate matter, or entrained water indicate that the fuel is unsuitable and point to a probable breakdown in fuel handling equipment or procedures. If contamination limits are exceeded, delivery of fuel to aircraft shall be stopped and corrective measures completed before resuming fueling operations. CAUSES OF CONTAMINATION Steps should be taken to find the source of trouble and corrective measures taken immediately. See figure 3-2 for the various types of contamination that may be detected visually. The first sample of fuel in this illustration is an acceptable fuel. Water Water in fuels may be either fresh or salt and may be present either as dissolved or free water. Dissolved 3-6