Aircraft Systems

the length of fuel line from the tank to the drain. If water or Another condition of undissolved water is free water that other contaminants are found in the first sample, drain further may be introduced as a result of refueling or the settling of samples until no trace appears. entrained water that collects at the bottom of a fuel tank. Free water is usually present in easily detected quantities at the Water may also remain in the fuel tanks after the drainage bottom of the tank, separated by a continuous interface from from the fuel strainer has ceased to show any trace of water. the fuel above. Free water can be drained from a fuel tank This residual water can be removed only by draining the fuel through the sump drains, which are provided for that purpose. tank sump drains. Free water, frozen on the bottom of reservoirs, such as the fuel tanks and fuel filter, may render water drains useless Water is the principal fuel contaminant. Suspended water and can later melt releasing the water into the system thereby droplets in the fuel can be identified by a cloudy appearance causing engine malfunction or stoppage. If such a condition of the fuel, or by the clear separation of water from the colored is detected, the aircraft may be placed in a warm hangar to fuel, which occurs after the water has settled to the bottom reestablish proper draining of these reservoirs, and all sumps of the tank. As a safety measure, the fuel sumps should be and drains should be activated and checked prior to flight. drained before every flight during the preflight inspection. Entrained water (i.e., water in solution with petroleum fuels) Fuel tanks should be filled after each flight or after the last constitutes a relatively small part of the total potential water flight of the day to prevent moisture condensation within the in a particular system, the quantity dissolved being dependent tank. To prevent fuel contamination, avoid refueling from on fuel temperature and the existing pressure and the water cans and drums. volubility characteristics of the fuel. Entrained water freezes in mid fuel and tends to stay in suspension longer since the In remote areas or in emergency situations, there may be no specific gravity of ice is approximately the same as that of alternative to refueling from sources with inadequate anti- AVGAS. contamination systems. While a chamois skin and funnel may be the only possible means of filtering fuel, using Water in suspension may freeze and form ice crystals of them is hazardous. Remember, the use of a chamois does sufficient size such that fuel screens, strainers, and filters not always ensure decontaminated fuel. Worn-out chamois may be blocked. Some of this water may be cooled further as do not filter water; neither will a new, clean chamois that is the fuel enters carburetor air passages and causes carburetor already water-wet or damp. Most imitation chamois skins metering component icing, when conditions are not otherwise do not filter water. conducive to this form of icing. Fuel System Icing Prevention Procedures Ice formation in the aircraft fuel system results from the The use of anti-icing additives for some aircraft has been presence of water in the fuel system. This water may be approved as a means of preventing problems with water undissolved or dissolved. One condition of undissolved and ice in AVGAS. Some laboratory and flight testing water is entrained water that consists of minute water indicates that the use of hexylene glycol, certain methanol particles suspended in the fuel. This may occur as a result of derivatives, and ethylene glycol mononethyl ether (EGME) mechanical agitation of free water or conversion of dissolved in small concentrations inhibit fuel system icing. These tests water through temperature reduction. Entrained water settles indicate that the use of EGME at a maximum 0.15 percent out in time under static conditions and may or may not be by volume concentration substantially inhibits fuel system drained during normal servicing, depending on the rate at icing under most operating conditions. The concentration which it is converted to free water. In general, it is not likely of additives in the fuel is critical. Marked deterioration in that all entrained water can ever be separated from fuel under additive effectiveness may result from too little or too much field conditions. The settling rate depends on a series of additive. Pilots should recognize that anti-icing additives are factors including temperature, quiescence, and droplet size. in no way a substitute or replacement for carburetor heat. Aircraft operating instructions involving the use of carburetor The droplet size varies depending upon the mechanics heat should be adhered to at all times when operating under of formation. Usually, the particles are so small as to be atmospheric conditions conducive to icing. invisible to the naked eye, but in extreme cases, can cause slight haziness in the fuel. Water in solution cannot be removed except by dehydration or by converting it through temperature reduction to entrained, then to free water. 7-28