In addition, an engine must be capable of sustained operation when the fuel is cooled to its most critical condition for icing. 75 cubic centimeter of free water per gallon of fuel at 80 degrees Fahrenheit. Turbine-powered aircraft must be capable of sustained operation with 0.Each fuel system must be arranged so that no pump can draw fuel from more than one tank at a time, or provisions must be made to prevent air from being drawn into the fuel supply line.Each fuel system must be constructed and arranged to ensure fuel flow at a rate and pressure established for proper engine and auxiliary power unit functioning under all likely operating conditions.To prevent the small amount of vapor that remains in the fuel from upsetting its metering action, vapor eliminators are installed in some fuel systems ahead of the metering device or are built into this unit. The vapor moves upward through the fuel in the tank and out the tank vents. The boost pump also releases vapor from the fuel as it passes through the pump. ![]() The pressure on the fuel reduces vapor formation and aids in moving a vapor pocket along. These booster pumps, which are used widely in most modern aircraft, keep the fuel in the lines to the engine-driven pump under pressure. The major improvement in reducing vapor lock, however, is the incorporation of booster pumps in the fuel system. In addition, the volatility of the fuel is controlled in manufacture so that it does not vaporize too readily. To reduce the possibility of vapor lock, fuel lines are kept away from sources of heat also, sharp bends and steep rises are avoided. Even small amounts of vapor in the inlet line restrict the flow to the engine-driven pump and reduce its output pressure. Vapor lock can become serious enough to block the fuel flow completely and stop the engine. Turbulence in the fuel pump often combines with the low pressure at the pump inlet to form a vapor lock at this point. As this mixture passes through the lines, the trapped air separates from the fuel and forms vapor pockets at any point where there are abrupt changes in direction or steep rises. Sloshing in the tank tends to mix air with the fuel. The chief causes of fuel turbulence are sloshing of the fuel in the tanks, the mechanical action of the engine-driven pump, and sharp bends or rises in the fuel lines. If the fuel is warm enough at takeoff, it retains enough heat to boil easily at high altitude. As the aircraft climbs, the outside temperature drops, but the fuel does not lose temperature rapidly. This is most apt to occur during a rapid climb on a hot day. High fuel temperatures often combine with low pressure to increase vapor formation. This tendency is increased if the fuel in the tank is warm. Transfer of heat from the engine tends to cause boiling of the fuel in the lines and the pump. Electronic controls have become quite common with gas turbines and have increased the capabilities of the fuel system, making it less complicated for the technician and decreasing maintenance problems. ![]() This signal sent to the metering device determines the correct amount of fuel needed by the engine. The computer determines the amount of fuel needed and then sends a signal to the metering device. Many engines use an interactive system that senses engine parameters and feeds the information to the onboard computer (electronic engine control). ![]() Fuel cost and availability have also become factors in providing engines with fuel systems that are efficient and very precise in scheduling fuel flow to the engine. By the use of electronic sensors and computer logic built in to electronic controls, the engines can be controlled with much more accuracy. Improvements in electronics and the use of digital computers have enabled the aircraft and engines to be electronically interfaced together. Gas turbine fuel controls have also greatly improved the ability to schedule (meter) the fuel correctly during all flight regimes. Engine fuel systems have become very accurate at providing the correct mixture of fuel and air to the engines. Electronic engine controls have allowed great increases in controlling the metered fuel flow to the engine.
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