Turbine Engine Fuel System—General  Requirements Main Line Strainers Fuel Quantity Gauges Fuel Pressure Gauge Pressure Warning Signal Water Injection Systems for Reciprocating Engines Drain water and sediment from the main line strainer at  each preflight inspection. Remove and clean the screen at  the periods specified in the airplane maintenance manual.  Examine the sediment removed from the housing. Particles of  rubber are often early warnings of hose deterioration. Check  for leaks and damaged gaskets. If a sight gauge is used, be sure that the glass is clear and that  there are no leaks at the connections. Check the lines leading to  it for leaks and security of attachment. Check the mechanical  gauges for free movement of the float arm and for proper  synchronization of the pointer with the position of the float. On the electrical and electronic gauges, be sure that both the  indicator and the tank units are securely mounted and that  their electrical connections are tight. Check the pointer for zero tolerance and excessive oscillation.  Check the cover glass for looseness and for proper range  markings. Check the lines and connections for leaks. Be sure  that there is no obstruction in the vent. Replace the instrument  if it is defective. Inspect the entire installation for security of mounting  and condition of the electrical, fuel, and air connections.  Check the lamp by pressing the test switch to see that it  lights. Check the operation by turning the battery switch on,  building up pressure with the booster pump, and observing  the pressure at which the light goes out. If necessary, adjust  the contact mechanism. These systems have very limited use in modern aircraft  engines. Water injection was used mostly on large radial  engines. The water injection system enabled more power  to be obtained from the engine at takeoff than is possible  without water injection. The carburetor (operating at high  power settings) delivers more fuel to the engine than it  actually needs. A leaner mixture would produce more  power; however, the additional fuel is necessary to prevent  overheating and detonation. With the injection of the  antidetonant fluid, the mixture can be leaned out to that  which produces maximum power, and the vaporization of  the water-alcohol mixture then provides the cooling formerly  supplied by the excess fuel. Turbofan, turbojet, turboshaft, and turboprop engines are  equipped with a fuel control unit which automatically  satisfies the requirements of the engine. Although the basic  requirements apply generally to all gas turbine engines, the  way in which individual fuel controls meet these needs cannot  be conveniently generalized.  The fuel system must deliver fuel to the combustion chambers  not only in the right quantity, but also in the right condition  for satisfactory combustion. The fuel nozzles form part of  the fuel system and atomize or vaporize the fuel so that it  ignites and burns efficiently. The fuel system must also  supply fuel so that the engine can be easily started on the  ground and in the air. This means that the fuel must be  injected into the combustion chambers in a combustible  condition during engine starting, and that combustion must  be sustained while the engine is accelerating to its normal  idling speed. Another critical condition to which the fuel  system must respond occurs during a rapid acceleration.  When the engine is accelerated, energy must be furnished to  the turbine in excess of that necessary to maintain a constant  rpm. However, if the fuel flow increases too rapidly, an over  rich mixture can be produced, with the possibility of a rich  blowout or compressor stall. The quantity of fuel supplied must be adjusted automatically  to correct for changes in ambient temperature or pressure.  If the quantity of fuel becomes excessive in relation to mass  airflow through the engine, the limiting temperature of the  turbine blades can be exceeded, or it will produce compressor  stall and a condition referred to as rich blowout. Rich blowout  occurs when the amount of oxygen in the air supply is  insufficient to support combustion and when the mixture is  cooled below the combustion temperature by the excess fuel.  The other extreme, lean flame-out, occurs if the fuel quantity  is reduced proportionally below the air quantity. The engine  must operate through acceleration and deceleration without  any fuel-control-related problems.  The fuel system is one of the more complex aspects of the gas  turbine engine. It must be possible to increase or decrease the  power at will to obtain the thrust required for any operating  condition. In turbine-powered aircraft, this control is provided  by varying the flow of fuel to the combustion chambers.  However, some turboprop aircraft also use variable-pitch  propellers; thus, the selection of thrust is shared by two  controllable variables, fuel flow and propeller blade angle. 2-34