Figure 14-21.  A central pressure refueling station on a transport  category aircraft allows all fuel tanks to be filled from one position. Figure 14-22  shows the fuel system diagram for a DC-10.  Dedicated transfer boost pumps move fuel into a transfer  manifold. Opening the fuel valve on one of the tanks transfers  the fuel into that tank. The transfer manifold and boost  pumps are also used to jettison fuel overboard by opening the  proper dump valves with a transfer boost pump(s) operating.  Additionally, the transfer system can function to supply the  engines if the normal engine fuel feed malfunctions. The fuel feed subsystem is sometimes considered part of the  fuel distribution system. It is the heart of the fuel system since  it delivers fuel to the engines. Jet transport aircraft supply  fuel to the engines via in-tank fuel boost pumps, usually two  per tank. They pump fuel under pressure through a shutoff  valve for each engine. A manifold or connecting tubing  typically allows any tank to supply any engine through the  use of crossfeed valves. Boost pump bypasses allow fuel  flow should a pump fail. Note that the engines are designed  to be able to run without any fuel boost pumps operating.  But, each engine’s shutoff valve must be open to allow flow  to the engines from the tanks.  Most jet transport fuel feed systems, or the engine fuel  systems, have some means for heating the fuel usually  through an exchange with hot air or hot oil taken from the  engine.  Figure 14-23  shows the fuel cooled oil cooler (FCOC)  on a Rolls Royce RB211 engine, which not only heats the  fuel but also cools the engine oil. Fuel indicating systems on jet transport aircraft monitor a  variety of parameters, some not normally found on general  aviation aircraft. Business jet aircraft share many of these  features. True fuel flow indicators for each engine are used  as the primary means for monitoring fuel delivery to the  engines. A fuel temperature gauge is common as are fuel  A slightly more complex system for a light turbine-powered  helicopter is shown in  Figure 14-25 . Two in-tank electric boost  pumps send fuel through a shutoff valve rather than a selector  valve, since there is only one fuel tank. It flows through an  airframe filter to an engine filter and then to the engine-driven  fuel pump. The fuel tank is vented and contains an electrically  operated sump drain valve. A pressure gauge is used to monitor  boost pump output pressure and differential pressure switches  warn of fuel filter restrictions. Fuel quantity is derived through  the use of two in-tank fuel probes with transmitters.  Fundamentally, helicopter fuel systems differ little from those  on fixed-wing aircraft. Gravity-feed systems have vented  fuel tanks with an outlet strainer and shutoff valve. Fuel  flows from the tank through a main filter to the carburetor.  [Figure 14-24]  Typically, a helicopter has only one or two fuel tanks located  near the center of gravity (CG) of the aircraft, which is near  the main rotor mast. Thus, the tank, or tanks, are usually  located in or near the aft fuselage. Some helicopter fuel tanks  are mounted above the engine allowing for gravity fuel feed.  Others use fuel pumps and pressure feed systems. Helicopter fuel systems vary. They can be simple or complex  depending on the aircraft. Always consult the manufacturer’s  manuals for fuel system description, operation, and  maintenance instructions. The location of fuel instrumentation varies depending on the  type of cockpit displays utilized on the aircraft. Fuel quantity gauges are important features on all aircraft.  Indications exist for all tanks on a transport category aircraft.  Often, these use a capacitance type fuel quantity indication  system and a fuel totalizer as is discussed later in this chapter. Low fuel pressure warning lights are also common on jet  transport aircraft. The sensors for these are located in the  boost pump outlet line. They give an indication of possible  boost pump failure.  filter bypass warning lights. The temperature sensor is usually  located in a main fuel tank. The indicator is located on the  instrument panel or is displayed on a multifunction display  (MFD). These allow the crew to monitor the fuel temperature  during high altitude flight in extremely frigid conditions. The  fuel filters have bypasses that permit fuel flow around the  filters if clogged. Indicator light(s) illuminate in the cockpit  when this occurs.  Helicopter Fuel Systems 14-17