Small Multiengine (Reciprocating) Aircraft Fuel  Systems Low-Wing Twin High-Wing Twin Figure 14-15.  A fuel distribution manifold for a fuel-injected engine. 14-12 The engine-driven fuel pump intakes the pressurized fuel  from the electrically driven pump or from the reservoir tanks  if the electric pump is not operating. It supplies a higher-than- needed volume of fuel under pressure to the fuel control.  Excess fuel is returned to the pump, which pumps it through  the selector valve into the appropriate reservoir tank. Fuel  vapor is also returned to tanks by the pump. The fuel control  unit meters the fuel according to engine rpm and mixture  control inputs from the cockpit. The fuel control delivers the fuel to the distribution manifold,  which divides it and provides equal, consistent fuel flow for  individual fuel injector in each cylinder.  [Figure 14-15]  A  fuel flow indicator tapped off of the distribution manifold  provides feedback in cockpit. It senses fuel pressure but is  displayed on a dial calibrated in gallons per hour. The fuel system on a small, multiengine aircraft is more  complicated than a single-engine aircraft but contains many  of the same elements. An example system used on a low-  wing aircraft is illustrated in  Figure 14-16 . It features the  main fuel tanks in the wing tips and auxiliary tanks in the  wing structure. A boost pump is located at the outlet of each  main tank. This pressurizes the entire fuel system from the  tank to the injectors eliminating the possibility of vapor lock.  An engine can operate with just its boost pump running in the  event the engine-driven injection pump fails. Typically, the  boost pumps are used to prime and start the engine. Two selector valves are required on twin-engine aircraft,  one for each engine. The right selector valve receives fuel  from a main tank on either side of the aircraft and directs it  to the right engine. The left selector valve also receives fuel  from either main tank and directs it to the left engine. This  allows fuel to crossfeed from one side of the aircraft to the  opposite engine if desired. The selector valves can also direct  fuel from the auxiliary tank to the engine on the same side.  Crossfeed of fuel from auxiliary tanks is not possible. From  the outlet of the selector valve, fuel flows to the strainer. On  some aircraft, the strainer is built into the selector valve unit.  From the strainer, fuel flows to the engine-driven fuel pump. The engine-driven fuel pump is an assembly that also  contains a vapor separator and a pressure regulating valve  with an adjustment screw. The vapor separator helps  eliminate air from the fuel. It returns a small amount of  fuel and any vapor present back to the main fuel tank. The  pump supplies pressurized fuel to the fuel control. The fuel  control, one for each engine, responds to throttle and mixture  control settings from the cockpit and supplies the proper  amount of fuel to the fuel manifold. The manifold divides  the fuel and sends it to an injector in each cylinder. A fuel  pressure gauge is placed between the fuel control unit outlet  and the manifold to monitor the injector-applied pressure  that indicates engine power. A simplified system on a high-wing, twin-engine aircraft that  combines gravity feed with an electric fuel pump is illustrated  in  Figure 14-17 . Directly downstream of the selector valves  are the fuel strainers and then an electric fuel pump for each  engine. This pump draws fuel from the selected tank and  sends it under pressure to the inlet side of the fuel injection  metering unit. The metering unit for each engine provides  the proper flow of fuel to the distribution manifold which  feeds the injectors.  Large, multiengine transport aircraft powered by reciprocating  radial engines are no longer produced. However, many are  still in operation. They are mostly carbureted and share many  features with the light aircraft systems previously discussed.  Figure 14-18  shows the fuel system of a DC-3. A selector  valve for each engine allows an engine-driven pump to  pull fuel from the main tank or an auxiliary tank. The fuel  passes through a strainer before reaching the pump where it  is delivered to the engine. The outlet of the pump can feed  either engine through the use of a crossfeed line with valves  controlled in the cockpit. A hand-operated wobble pump  located upstream of the strainer is used to prime the system for  starting. Fuel vapor lines run from the pressure carburetor to  the vent space in the main and auxiliary tanks. Fuel pressure  gauges are tapped off of the carburetor for power indication.  The hand-operated wobble pumps were replaced by electric  pumps on later model aircraft. A fuel pressure warning light  Large Reciprocating-Engine Aircraft Fuel Systems