design of the aircraft. From each tank, a line leads to the  selector valve. This valve is set from the cockpit to select  the tank from which fuel is to be delivered to the engine. The  boost pump forces fuel through the selector valve to the main  line strainer. This filtering unit, located in the lowest part of  the system, removes water and dirt from the fuel. During  starting, the boost pump forces fuel through a bypass in the  engine-driven pump to the metering device. Once the engine- driven pump is rotating at sufficient speed, it takes over and  delivers fuel to the metering device at the specified pressure.  The airframe fuel system begins with the fuel tank and ends  at the engine fuel system. The engine fuel system usually  includes the engine-driven pumps and the fuel metering  systems. In aircraft powered with a reciprocating engine, the  engine-driven fuel pump and metering system consists of the  main components from the point at which the fuel enters the  first control unit until the fuel is injected into the intake pipe  or cylinder. For example, the engine fuel system of a typical  engine has an engine-driven fuel pump, the fuel/air control  unit (metering device), the fuel manifold valve, and the fuel  discharge nozzles. The fuel metering system on current  reciprocating engines meters the fuel at a predetermined  ratio to airflow. The airflow to the engine is controlled by  the carburetor or fuel/air control unit. The fuel metering system of the typical gas turbine engine  consists of an engine-driven pump, fuel flow transmitter,  fuel control with an electronic engine control, a distribution  system or manifold, flow divider, and fuel discharge nozzles.  On some turboprop engines, a fuel heater and a start control is  a part of the engine fuel system. The rate of fuel delivery can  be a function of air mass flow, compressor inlet temperature,  compressor discharge pressure, compressor revolutions per  minute (rpm), exhaust gas temperature, and combustion  chamber pressure. atomize and distribute the fuel from the carburetor into the  mass airflow. This must be accomplished so that the fuel/air  charges going to all cylinders holds equal amounts of fuel.  Each one of the engine’s cylinders should receive the same  quantity of fuel/air mixture and at the same fuel/air ratio. Basic principles of operation are discussed here with no attempt  being made to give detailed maintenance instructions. For the  specific information needed to inspect or maintain a particular  installation or unit, consult the manufacturer’s instructions. The basic requirement of a reciprocating fuel metering system  is the same, regardless of the type of system used or the model  engine on which the equipment is installed. It must meter fuel  proportionately to air to establish the proper fuel/air mixture  ratio for the engine at all speeds and altitudes at which the  engine may be operated. In the fuel/air mixture curves shown  in  Figure 2-1,  note that the basic best power and best economy  fuel/air mixture requirements for reciprocating engines are  approximately the same. The fuel metering system must  Due to the drop in atmospheric pressure as altitude is  increased, the density of the air also decreases. A normally- aspirated engine has a fixed amount or volume of air that  it can draw in during the intake stroke, therefore less air is  drawn into the engine as altitude increases. Less air tends to  make carburetors run richer at altitude than at ground level,  because of the decreased density of the airflow through  the carburetor throat for a given volume of air. Thus, it  is necessary that a mixture control be provided to lean  the mixture and compensate for this natural enrichment.  Some aircraft use carburetors in which the mixture control  is operated manually. Other aircraft employ carburetors  which automatically lean the carburetor mixture at altitude  to maintain the proper fuel/air mixture. 2-3 Fuel Metering Devices for Reciprocating  Engines Figure 2-1.  Fuel/air mixture curves. Typical F/A mixture curve—float-type carburetor Rich F/A Lean Takeoff Maximum cru ise Minimum cruise Auto rich Auto lean Climb Idle Typical F/A mixture curve—pressure injection carburetor Airflow in lb/hr Low High Rich F/A Lean Idle Takeoff Maximum cru ise Minimum cruise Rich Manual Lean Climb Airflow in lb/hr Low High