accuracy in fuel gauges only when they read “empty.” Any  reading other than “empty” should be verified. Do not depend  solely on the accuracy of the fuel quantity gauges. Always  visually check the fuel level in each tank during the preflight  inspection, and then compare it with the corresponding fuel  quantity indication.  If a fuel pump is installed in the fuel system, a fuel pressure  gauge is also included. This gauge indicates the pressure in  the fuel lines. The normal operating pressure can be found  in the AFM/POH or on the gauge by color coding.  The fuel selector valve allows selection of fuel from various  tanks. A common type of selector valve contains four  positions: LEFT, RIGHT, BOTH, and OFF. Selecting the  LEFT or RIGHT position allows fuel to feed only from the  respective tank, while selecting the BOTH position feeds  fuel from both tanks. The LEFT or RIGHT position may be  used to balance the amount of fuel remaining in each wing  tank.  [Figure 7-31]  Fuel placards show any limitations on fuel tank usage, such  as “level flight only” and/or “both” for landings and takeoffs.  Regardless of the type of fuel selector in use, fuel  consumption should be monitored closely to ensure that a  tank does not run completely out of fuel. Running a fuel tank  dry does not only cause the engine to stop, but running for  prolonged periods on one tank causes an unbalanced fuel load  between tanks. Running a tank completely dry may allow air  to enter the fuel system and cause vapor lock, which makes  it difficult to restart the engine. On fuel-injected engines, the  fuel becomes so hot it vaporizes in the fuel line, not allowing  fuel to reach the cylinders.  The tanks are vented to the outside to maintain atmospheric  pressure inside the tank. They may be vented through the  filler cap or through a tube extending through the surface  of the wing. Fuel tanks also include an overflow drain that  may stand alone or be collocated with the fuel tank vent.  This allows fuel to expand with increases in temperature  without damage to the tank itself. If the tanks have been  filled on a hot day, it is not unusual to see fuel coming from  the overflow drain.  The fuel quantity gauges indicate the amount of fuel  measured by a sensing unit in each fuel tank and is displayed  in gallons or pounds. Aircraft certification rules require  Fuel Gauges  Fuel Selectors  Figure 7-30.  Gravity-feed and fuel-pump systems.  Figure 7-31.  Fuel selector valve. 7-26  Gravity-feed system  Fuel-pump system  Engine-driven pump  Right tank  Right tank  Left tank  Left tank  BOTH  LEFT RIGHT OFF  BOTH  LEFT RIGHT OFF  Selector valve  Carburetor  Primer  Strainer  Vent  Selector valve  Carburetor  Electric pump  Strainer  Primer   LEFT  19 gal  LEVEL  FLIGHT  ONLY  RIGHT  19 gal LEVEL FLIGHT ONLY  OFF  T A K E O F F  L A N D I N G  A L L F L I G H T  38  G A L T I T U D E S  B O T H 