All aircraft fuel systems have filters and strainers to  ensure that the fuel delivered to the engine(s) is free from  contaminants. The first of these is encountered at the outlet  of the fuel tank. A sump is used to encourage the collection  of debris in the lowest part of the tank, which can then be  drained off before flight. The actual tank outlet for the fuel  is positioned above this sump. Some type of screen is used  to trap contaminants attempting to flow out of the tank into  the fuel system. Finger screens are common on light aircraft.  They effectively increase the area of the fuel tank outlet,  allowing a large amount of debris to be trapped while still  permitting fuel to flow.  Figure 14-58  illustrates finger screens  that are screwed into a fitting welded in the tank outlet. Fuel tank outlet screens on aircraft with more complex fuel  systems are similarly designed. When in-tank boost pumps  are used, the tank outlet strainer is located at the inlet to the  boost pump as was shown in  Figure 14-50.  The screen’s large  area allows debris capture while still permitting sufficient  fuel flow for operation. Regularly scheduled inspection and  cleaning of these strainers are required. An additional main strainer for the aircraft fuel system is  required between the fuel tank outlet and the fuel metering  device (in a carburetor or fuel-injection system). It is normally  located between the fuel tank and the engine-driven fuel  pump at the low point in the fuel system and is equipped  with a drain for prefight sampling and draining. On light  aircraft, the main strainer may be in the form of a gascolator.  A gascolator is a fuel strainer, or filter, that also incorporates  a sediment collection bowl. The bowl is traditionally glass to  Figure 14-58.  Fuel tank outlet finger strainers are used in  light aircraft. 14-34 Figure 14-59.  A gascolator is the main fuel strainer between the fuel  tanks and the fuel metering device on many light aircraft. allow quick visual checks for contaminants; however, many  gascolators also have opaque bowls. A gascolator has a drain,  or the bowl can be removed to inspect and discard trapped  debris and water.  [Figure 14-59] The main fuel strainer is often mounted at a low point on the  engine firewall. The drain is accessible through an easy-access  panel, or it simply extends through the bottom engine cowling. As with most filters or strainers, fuel is allowed to enter the  unit but must travel up through the filtering element to exit.  Water, which is heavier than fuel, becomes trapped and  collects in the bottom of the bowl. Other debris too large to  pass through the element also settles in the strainer bowl. Higher performance light aircraft may have a main filter/ strainer.  [Figure 14-60]  On twin-engine aircraft, there is a  main strainer for each engine. As with single-engine aircraft,  a strainer is often mounted low on the engine firewall in  each nacelle.  Other larger fuel filters have double-screen construction.  A cylindrical structural screen is wrapped with a fine mesh  material through which inlet fuel must pass. Inside the  cylinder is an additional cone-shaped screen. Fuel must pass  up through the cone to get to the filter outlet. The mesh used in  this filter assembly prevents water and particles from exiting  the filter bowl. The contaminants collect at the bottom to be  drained off through a drain valve.  [Figure 14-61] Fuel outlet Transparent bowl Thumb nut Fuel intake Clamp Cast metal cover Screen Primer line connection