After leaving the fuel tank and before it enters the carburetor,  the fuel passes through a strainer that removes any moisture  and other sediments in the system. Since these contaminants  are heavier than aviation fuel, they settle in a sump at the  bottom of the strainer assembly. A sump is a low point in a fuel  system and/or fuel tank. The fuel system may contain a sump,  a fuel strainer, and fuel tank drains, which may be collocated.  performs the same as grade 100, the “LL” indicates it has  a low lead content. Fuel for aircraft with turbine engines is  classified as JET A, JET A-1, and JET B. Jet fuel is basically  kerosene and has a distinctive kerosene smell. Since use of  the correct fuel is critical, dyes are added to help identify the  type and grade of fuel.  [Figure 7-32]  The fuel strainer should be drained before each flight. Fuel  samples should be drained and checked visually for water  and contaminants.  In addition to the color of the fuel itself, the color-coding  system extends to decals and various airport fuel handling  equipment. For example, all AVGAS is identified by name,  using white letters on a red background. In contrast, turbine  fuels are identified by white letters on a black background.  Water in the sump is hazardous because in cold weather the  water can freeze and block fuel lines. In warm weather, it  can flow into the carburetor and stop the engine. If water is  present in the sump, more water in the fuel tanks is probable,  and they should be drained until there is no evidence of water.  Never take off until all water and contaminants have been  removed from the engine fuel system.  Special Airworthiness Information Bulleting (SAIB)  NE-11-15 advises that grade 100VLL AVGAS is acceptable  for use on aircraft and engines. 100VLL meets all  performance requirements of grades 80, 91, 100, and 100LL;  meets the approved operating limitations for aircraft and  engines certificated to operate with these other grades of  AVGAS; and is basically identical to 100LL AVGAS. The  lead content of 100VLL is reduced by about 19 percent.  100VLL is blue like 100LL and virtually indistinguishable.  Because of the variation in fuel systems, become thoroughly  familiar with the systems that apply to the aircraft being flown.  Consult the AFM/POH for specific operating procedures.  Aviation gasoline (AVGAS) is identified by an octane or  performance number (grade), which designates the antiknock  value or knock resistance of the fuel mixture in the engine  cylinder. The higher the grade of gasoline, the more pressure  the fuel can withstand without detonating. Lower grades of  fuel are used in lower-compression engines because these  fuels ignite at a lower temperature. Higher grades are used  in higher-compression engines because they ignite at higher  temperatures, but not prematurely. If the proper grade of fuel  is not available, use the next higher grade as a substitute. Never  use a grade lower than recommended. This can cause the  cylinder head temperature and engine oil temperature to exceed  their normal operating ranges, which may result in detonation.  Several grades of AVGAS are available. Care must be  exercised to ensure that the correct aviation grade is being  used for the specific type of engine. The proper fuel grade is  stated in the AFM/POH, on placards in the flight deck, and  next to the filler caps. Automobile gas should NEVER be  used in aircraft engines unless the aircraft has been modified  with a Supplemental Type Certificate (STC) issued by the  Federal Aviation Administration (FAA).  The current method identifies AVGAS for aircraft with  reciprocating engines by the octane and performance number,  along with the abbreviation AVGAS. These aircraft use  AVGAS 80, 100, and 100LL. Although AVGAS 100LL  Fuel should be drained from the fuel strainer quick drain and  from each fuel tank sump into a transparent container and  then checked for dirt and water. When the fuel strainer is  being drained, water in the tank may not appear until all the  fuel has been drained from the lines leading to the tank. This  indicates that water remains in the tank and is not forcing the  fuel out of the fuel lines leading to the fuel strainer. Therefore,  drain enough fuel from the fuel strainer to be certain that  fuel is being drained from the tank. The amount depends on  Accidents attributed to powerplant failure from fuel  contamination have often been traced to:  Fuel Contamination  Fuel Grades  • Inadequate preflight inspection by the pilot  • Servicing aircraft with improperly filtered fuel from  small tanks or drums  • Storing aircraft with partially filled fuel tanks  • Lack of proper maintenance  Figure 7-32.  Aviation fuel color-coding system.  7-27  Fuel Strainers, Sumps, and Drains  80  AVGAS  100  AVGAS  100LL  AVGAS  JET  A  AVGAS  100LL  AVGAS  80  AVGAS  100  JET A  RED  GREEN  BLUE  COLORLESS  OR STRAW 