after learning about how the engine works we remember that there are two things that are mixed together for combustion fuel and air let's learn some things about the fuel system the fuel system is designed to bring fuel from the fuel tanks to the engine and general aviation airplanes fuel systems can be broken down into two sub systems gravity fed systems and fuel pump systems high wing airplanes such as the Cessna 172 are normally designed to be gravity fed systems because the fuel tank is inside the wing which is located above the engine gravity can naturally pull the fuel from the tank to the engine although in normal operation a fuel pump is not required many high wing airplanes are still equipped with some sort of fuel pump either an engine driven electrically powered or both as a backup for emergency operations low wing airplanes such as the piper arrow a fuel pump is required to send the fuel from the tanks to the engine since the fuel is located below the engine it is simply impossible for gravity to send the fuel upwards for this reason blowing airplanes are designed with a fuel pump system normally there are two fuel pumps one of the fuel pumps is engine driven and the backup is electrically driven some airplanes are equipped with a fuel primer the fuel primer allows fuel to be injected straight into the cylinder prior to engine start fuel priming is especially useful during cold operations while some airplanes are equipped with a fuel primer others like the Cessna 172 are not however we'd still like to enjoy the benefits of priming the engine therefore it is possible to prime the engine of the Cessna 172 using the electric fuel pump fuel is stored in fuel tanks which are usually located inside of the wings these tanks are filled from the top there is a cap on the top that has easily twisted off and gas is pumped into that tank until it is visually full this cap is usually vented to allow outside air pressure to enter the fuel tank in order to prevent a vacuum from being created inside of the fuel tank if the fuel tank did not have a vent fuel would initially flow from the tank but as it empties there would be nothing to replace that space which would create a vacuum and eventually stop fuel flow to the engine therefore manufacturers vent the tanks to allow air to replace the fuel that is leaving the tank to the engine some airplanes not only have vented fuel caps but they also have fuel vents that protrude from the wing as the airplane flies through the air air enters this vent and is pushed into the fuel tank also inside the fuel tanks our fuel sensors these fuel sensors send information to the fuel gauge which shows the pilot the amount of fuel in the tank these sensors work in many different ways depending on the manufacturer of the airplane it is absolutely critical to visually verify the amount of fuel in the tank never entirely trust the fuel gauges after visually verifying the quantity of fuel in the tanks it's also equally important to confirm that there are no contaminants in the fuel such as water the fuel tanks are equipped with fuel sumps or drains the pilot uses a special fuel sump Cup that allows fuel to flow out of the tank and into the cup at that point the pilot visually verifies that there are no impurities in the fuel most general aviation airplanes utilize 100 low lead fuel which is blue in color therefore not only should the pilot check for impurities the color of the fuel should also be inspected to ensure that the proper fuel was pumped into the tank another component found in the fuel system is the fuel selector the fuel selector usually has four different positions some manufacturers may have fewer positions the four positions are left right both and off if the fuel selector is in the left position then fuel is only drawn from the left tank similarly if the fuel selector is in the right position then fuel is only drawn from the right tank in normal operation the fuel selector is in the both position which allows fuel from both the left and right tanks to be sent to the engine in the off position fuel cannot pass into the engine which stops the fuel flow to the engine ordinarily the fuel selector is only placed on the off position in an emergency situation such as an engine fire the Cessna 172 only has three positions on its fuel selector left right and both a fuel shutoff valve is instead used to shut off the fuel supply this valve is located just above the fuel selector and can be activated by pulling on the knob