I’m starting a new series called, appropriately, “Learn A System”. I’m currently coming close to my private license flight test, and in an attempt to learn all of the systems for the oral portion of my test, I plan to teach them to you.
In my mind, if I explain it to someone else, even if no one reads it, I’ll somehow magically absorb it and know it a little better than I did before. I fly the Cessna 172N, so here we go: the fuel system. (see the diagram at the bottom of post)
To begin, the C172 has the option of two different types of fuel tanks: standard and long range. Standard tanks are 21.5 gallons each totaling 43 gallons. However, 3 of these gallons are unusable (meaning 3 gallons will sit in the bottom of the tanks and can’t be sucked into the fuel lines). Therefore, standard tanks have 40 gallons of usable fuel.
Long range fuel tanks hold slightly more fuel in each tank at 27 gallons per tank. In total, both tanks can hold 54 gallons between them with 4 gallons unusable, meaning long range tanks can hold 50 gallons of usable fuel.
If you haven’t already deduced it yet, the C172 has two fuel tanks located in each wing near the root. From there, the fuel is gravity fed to the fuel selector valve, located at your feet in between the two seats in the cockpit. You can manually select to feed the fuel from the left, right, or both tanks. The off position stops the fuel feed to the engine.
From the fuel selector valve, the fuel flows to a fuel strainer which filters the fuel for any debris or foreign particles. This is also where the primer draws the fuel from where it is injected directly into the cylinder intake ports.
From the strainer, the fuel flows to the carburetor where it is mixed with air and then to the cylinders through intake manifold tubes.
So now you know how the fuel gets from fuel truck to cylinder, but the fuel system includes a few more things. There is space for air to move between fuel tanks, and in the C172N, the right fuel cap is vented to allow air to flow freely. Ventilation is essential to the fuel system as it allows fuel to flow freely.
So how is fuel measured you ask? Well the same way your toilet knows to stop filling once you’ve flushed it. A float sits in each tank and is connected to electrically operated fuel quantity indicators in the cockpit. These indicators are known to be very inaccurate, so always dip your tanks before you fly!
This sums up the basics of the fuel system. Sure, you could get into the locations of the drain valves or the proper position of the fuel selector during certain maneuvers, but I won’t bore you with things that you’ll learn in the cockpit anyways. This first lesson is supposed to be directed at the theory of it all, so I hope this helps.
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