How Does an Aircraft Fuel System Work?

The fuel system is the power behind aircraft, so it is important that fuel follows a safe, clean path from the tank to the engine nozzle. There are a few types of fuel systems used depending on certain aircraft design aspects, but for the sake of this blog, we will examine the Cessna 172S which is a high-wing aircraft relying on two tanks and gravity to pump fuel at the right pressure. This blog will cover the Cessna 172S fuel system and the functions of its main components so that you can better understand your own aircraft.

In this system, fuel is stored in two tanks, each of which are mounted to a wing and hold 28 gallons of fuel. Any contaminants, such as dirt or debris, threaten the system, so the lines that draw fuel from the tank to the engine are situated above the bottom surface of each tank, rendering 3 total gallons unusable. The fuel beneath these lines contains all contaminants that settle to the bottom of the tank. A fuel selector valve selects which tanks to open, either left or right, or in the case of 172S systems, both. Opening both at the same time risks a pump sucking air, rather than fuel, from an empty fuel tank; however, this can be avoided with proper care.

Fuel then travels through a filter and a drain, which serve the purpose of removing additional debris, and sumping the drain allows pilots to manually check for contaminants. This is the lowest point in the fuel system; however, gravity does most of the work for 172S models by pressurizing the fuel with the help of an engine-driven fuel pump. Certain operations require additional force, such as starting up, which is provided by an electric boost pump. The engine must be powered up in order for the fuel pump to operate, so the electric boost pump functions to increase pressure when the engine is not running. This is important for operations such as priming, which is when fuel is pumped into the manifold or valve chamber while the engine is off. The electric boost pump can also serve as a backup to the engine-driven fuel pump.

Next, the fuel injector servo regulator works to measure pressure levels and maintain a proper air-to-fuel ratio by comparing the inlet air pressure to the inlet fuel pressure. Increased throttle lowers pressure in the venturi, creating suction while the impact pressure of the airflow through the engine increases. Meanwhile, the suction then pulls a ball valve open and allows for more fuel flow to balance the increase of airflow. Next, the fuel distributor measures appropriate amounts of metered fuel flow for the fuel injection process. Afterwards, the same amount of air-fuel mixture is pumped into each cylinder for injection via a fuel nozzle. A diaphragm is used to maintain this pressure and provide simultaneous shut downs of each cylinder during the engine shut down process.

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