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How Aircraft Fuel Systems Work: Cessna 172S

Boldmethod

A few weeks ago we released an article breaking down the fuel system of the Piper Archer TX. Today, we're covering the fuel system of the Cessna 172S. While many components and principles of operation could be similar to the plane you fly, check your aircraft's POH or AFM for the most accurate information.

Fuel System Overview

We'll start by tracing the path of a single drop of fuel through the system, from the fuel tanks to the fuel nozzles.

Fuel Tanks

The Cessna 172S has two fuel tanks. One tank is mounted in each wing. Each tank holds 28 gallons, for a total fuel capacity of 56 gallons. However, only 53 gallons of fuel is usable.

Why is there unuseable fuel? To prevent fuel contamination.

While you do your best to prevent dirt, etc., from entering the tanks, if your fuel is contaminated, the contaminant will settle to the bottom. The lines that draw fuel to your engine are intentionally slightly above the bottom to avoid picking up these contaminates, making the fuel below it unusable.

Fuel Selector Valve

Unlike a low wing aircraft, the 172's fuel selector valve can be placed in a "both" position, in addition to the "Left" and "Right" settings you find on most low-wing aircraft. The "both" position allows fuel to flow from both the left and right tanks at the same time.

Under FAR 23.951, aircraft manufacturers are not permitted to design "[a] fuel pump [that] can draw fuel from more than one tank at a time." This is done to avoid the possibility of the pump sucking in air from an empty fuel tank, causing vapor lock in flight. An exception to this rule is made under 23.951 (2), which states, "There are means to prevent introducing air into the system."

Filter and Drain

This is the lowest point in the fuel system, while the fuel supply lines in the tanks have a mesh screen to help eliminate sediment from the tanks, a finer filter and sump exist here. Sometimes referred to as a "gascolator" the drain can be sumped to check for contaminants within the fuel system.

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Fuel Pumps

Unlike low wing aircraft, the high wing Cessna 172 lets gravity do a lot of the work. Gravity helps pressurize the system, meaning it only needs the help of an electric boost pump in addition to the engine-driven fuel pump for operations like starting.

Before you start your engine you (usually) have to prime it. This is done by pumping fuel into the intake manifold or valve chamber, and the fuel pressure is provided by the electric pump.

Remember that the engine-driven fuel pump is powered by your engine's accessory case, so if your engine isn't running it can't provide fuel pressure. That's why you need to turn on the electric fuel pump.

The fuel flow of a gravity fuel system (high wing) must be able to provide 150% of the takeoff fuel consumption of the engine, while a pump system (low wing) must be able to provide 125%. (FAR 23.955) In high wing aircraft, this pressure is achieved by a combination of the engine-driven fuel pump and gravity.

The final purpose of the electric fuel pump is redundancy. If your engine-driven pump fails, you have a backup to provide enough fuel pressure to keep your engine operating with normal power output.

Fuel Injector Servo Regulator

The fuel injector servo regulator works to create the right ratio of air-to-fuel. This is done by comparing the inlet air pressure to the fuel inlet pressure. This allows your plane to determine a metered fuel pressure.

As you increase throttle, airflow through the engine increases, causing pressure to drop in the neck of the venturi. This drop in pressure creates suction, while the impact pressure of the air increases. This pressure difference causes the diaphragm to move to the left, pulling the ball valve open allowing more fuel flow. (AMT Handbook, Powerplant Volume I, 2-22)

The key takeaway from the fuel servo regulator is that the metered fuel pressure is determined by comparing the inlet air pressure to the inlet fuel pressure.

Fuel Distributor

One of the most significant advantages of fuel injection systems is that the fuel-air ratio in each cylinder is more uniform. This is achieved through the use of a fuel distributor. The fuel distributor takes the metered fuel flow from the servo and separates it equally into the fuel lines leading to the fuel nozzles (sometimes known as "the spider" for its' shape).

In order to keep the fuel at the metered fuel flow, a diaphragm maintains constant pressure via a spring. When you shut down your engine, the diaphragm's pressure ensures a simultaneous shutdown of fuel flow to each cylinder.

Any Differences In Your Fuel System?

While most aircraft fuel systems are similar, not every system operates the same way. Is there anything unique about your fuel system? Tell us about it in the comments below.

Nicolas Shelton

Nicolas is a private pilot from Southern California. He is currently studying at Purdue University, where he is working on advanced pilot ratings. You can reach him at nicolas@boldmethod.com.

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