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How Aircraft Fuel Systems Work

Nicolas Shelton

You put fuel in your tanks, and eventually it makes its way to your engine. But how does it get from "Point A" to "Point B"?

In this article, we'll reference the fuel system of the Piper Archer TX (Don't worry Cessna flyers, we'll have an article for you soon!). 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.

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Fuel Tanks

The Archer TX has two fuel tanks. One fuel tank is in each wing, and each tank has 25 gallons of capacity (50 gallons total). However, only 24 gallons are usable in each tank (48 gallons total useable).

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 contaminent 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

The fuel selector valve allows you to select which tank you would like to draw fuel from. In the Piper Archer TX, like many aircraft, you can only draw fuel from one tank at a time. So why not both?

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. Manufacturers can design sytems to draw from both tanks as long as "There are means to prevent introducing air into the system.", however, in the case of the Archer, you can only select one tank at a time.

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

In a high wing aircraft like a Cessna 172, gravity helps pressurize the system, but the Archer TX is a low wing aircraft, meaning it needs the help of an electric fuel pump in addition to the engine-driven fuel pump for operations like starting, take-off, and landing.

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)

Having the boost pump on for take-off makes sense. You need more fuel and steady pressure, but why when you're landing? The answer is simple: go-arounds.

Even though most landings require a small amount of power the last thing you want on a go-around is to have to flip a switch before you advance the power, or worse, have your engine sputter during a go-around.

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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