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How It Works: Aircraft fuel system

Getting gas to the engine

The fuel system in the aircraft that you fly has one purpose: Delivering a steady flow of fuel from the tanks to the engine so it doesn’t miss a beat, regardless of power settings, the aircraft’s altitude, or attitude. Systems are either gravity fed—primarily used in high-wing aircraft—or employ fuel pumps, required by low-wing aircraft and those with fuel-injected engines.

In most light aircraft, the fuel tanks are located inside the wings. A filler cap on top of the wing allows them to be filled; drains in the bottom allow fuel samples to be drawn for inspection and any moisture to be removed. Vents allow internal pressures to equalize. A sensing unit in each tank measures the fuel quantity, which is displayed on the fuel quantity gauges in the cockpit.

In high-wing aircraft, gravity propels the fuel from tank to carburetor, and fuel pumps might not be necessary. However, a low-wing airplane—and high-wing craft with fuel-injected engines—will need an engine-driven pump to move the fuel. There should be an electrically powered pump for engine start and use as a backup. You’ll also see a fuel pressure gauge, so you can know that the pumps are working properly.

The fuel selector valve allows the pilot to choose which tank is feeding fuel to the engine. Some systems require you to alternate between Left and Right tanks, while others offer a Both position. Some aircraft might favor one side over the other when Both is selected; select the appropriate side to rectify any fuel imbalance. The selector also has an Off position.

Aircraft with carbureted engines will have a manual primer that pushes additional fuel into the engine to help in starting. (The electric fuel pump fills this role for fuel-injected powerplants.)

Before the fuel moves into the carburetor or fuel injector, it will pass through a strainer that’s normally located at the lowest point in the fuel system, so that it can collect water and any other contaminants heavier than avgas. The fuel strainer’s sump is below the engine; you’ll take a fuel sample from here, too, during preflight.

How It Works
Illustration by Illo Concept Idea
Mike Collins

Mike Collins

Technical Editor
Mike Collins, AOPA technical editor and director of business development, died at age 59 on February 25, 2021. He was an integral part of the AOPA Media team for nearly 30 years, and held many key editorial roles at AOPA Pilot, Flight Training, and AOPA Online. He was a gifted writer, editor, photographer, audio storyteller, and videographer, and was an instrument-rated pilot and drone pilot.

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