Fuel System Basics

Understanding how fuel is stored and delivered to the engine is one of the most important things a pilot must understand about his aircraft.

That’s because without fuel, things get eerily quiet in a hurry.

We can begin our brief overview of the fuel system of the typical light aircraft at the fuel tank. There could be one tank or several. Typically there is a tank in each wing; older designs might have just one tank in the fuselage. Aircraft optimized for cross-country travel often have several tanks in the wings and fuselage.

A very common and simple fuel system is found in high-wing aircraft such as Cessna 172s. Here there is one tank in each wing. A fuel line runs from each tank to the fuel selector valve. It does what you think—allows selecting which tank the fuel is drawn from, plus offers an off position that stops all fuel flow past the fuel selector valve. This is useful during maintenance or when you’re on fire.

From the fuel selector valve, a single line runs to the gascolator, which old farmers might call a sediment bowl. It’s pretty much a small vessel—often glass so you can see inside but not necessarily—where the fuel spends a few quiet moments before moving on to the carburetor or fuel injection system at the engine. Dirt and water contaminants thus congregate in the bottom of the gascolator bowl, which is far better than getting into the carburetor or fuel injection where they can clog passages or put out the desired fire in the cylinders.

When you “check the gascolator” by releasing a bit of fuel into a glass container for examination, you’re checking the bottom contents of the gascolator bowl. It’s also opened and inspected by the mechanic during the annual or condition inspection.

On the simplest high-wing aircraft, gravity is the sole motivator of the fuel. The wings are high enough above the engine that gravity alone provides enough flow and pressure (not much) to keep a carburetor fed. Obviously this doesn’t work well during aerobatics, but it’s OK for upright flight.

It is far more common to have a fuel pump, however. In fact, most systems use two fuel pumps. These come in two styles: engine-driven and electric. An engine-driven pump uses mechanical motion from the engine to exercise a diaphragm. This is sort of like flexing the bottom of an old oil can. Given a couple of check valves—those are just small balls that move back and forth with fuel flow to alternately open and close a fuel line—a diaphragm pump can easily move large volumes of fuel but doesn’t build much fuel pressure. Obviously the engine must be rotating (either running or turning over via the starter motor) for an engine-driven pump to deliver fuel.

Electric fuel pumps utilize a compact electric motor turning a paddle wheel or other arrangement. These have the advantage of operating any time they are electrically energized, which is easy to do with a switch before starting the engine. Electric pumps can also be sized to move generous amounts of fuel, and, especially, supply elevated fuel pressure. Such higher fuel pressures are necessary with fuel injection systems.

It is normal to have one engine-driven pump and one electric pump. The engine-driven pump runs at all times and can be considered the main fuel pump. The electric pump is typically used to build fuel pressure when starting a fuel-injected engine, or as a secondary or backup pump should the engine-driven pump fail. It is common to refer to such an electric unit as a “boost pump” and turn it on for takeoff and landing, but shut it off during cruise as it isn’t needed. Of course, the designer could specify two electric pumps if desired, but in light aircraft the engine-driven/electric combination is typical. It divides your fuel supply eggs between the mechanical and electric worlds.

Low-wing airplanes must have a fuel pump to move fuel “uphill” against gravity because the fuel tanks are almost always lower in the aircraft than the engine. Furthermore, these airplanes normally don’t have a “both” setting on the fuel selector valve because if one tank is low, attempting to draw from both tanks simultaneously could draw air and not fuel from one tank (if the tanks are low on fuel or the airplane is banking).

Fuel-injected engines must have a fuel pump to pressurize the injection system for starting, if nothing else.

Naturally, there is a gasoline filter somewhere in the fuel system. Often these are simple fine-mesh screens just before the gascolator, but they can be elsewhere.

Often overlooked but very important is fuel tank venting. If a fuel tank were sealed airtight, it would take a very powerful pump to suck air out of it. This suction could be strong enough to collapse the tank, in fact. Thus, all fuel systems are vented to the atmosphere so that as the fuel level drops in the tank, air can flow in. Much mischief results when the venting system is clogged by mud or bugs.

Venting systems are often built into the fuel tank caps, but tanks are also typically given separate vent lines from the top of the tank to the atmosphere. Aerobatic aircraft have two vent lines, one for upright flight, the other for inverted.

Fuel systems become rather complex on larger, faster, higher-flying, long-distance, multi-engine cross-country aircraft. Pilots can expect to spend hours reviewing technical manuals on understanding and operating such fuel systems, but for the beginner, the basics described here will start the necessary education.

Above all, the pilot must ensure there is sufficient fuel in the tanks before taking off, but you’d be surprised how often that doesn’t happen. Don’t be that pilot.