Why Is There No Both Fuel Selector on a Low Wing Airplane?

Why is there no both fuel selector on a low-wing airplane? A short answer to this question is that it has to do with fuel system construction on low-wing aircraft.

Many low-wing aircraft are unable to use gravity to fuel their carburetors. A result of this is that, if both tanks were in operation and one was empty, air would be pulled from the empty tank.

Measures such as not having a ‘Both’ fuel selector help keep the plane’s engines fully operational and operating safely. Understanding and appreciating how the fuel system works helps ensure better operation at all times.

Why Is There No Both Fuel Selector on a Low Wing Airplane? What You Need to Know

The best way to understand the answer to this question is to understand how the plane’s fuel system works. Planes have fuel systems that work differently based on whether they have high or low-wing systems, with variations in the systems that low-wing planes use.

If you’re in the market for a plane or want to know the benefits of flying different types, knowing as much as possible is always helpful. Read on to learn more about the fuel systems in planes work and the impact they have on different plane types.

What is the Difference Between High Wing and Low Wing Planes?

The simple answer to this question is that the difference is in the location of the wings in relation to the fuselage. High-wing planes have their wings above the fuselage, while low-wing planes have their wings below the fuselage’s mid-point.

High wings are common in cargo, military, and passenger aircraft with higher clearance and lighter aircraft, while low wings are common in commercial planes. However, there can be exceptions to these norms.

Do High, and Low Wing Planes Have Different Fuel Systems?

A short answer to this question is – yes, they do! High-wing planes may use a gravity-feed or fuel injection system, while low-wing planes use pump-feed systems more often than not.

Gravity-Feed Systems

Gravity-feed systems are more commonly found on high-wing planes with single engines. Most of these systems use two engines, with one mounted on each wing.

Both tanks are capable of feeding the engine at the same time. Alternately, a selector valve allows the tank outlets to draw from either tank.

Fuel selectors on planes that use these systems also allow all fuel to be turned off. With both tanks potentially feeding the engine, the space that exists above both tanks’ fuel must be interconnected and vented.

The vent line for the fuel usually ends on the wing’s underside in a siphon-proof position. After the fuel exits the selector valve, it goes through the primary filter and into the carburetor inlet, with fuel for the primer coming from the strainer.

Fuel Injection Systems

This fuel system uses an engine-driven fuel pump utilizing technology that prevents engine problems by returning some of the fuel back to the fuel tank. Fuel vapors could block the system, and by returning some of the fuel to the tank, these vapors get purged.

This fuel system also uses gravity, with fuel flowing through the wing tanks by way of two feed lines at the front and rear of each inboard tank end. Fuel flows from the desired tank to the engine, with fuel vapor going back to the wing tank supplying the reservoir tank.

Pump-Feed Systems

Pump-feed systems are the systems where planes are most likely to have no ‘Both’ fuel selector setting. Rather than allowing pilots to select both tanks, these systems allow one tank to be selected.

The alternative to the individual tank in these systems is to have all fuel flow to the engine shut off. Once fuel exits through the selector valve, it makes its way through the filter to the electric fuel pump.

The engine-driven pump and electric pump are parallel to each other, allowing fuel to move through either pump without a bypass feature. These pumps serve different purposes, with the electric pump starting the engine and acting as a backup if the engine-driven pump should fail.

Are There Ways a Low Wing Aircraft Has an Advantage?

Low-wing planes can have advantages over their high-wing counterparts. These advantages can also influence how the fuel selector options work.

Safety Advantages

Although no pilot wants to think about emergency landings or crashes, being prepared is always better. You’ll find it better to know what to do if the worst happens than have to scramble to find a solution.

Low-wing planes have a safety advantage in the event of an emergency. When a plane has to make an emergency landing, the wings will absorb a lot of the impact with the ground.

If a plane should have to make an emergency landing on water, low wings are an advantage here as well. Low-wing planes may be able to float for a short time to allow a few minutes for an escape.

Easier Fueling

Low-wing aircraft are easier to fuel because of the fuel tank position. Most pilots can reach the fuel tank fairly easily.

Final Thoughts

Why is there no Both fuel selector on a low-wing airplane? This option relates to the fuel selector system used for many low-wing aircraft, and helps prevent damage.