Airplanes rely on fuel to power their engines. Most airplanes use a specific type of kerosene-based jet fuel, such as Jet A, Jet A1 or Jet B. Kerosene-based fuels such as these have a higher flash point than gasoline, meaning they are less likely to combust unintentionally.
Not all the fuel stored in an airplane’s tanks is usable, however. A typical Boeing 747 Jumbo Jet can hold over a quarter-million liters of fuel. There are storage tanks in the 747’s wings and tail. But not all the fuel stored in these tanks is usable. Most airplanes have both usable fuel and unusable fuel.
What Is Unusable Fuel?
Unusable fuel is exactly what it sounds like: fuel stored in an airplane’s tanks that is inaccessible and unusable. It’s stored in the same places as usable fuel. Both usable and unusable fuel is commonly found in storage tanks in the wings and tail. Fuel pumps are designed to pump and move fuel from these tanks to the airplane’s engines.
Fuel pumps, though, will only transfer some of the stored fuel to the airplane’s engines. A small amount of fuel will remain in the tanks. Known as unusable fuel, it can’t be accessed and used by the airplane’s engines.
Why Airplanes Have Unusable Fuel
Airplanes have unusable fuel for several reasons, one of which is the tank design. Fuel tanks are complete. They are typically integrated into the wings, tail and even the fuselage to conserve space. Because of their intricate design, there are small spaces and pockets in the tanks that can trap fuel. Any fuel that settles in these spaces and pockets is unusable.
Another reason airplanes have unusable fuel is for safety purposes. Over time, moisture can accumulate in an airplane’s fuel tanks. Water, of course, doesn’t have the same combustible properties as fuel, so it won’t burn if it enters the airplane’s engines. Instead, it will interfere with the combustion process while increasing the risk of mechanical failure. Water, however, is heavier than fuel, so it will settle at the bottom of the airplane’s fuel tanks.
In Conclusion
Airplanes can’t use all of the fuel stored in their tanks. Because of the way in which the tanks are designed, and for safety purposes, they have a small amount of unusable fuel. Unusable fuel consists of fuel that settles at the bottom of the tanks where the pumps can’t reach it. Unusable fuel may also contain moisture, so it needs to be kept out of the engines.
