What it Looks Like

Fuel strainer

When it comes to putting fuel in an aircraft, the most important thing to check is not price per gallon, but whether you have more than enough for the planned flight (an hour's reserve to be on the safe side), and the fuel is clean and contaminant-free.

Once fuel has been added, the pilot has a couple of opportunities to check its purity. Most piston singles have fuel drains at the low point in each fuel tank, and for a final check, a drain at the fuel strainer.

The strainer is a small metal bowl located in the bottom of the engine plenum, ahead of the firewall. The bowl incorporates a fine-mesh metal screen and a spring-loaded drain valve. Fuel flows from the tanks into the strainer bowl, through the mesh screen, and into another fuel line leading to the mechanical engine-driven fuel pump and auxiliary electric pump. From there it goes to the carburetor or, in the case of a fuel-injected engine, the fuel servo to be metered to the individual cylinders.

If water or solids contaminating the fuel make it past the pilot's check of the fuel tank sump drains, it will collect in the fuel strainer bowl. Since water is heavier than avgas, it will separate from the fuel and collect in the bottom of the strainer bowl before passing into the fuel line leading to the fuel pumps. The mesh screen prevents solids from being carried to the fuel pumps.

Access to the fuel strainer drain differs with different aircraft. On single-engine Pipers, the strainer quick-drain valve protrudes from the aft end of the lower engine cowl. On older single-engine Cessnas, the pilot drains the strainer by pulling on a knob located under a hatch on the upper engine cowl. Newer Cessnas employ a fuel strainer quick-drain valve - similar to those used to sample fuel from the wing tanks - under the engine cowl.

Check the pilot's operating handbook for the proper procedure to follow when checking the fuel for contaminants. For example, Piper says that after draining the sumps on the wing tanks, the fuel strainer should be drained with the fuel selector on one tank, and then again after switching to the other tank.

The fuel strainer bowl is removed and checked for sediment during regular maintenance checks of the aircraft.