The gascolator is located at the airplane's lowest point so as to trap water and debris before it can enter the fuel system.
The gascolator is a catch-all of sorts, a small sump that is designed to catch and trap water from fuel tanks before it can travel into the carburetor of your airplane’s engine.
From your very first preflight you most likely learned that water in the fuel system is very undesirable, which is why you sump small samples from the tanks through fuel drains located under the wings and on the engine side of the firewall. You then look for the presence of dirt or water—which is heavier than aviation fuel—in your fuel tester.
On many airplanes, that drain on the engine side of the firewall is connected to the gascolator, which is located at the lowest point of the fuel system. (Again, because water is heavier than aviation fuel, so if its present the water will collect there.)
The bowl in the gascolator includes a fuel filter screen, an in port and an out port. The bowl is generally made of metal, though much older aircraft had gascolators whose bowls were made of glass—presumably so that the pilot could see any evidence of fuel contamination.
During an airplane’s annual inspection, the gascolator screen should be inspected and the bowl itself should be checked to ensure no corrosion has begun to occur.
