A gascolator is a combination filter and water trap device placed inline with the fuel supply on light aircraft. The primary function of the gascolator is the isolation and removal of any water which accumulates in the aircraft fuel system. To this end, the devices are located at the lowest point in the fuel system and equipped with a drain cock through which water is purged from the fuel. The gascolator also serves as a standard inline fuel filter and is fitted with an internal mesh baffle to trap any suspended sediment. Gascolators are critical parts of any light aircraft system and should be checked regularly to remove any accumulated moisture because water contamination is a frequent cause of flight engine failure.
Water contamination in aircraft fuel is a major concern particularly for light piston engined aircraft operators. Water in aviation fuel interrupts correct flame propagation with a resultant partial or complete loss in power and is a regular cause of engine failure during flight. Most light aircraft feature drain cocks on their fuel tanks that may be used to remove any accumulated water, but this is not always completely effective. Water is heavier than fuel and will always collect in the lowest point in the fuel system. This makes some aircraft designs susceptible to the collection of water in the fuel lines as well as in the tanks.
The fuel tanks in most light planes are situated in the wings; in high wing designs such as the Cessna 172/182, the tanks are a fair distance above the rest of the fuel system. This means that water in the system tends to collect in the fuel lines below the tank, thus rendering the tank drains ineffective for removing all the moisture. The gascolator is intended to remedy this problem by presenting a combination of water trap and drain point in the fuel lines. This is achieved by placing the gascolator in series with the fuel lines at the lowest possible point and in a position where they can be easily accessed to drain trapped water.
Gascolators typically consist of a bowl shaped fitting with inlet and outlet fittings on the top of the body and a drain tap on the bottom of the bowl. Common materials used in the construction of gascolators include brass and aircraft grade aluminum; some designs feature insulation material around the body of the device. The devices generally also contain an internal filter element consisting of a mesh disc that acts as an additional filter to remove solid sediment from the fuel. As with all inline filters, it is important to regularly check the elements and replace them where necessary to prevent loss of performance.
The placement of the gascolator is also important. It is good practice to try to keep the unit as far as possible from engine components and exhaust manifolds. Although some examples feature insulation or heat shields, exposure to excessively high temperatures can cause vaporization of the fuel in the gascolator, thereby causing vapor lock in the fuel lines. The units should also be placed where they are readily accessible to facilitate ease of access for preflight checks.