a.	Fuel tank sealant blocking water flow to the sumps;
b.	Sump drains that protrude into the bottom of the sump area so deeply that all water cannot be drained;
c.	Fuel tank bladders that develop deep wrinkles that trap water;
d.	Improperly placed or non-existent fuel sump drains;
e.	Clogged fuel filter bowls;
f.	Sump drain bowls with too small a capacity;
g.	Aircraft operation manuals with incomplete sump draining instructions; and
h.	Ice from condensation or trapped water present in the tanks.

Some aircraft water/fuel contamination problems can be created by poor maintenance or repair procedures. Maintenance personnel should pay special attention when resealing wet wing tanks to ensure that excess sealant is not trapping water by preventing it from passing around the lower wing stiffeners, and hence to the sumps. Other improper repair practices (e.g., the failure to seal the top fuel tank inspection plates properly, the installation of the wrong sump drains, i.e., ends protruding too far into the sump to completely drain, installation of fuel tank bladders with disconnected snaps, large wrinkles, or a partially collapsed tank condition) have all contributed to water contamination problems.

Sump draining procedures should include the draining of both the main and auxiliary tanks. Pilots should be aware that the pre-flight section of the aircraft operations manual may not always give explicit details for draining the fuel system. If in doubt, refer to the operations manual's fuel system schematic. Don't forget; on multi-engine airplanes, the cross-feed sump should be drained as well.

When considering the correct amount of fuel to drain during pre-flight, remember that long fuel lines and the sump drain bowl can often hold 1/2 pint or more of liquid. In many cases, a plastic fuel drain cup holding an ounce or so just won't do a proper job.

Aircraft that have been out of service for extended periods may have the greatest potential for fuel/water contamination. Often these aircraft are exposed to rain or snow during those long periods of storage, and even when stored under cover, significant water/fuel contamination can result from condensation.

When significant water contamination is suspected, purging water from the fuel tanks may be most effectively accomplished by holding the aircraft's tail section to the ground and physically shaking the aircraft to dislodge any water from tank crevices. Using the fuel drain procedures previously discussed, sufficient fuel should be drained from the sumps to ensure several consecutive water-free samples. At this point, all the fuel tanks should be filled and the aircraft should remain motionless for approximately 1/2 hour. The purging process should be repeated again until clear fuel samples are collected at all sumps. In some cases, the amount of water taken from each sump can exceed several quarts.

Winter weather presents a whole new set of problems with respect to water contamination. When the ambient air temperature is below freezing, ice can form in the fuel tanks before takeoff. When an airplane begins a flight in temperatures below freezing and flies into warmer weather, ice will melt and water can then enter the rest of the fuel system. Sometimes, however, ice in the fuel system may be detected during the pre-flight.

If fuel draws slowly from your sumps, there could be ice inside your tank. Vigorous wing rocking may dislodge some ice, and you can hear the loose ice banging around in the tank. The only solution to this problem is to place the aircraft in a heated hangar, melt the ice, and drain the water.