The fuel caps on your airplane keep contaminants out of your fuel tank — provided they’re new. Most caps use rubber or cork seals or o-rings, which act to seal out moisture and dirt, and by doing so, keep your fuel safely clean. Unfortunately, over time, those seals wear out. Add some simple tasks to your standard pre-flight to catch most problems before they catch you.
CLOSURE: No, not about your long-lost dog… Do your fuel caps settle into the right position when you close them, or do they hang up just a little bit from being fully engaged? Proper closure of your fuel caps helps keep out moisture by applying the right amount of pressure to the sealing surfaces.
CRACKS: While the o-rings and elastomers, which are used to seal your fuel caps, are fortified for their task, exposure to the elements (most notably sunlight), along with exposure to fuel, will cause these devices to crack and ‘weather check.’ Once they are cracked, your level of protection has been drastically reduced. The more cracks you see, the less protection!
SEEPING: If your plane is exposed to the rain, like when it’s left on the ramp during a spring shower, water will seep into the space around the caps. If there is a gap between the cap and the surface of the wing, the water will wick into that space, and will then seep into your fuel tank through the cracks in the sealing devices. This water carries whatever dirt that happens to be in the area (and can fit through the cracks in the seals) into your tank when it runs inside. Little cracks will allow little specks of dirt into your fuel system. Big cracks — well, you get the idea — big specks of dirt into your fuel system.
Caution: If you see water when you check your fuel sumps, drain them again until they run clean of water or other contaminants. Grab the wing tips and rock them gently. This causes more moisture and debris to migrate to the wing tank’s lowest point, the drain area. Also, take a good close look at your fuel caps — the water didn’t beam itself into your fuel tank, it likely seeped through your fuel cap seals!
SWOLLEN SEALS? On occasion, a bad lot of sealing material unknowingly leaves the factory, or was used by a well-meaning pilot to replace a leaky seal. The sealing material reacts to the avgas, and swells up. As the sealing material swells, it loses some of its sealing properties, and can even become sticky. If your seals are swelling, have your A&P check them out and replace them promptly.
BOTTOM LINE: Your fuel caps aren’t made by rocket scientists, so you should take a closer look at them during your pre-flight inspection. Keep an eye on your fuel caps, and actively look for problems. If you do, you will be able to find those problems before they result in water and dirt contamination of your fuel system.