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Preflight Your Fuel: 5 Things You Don't Want To See

Your engine may not fail right away when running on contaminated fuel. The first indications will likely be sputtering and a generally rough-running engine. Once enough water is mixed with fuel, combustion is no longer possible.

1) Water In Your Sump Cup

Water is the most common contaminant in aviation fuel. Because water it's denser than 100LL, you'll find water settling to the lowest part of the tank. Here's what it may look like in your sump cup...

Dror Artzi

2) Cracked Fuel Caps

When the filler neck of a fuel tank isn't sealed properly by a fuel cap, water can seep in quite quickly. After heavy rain, you could find literally gallons of water in your tanks if you have bad seals.

Dror Artzi

3) Clogged Sump

If your sump point won't drain, you may have contaminants in the fuel large enough to clog the drain hole. There could be a problem with the drain mechanism itself, too.


4) "Koolaid" Blue Avgas Color

Avgas is usually a light blue to nearly clear color. If you sump the tanks of an airplane and the color is initially a deep, dark blue, you make have a slow leak. If the leak around the drain is slow enough, the blue dye from the avgas can build up at the drain port, causing the avgas to come out unusually blue compared to gas from another tank.

After a few sumps, the color will generally return to normal. Get your plane inspected by a mechanic for possible leaks.


5) Incorrect Fuel Types

Whether you're flying a piston or a jet, know the characteristics of your fuel. There have been dozens of accidents over the years from pilots attempting to fly with the wrong fuel in their tanks. The plane may take off, but engine failures shortly thereafter are common.

Have you ever noticed a problem with your fuel on preflight? Tell us about your experience in the comments below.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and an Embraer 145 First Officer for a regional airline. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), and is a former pilot for Mokulele Airlines. He's the author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at, and follow his flying adventures on his YouTube Channel.

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