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When Contaminants Show Up to the Party
Few things can ruin your day as quickly as the big fan turning off because there is something wrong with the fuel (or running out of it, but that’s a discussion for another time). Hence one of the earliest lessons you learn about pre-flight is to sump the fuel tanks for water or other contaminants which might impair your engine’s ability provide thrust, or to run altogether. And while I’ve yet to ever sump water from a fuel tank, I know others who have, and it was a good thing they caught it before takeoff.
Just the other day I was sitting with an Airframe and Powerplant (A&P) mechanic as he discussed with an airplane owner the existence of some water in one of the fuel tanks on his airplane. The pilot described taking an entire sump tube of water out of one tank, followed by less and less in each progressive sample, but never reaching the point where there wasn’t any water in the sample. After bringing the airplane over to the shop, the A&P’s pulled even more water from the tank, sump by sump, until they finally got straight fuel. In addition to the amount of water being rather substantial, the question was “how did it get there?” Having just changed the seals on the fuel caps and verifying they were in excellent condition, there was no clear explanation as to how the water had gotten into the tank. The pilot stated he had not encountered any storms on his trip East, there had not been any rain at the airport where the plane was currently located, and there had been no water found in any of the tanks prior to his departure for this trip. A true mystery.
But that got me thinking toward what could have happened if the pilot had assumed that because of these factors, namely the new fuel cap and lack of rain, that there simply couldn’t be water in his fuel tanks and had elected to take off. Would the engine have continued to run, or would the water have been sucked into the fuel line leading to an engine out scenario or significant loss of thrust? Fortunately, we’ll never know because he did things right and checked beforehand.
I think we can all agree that any water in the tank is a problem, but how much water is too much? I’m not sure that anyone has the answer to that, as it probably depends on a number of varying factors. But if we agree that any water in the tank is an issue, then a fuel sample with ZERO water is what we’re looking for. As in, any water is too much water and needs to be mitigated.
And if we find water, what next? I think that really depends on how much is found, and whether the problem is mitigated by disposing of that fuel sample, and taking another. It also depends on your level of comfortability with what was found and whether you think the problem has been mitigated once you have drawn straight fuel. Are you wrong to ground the airplane until you can get it looked at? Probably not. Similarly, you’re probably not wrong, especially if the amount is small, to sump it out and move on with your flight. Whether or not you do this is totally up to you, and any policies or personal rules that apply.
Water also isn’t the only thing we should be looking for. Sumping could reveal other contaminants beyond just water in the tank. Things such as dirt and debris, bits of tank sealant, fuel that is the wrong color (expecting Blue for 100 Octane Low Lead and instead getting a straw-colored liquid indicative of Jet A), or fuel that is too dark (demonstrating a potential tank leak allowing fuel to evaporate but leaving the colorant behind). Any of these things could cause just as big an issue as water in the tank.
Getting back to the example of that airplane owner, would anything have been different if the airplane had been hangered? Probably not, because there was nothing to indicate that the airplane had been exposed to water at any degree that would have suggested the tanks could have compromised. And yet I have talked to pilots recently who state that if the airplane has been hangered and they haven’t added any fuel, that they don’t sump because there couldn’t possibly be water in the tanks. But we’ve established that water isn’t the only thing we’re looking for, and also that apparently water can find a way in through other means. Condensation perhaps? It seems a risky gamble, given this recent scenario, to skip the process of sumping solely because the airplane is hangered.
So now that we know what we're looking for, perhaps a brief refresher on how to sump is in order. What follows is my process, although you can adapt any process that works well for you, the airplane you’re flying, and is one that you will go through every time you fly. Both of the airplanes that I fly currently use a sump cup, although many will use the test tube style fuel tester. If I am fueling up the airplane, I wait until after the fueling is complete and then allow a short time to allow any water to sink to the bottom of the tank, prior to taking my samples.
1) I start with one sump point, usually one of the wing tanks, and take a sample. I take a large enough sample that any water or contaminants will readily make themselves apparent. Taking too small a sample might not allow you to see any problems which defeats this whole process. My sample is generally about 25% or so of the total cup.
2) I first hold the cup up straight and look, then tilt the cup to the side and look into the bottom edge. I will also use a flat color surface (such as the airplane fuselage) so that I can see contrast more easily. I look to see if water is in the bottom of the cup (oil floats on water of course), whether the color of the fuel looks correct (not overly light or overly dark, both which may indicate a problem), and look for anything floating on or in the fuel.
3) If the sample is good and there is enough room left in the sump cup, I will move methodically to the next nearest sump without emptying the cup. If the cup is getting too full, I will empty it prior to proceeding.
a. If the fuel is good, I will add it back to the tank as opposed to dumping it on the ground (especially with leaded fuel). It seems strange to dump known good fuel on the ground as opposed to back into the airplane).
b. If I need to dispose of the sample, I would prefer to find a disposal can as opposed to dumping it on the ground. Regardless of how you dispose of unusable fuel, please do so in accordance to any airport policy or applicable laws.
4) I make sure to hit all the sumps, including the one at the engine, after all you never know which one might reveal the problem.
5) As I proceed, and again once I have finished, I ensure that the sumps are not leaking fuel as one of the airplanes I fly has sumps that can be locked in the open position which would lead to fairly rapid fuel tank drain.
Like any part of my preflight procedure, if I become distracted during the process and cannot remember if I sumped from each point, I go back and run the procedure again. It may be redundant, but at least I have the peace of mind knowing that I checked them all.
Sumping before every flight and after every fuel up (allowing a short time for any water and the fuel to separate) is just good common sense. Even if the airplane was hangered. Even if it hasn’t rained. Even if you haven’t added any fuel. Just like any other preflight process, it might take effort to make sumping a habit but it will pay you back in dividends the day you find a problem.
