Fueled up a Cessna 150 on a Friday evening. Waited a few minutes, sumped both tanks, and found dirty water in the sample from the left wing tank. All three subsequent samples from that wing were clean, even after rocking the wings, so we went to go fly.
Fueled up this same Cessna 150 the following day (Sat). Found the same dirty water contamination (approximately the same amount, too) in the left wing.
There was no precipitation on Friday evening. Also, the tanks were nearly full after the Friday flight so I don't think condenstation was the issue.
I decided to scrub the Saturday flight because there was no obvious reason why more water should have appeared in the same wing after having taken a clean sample on Friday, under the good assumption that the fuel source is clean. My concern was that water is collecting somewhere which is not being drained by the sump, and perhaps adding the fuel was enough to slosh things around so that I saw the water in my sample. This seems to be a known issue with the Cessna 150 and other models (see www.sumpthis.com), but I have no proof that this was the issue.
Am I being overly conservative about this or is the repeating issue (over two successive flights) enough of a cause for concern? What actions can I take to be sure that my fuel tank is clear of contaminants? Any other similar experiences?
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