A time-honored pre-flight inspection ritual is sampling an aircraft’s fuel tanks. The idea, of course, is to drain a quantity from each sump, which usually mounted at a tank’s lowest point, into a clear container, then examine the fuel for contamination and quality: Is it the same stuff that’s supposed to be in the tanks? Are water or other contaminants present? Is it even fuel? In my flying career, at least, it’s rare to find a problem with a fuel sample. But I’ve been quite shocked on a few occasions to find nothing but some very nasty-looking water in a tank or two, which demanded further investigation. Yet, I’ve seen pilots walk up to an airplane for the first flight of the day, kick the tires, light the fires and launch without sumping. Truth be told, I’ve probably done it a couple of times, too. But it’s a bad idea. Here’s why, and what can happen when we fail to sump fuel tanks and ensure we have clean, correct fuel.