I was reading this checkup writeup and it says the pilot took fuel samples before the flight. So I Googled and found out, there are several fuel samplers/testers available.

Now I am wondering: Why is this necessary? Is it a mandatory procedure or do you just do this if you don't trust the supplier?

I can also imagine, that you check for water or contamination. However, we never do that with our cars.

Or am I completely off track?

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    $\begingroup$ As a partial answer, on many cars, especially diesel cars, you have a fuel filter with a bowl or a water separator that helps prevent water from entering the engine. Cars also have pretty good designs to prevent water from entering the fuel tank and fuel cap. $\endgroup$
    – Cody P
    Oct 20, 2017 at 23:58
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    $\begingroup$ With cars you don't do a forced landing if the engine dies due to water ingestion. $\endgroup$
    – GdD
    Oct 21, 2017 at 7:36
  • $\begingroup$ @GdD You are right. However, a stall on the Autobahn due to engine failure isn't not dangerous. It has happened to me on the very left lane (of three) into road works without a shoulder to stop. I had to keep rolling without servo steering until there was one. But I get the point. It is not comparable with a car. ;) $\endgroup$
    – basseur
    Oct 21, 2017 at 9:44
  • $\begingroup$ @CodyP If that was the case, then why would airplanes not have the same good design. It is way more crucial than in a car I guess. $\endgroup$
    – basseur
    Oct 21, 2017 at 9:46

3 Answers 3


On the first flight of the day, after every refueling, and if you have flown through precipitation or the plane is left in the rain, you check the fuel for water and to make sure you have the right kind of fuel. Since water is heavier than fuel, if there is just a little bit of water in the fuel it could cause the engine to stop. You can’t pull over to the side of the road like you can in a car, so you need to immediately deal with the problem. The most likely place for this to occur is on takeoff. If you think fast and switch tanks, you might get lucky and only have water in one tank so you can continue your takeoff. If not, you may need to find a place to land.

In 17 years of flying, I have only found water in the fuel once, and it wasn’t even my airplane. But since it was on Catalina Island, a stopped engine would not have been pretty.

The other reason to check the fuel is that airplanes designed for 100LL do not burn Jet Fuel. Jet fuel is denser than 100LL so it will sink to the bottom of the tank. It is straw colored and somewhat greasy, so you can tell if your plane has accidentally been filled with it, but you really have to be paying attention. It would be easy to miss if you just topped off. There have been incidents when twin-engine planes have mistakenly been filled with Jet A and have either crashed or required expensive fuel system cleaning. Mike Busch's article describes his experiences. After the misfueling of a Cessna 421s, someone did a test to see how easy it is to identify a combination of Jet A and Avgas. It’s fairly hard using standard pre-flight practices. That’s why many people recommend watching the fuel truck if you have a twin-engine plane or one that prominently says “Turbo” on the side.

Many aircraft have rubber bladders to hold the fuel. Most older Cessnas have them. Water can get trapped in the folds of the bladder and get into the engine later when the fuel is sloshed around.

Even if the fuel source is free of water, a tank that is not full can have water condense in it.

Most autogas has some ethanol in it. Water will dissolve in a gas-ethanol mixture and so a slug of water won’t get into the engine like it will in an aircraft engine.

Contamination is less of an issue and is one that is really hard for a GA pilot to determine. You really can’t tell if the fuel is contaminated with other petroleum products just by looking at it.

  • $\begingroup$ Let's say that my aircraft is re-filled with jet fuel instead of 100LL without any special equipment how can I realise it before flight? and there is any special equipment to buy (not expensive please) to bring with me during my flights? thank's $\endgroup$ Oct 22, 2017 at 9:12
  • $\begingroup$ @Ghilardi, fuels are dyed by the manufacturers exactly for this purpose. 100LL is blue. So if the sample has wrong color, or the color is faded, something else was mixed in and you should drain it and refuel fresh. $\endgroup$
    – Jan Hudec
    Oct 22, 2017 at 12:42
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    $\begingroup$ @Ghilardi As mentioned in the linked Mike Busch article it is difficult to see a color difference if a small percentage of the fuel is jet fuel. One sure way to determine whether jet fuel is in your sample is to put some on a paper towel. 100LL will evaporate quickly and leave no residue. Jet A leaves an oily residue. $\endgroup$
    – JScarry
    Oct 22, 2017 at 14:52
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    $\begingroup$ Jet fuel also has a distinctly different smell. It would be a good idea to learn to distinguish the two. I bet the local fuel supplier would be happy to give you a sample sniff if you asked. $\endgroup$ Nov 30, 2018 at 17:05
  • $\begingroup$ Here is an example of a plane being misfueled and crashing: kathrynsreport.com/2014/08/… $\endgroup$
    – DLH
    Mar 26, 2019 at 19:53

An anecdote, if I may.

Once when I was a student pilot, I was preflighting a Cessna 152 for a solo flight. I always sampled the fuel before a flight, as I had been taught. On this day, I noticed a tiny red speck in the sample from the right tank. Tiny, like maybe the size of a grain of sand, but red. It bothered me. I tried another sample and got a red flake, still small. The plane was white with red trim. A paint flake, perhaps? I got up on the wing, but there was no red trim near the fuel filler, nor was there any near the drain. I pulled the cap, thinking it might be from a deteriorating seal. The seal was black and in good condition. I tried the left tank and it looked fine. Tried more samples from the right tank and got a red flexible blob. At that point, I took the sample inside and asked the chief mechanic. He immediately grounded the plane.

As I recall, the determination was that someone had transferred fuel to and/or from the plane using a hose that wasn't made for gasoline. The lining of the hose had come out and into the tanks. Both tanks had gobs of the stuff, though I had found it in samples from only one. The plane had shop time logged just before my preflight, so it had probably happened then.

I was grateful I'd done a proper preflight inspection and paid attention to that little red speck. Had I not, the engine might have run long enough for me to get airborne before that stuff plugged up the fuel screens. I could just imagine being a few hundred feet up and faced with the "impossible turn" as a student pilot. Not a pleasant thought.


Sampling fuel is a mandatory procedure. It's purpose is to check if water has gotten into the fuel tanks.

There are many ways which water can enter a fuel tank: Heavy rain , an unsecured fuel cap, fuel contamination are some of them.

Cars are not sampled because an engine failure on a car is a minor event. On a plane it is a major emergency, and if you are in a single-engine aircraft you will become a glider.

A story from my instructor: In Mexico there was a situation where someone wanted a person dead. Remember these were real, professional assassins. The fuel tank was filled with water during a turn-around on the ground, and the pilot did not check fuel samples. The plane crashed minutes after it took off.

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    $\begingroup$ Mandatory by what authority? It's a good idea but is there a legal requirement to do so? $\endgroup$
    – Steve Kuo
    Oct 20, 2017 at 23:47
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    $\begingroup$ The Front Cover of my Pilot Operating Handbook is marked "THIS HANDBOOK INCLUDES THE MATERIAL REQUIRED TO BE FURNISHED TO THE PILOT BY FAR PART 23". Later on: "The limitations included in this section have been approved by the Federal Aviation Administration." In the Checklists "Before first flight and after each refueling, use sampler cup and drain small quantity of fuel from fuel tank sump quick-drain valve to check for water, sediment." So reads like it's sorta mandatory by the FAA. If you filled up with water and the NTSB determines you didn't check and thereupon crashed, it's on you. $\endgroup$
    – CrossRoads
    Mar 27, 2019 at 14:38
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    $\begingroup$ I was flying yesterday, couldn't get a sample from my right tank. Had to wait for temps to climb to melt what I guess was water frozen around the drain valve. When it finally flowed, it took several samples before I got one to come out water-free. Not sure where it came from; wash water after a recent repainting? Water in the fuel at the painting airfield? (Had some water there before leaving as well). Water in the fuel at my home field? Water from rain over the weekend? Plane is going in for an oil change, having the gas cap seals changed to new at the same time, just in case. $\endgroup$
    – CrossRoads
    Mar 27, 2019 at 14:47

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