Should I Pour Fuel into Tanks After Sumping During Preflight?

When you check the fuel during preflight, do you pour the fuel you sampled back into the fuel tank in the airplane?

My answer:  No!


C172_fuel_sampleAs with most decisions in aviation, ask yourself the reasons why you would do either option and evaluate the risks.  You are doing yourself a disservice if the reason why you are doing a specific task is because you were told to by another person (such as a CFI).

Pouring it Back into your Tanks

The most common reason I hear is that the pilot doesn’t know what to do with this fuel they just removed from the tank.  The pilot doesn’t want to pour it on the ground for environmental reasons.  And we’ve all be taught to always operate with as much fuel as we can.

In my opinion, fuel contamination is the biggest issue.  I typically use a shared fuel strainer that is left in the airplane.  It is normally dusty and dirty.   I don’t want to risk pouring contaminated fuel back into the airplane tanks.  Sure there is a fuel filter and a few things to protect us, but repeated addition of contaminates into your fuel tank is a bad idea.

Pouring it onto the ground

If you are operating off of paved surfaces, pouring the little bit of fuel onto the ground isn’t a big impact.  100LL evaporates quickly and is actually a good sign that it is truly 100LL.  If it was 100% Water, it wouldn’t evaporate like 100LL would.  If there are trace amounts of water in the fuel, it will bubble on the ground as it evaporates.

How I do It

When I strain my fuel this is how I do it:

  • I fill up the strainer about 25% of the way.  I try to minimize the amount of fuel I take from the tank and then have to discard of.
  • I inspect the fuel looking for: color, odor, and evidence of dirt or water
  • If I see the slightest concern, I stop and drain much more fuel and inspect in more detail.
  • I find a sunny spot in a well-ventilated area and dump the fuel and watch it evaporate for 5-20 seconds.

I do NOT:

  • Drain 1 tank, then drain the other tank then inspect once.  Each fuel drain is drained and evaluated by itself
  • Pour the fuel back into the tank

What to do when Water is Found

When you find water in your tank, the first thing you should tell yourself is that:

I am going to be here for a little while

water-in-fuel-624x832Do not rush.  If you find a small trace of water you should be thinking about how much more water is left in the tank.  Water can get into your tanks for many reasons.  Most common is when it rains and the water gets past the gasket in the fuel tanks;  Other times it can come from after you fuel and the tank is contaminated.

Whenever you add fuel, you should ALWAYS check your tanks.  Actually, anytime you walk away from your airplane and come back to fly, you should check them.  If you land on a clear blue sunny day, go have lunch and come back without fueling… Still check your tanks.  You never know if someone had a water gun fight next to your airplane while you were gone.

Steps to removing Water

  • Keep draining until you find no water
  • Rock the wings forward/backwards and up/down.
  • Give them 2-3 minutes to settle
  • Drain again!  I almost guarantee you will find more water
  • Repeat these steps until you find no more water

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