What is the shelf life of fuel?

One of the great things about living in a small town is the number of characters you meet. One of the characters in our area is a man who always has a foolproof way of beating the system. His latest scheme was to buy extra diesel fuel each year and put it in storage in case the price went way up or there was a shortage. He started this about 10 years ago and had several thousand gallons in storage when the price went up this year.


One of the great things about living in a small town is the number of characters you meet. One of the characters in our area is a man who always has a foolproof way of beating the system. His latest scheme was to buy extra diesel fuel each year and put it in storage in case the price went way up or there was a shortage. He started this about 10 years ago and had several thousand gallons in storage when the price went up this year.

I warned him that all hydrocarbon fuels deteriorate over time and he should periodically drain the water off the bottom and check the quality of the fuel to ensure that it was okay.

By now you have probably guessed that he did not monitor the quality of the fuel and that it has gone bad. When he filled up one of his tractors, within four hours the tractor would barely run. After he changed the fuel filters, it would only run a few hours before it plugged up again.

What he rediscovered is microbial growth. These little bugs are present in almost all areas and products. (They are rarely found in leaded avgas, but never say never.) When fuel sits for a long time, a small amount of water normally collects in the bottom of the tank. The bugs are attracted to the water and over time reproduce at a very rapid rate. When a significant amount of these little critters gets into a fuel system, they usually disarm a fuel/water separator, which allows the water with the bugs to enter a vehicle’s fuel tank. In the tank, they attack the surface, especially an aluminum tank, and corrode the daylights out of it. They also plug filters and other intake system orifices.

When this happened to my friend, his tractor was left sitting in the field and he had to call someone to pick him up. Other than the embarrassment of his grand scheme not working, the cost of several fuel filters and the cost of the unusable fuel, he had no major problem.

If this happened with a plane, the very best outcome would be an unscheduled landing, offloading of the fuel tanks, flushing, replacing fuel filters and proceeding with a great deal of concern. If the fuel had been in the tank for any length of time, it may be necessary to replace the tank. In the worst case we have the Earth rising up to smite you, and that is never a good thing.

So what is a pilot suppose to do? Use FBOs that follow recommended cleanliness and handling procedures. Always sump your water drains. If you find water and it is not clear, have your fuel tanks inspected before flying.

If you handle your own fuel, follow recommended handling and inventory controls. 100LL avgas can usually be stored for about a year with little or no concern. Auto gas should not be stored for more than six months. If you have a large tank, it should have a floating suction so that the fuel pumped out is clean and you do not disturb the sediment on the bottom of the tank. It is also important that a tank is allowed to settle after a delivery and prior to fueling.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. Contact him at Visser@GeneralAviationNews.com

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