Fuel is an item that is very important for the proper operation of our aircraft. Even though the world is full of “fuel experts,” it is still a bit of a mystery.
I recently received three questions that are actually interrelated. The first has to do with water in a plane’s fuel tank. The person changed out the fuel cap and gasket, stored the aircraft in an unheated hangar, and still found water in the fuel sump. Why? Any hydrocarbon fuel, such as 100LL, will absorb a small amount of water from the air. The amount of water suspended will depend on the temperature of the fuel. During the day, the fuel will absorb water from the air, then when it cools down at night some of the water can drop out and become free water. Because of surface area, the next day the fuel will absorb more water from the air and not the free water that had previously dropped out. The bottom line is that free water will always be present and all FBO tanks and aircraft tanks must be sumped daily or before every flight.
The second question deals with how long 100LL can be stored before being sold at an FBO. Although not specified in the ASTM D-910 spec for 100LL, the limits for the oxidation tests are designed to ensure that the fuel will be suitable for service after a year in proper storage.
There are two major concerns here. One is that not all storage is under ideal conditions. The other is turnover. At an FBO, the tanks are not usually emptied completely prior to the addition of fresh fuel, which means a part of the old fuel is left in the tank. When you consider that the fuel can sit awhile at the refinery, then at the distributor, then at the FBO, and finally in the fuel tank on an airplane, you can understand that, over time, especially if the storage is not up to par, there can be some problems with the fuel. This older fuel can allow gum formation and other problems, so it is very important that all parties who handle the fuel follow proper handling procedures and practice good inventory control.
The third question came from Bent Esbensen, who stated that the Danish government is demanding that all mogas contain at least 5% ethanol. He was wondering if this fuel would be OK to use in his plane. I understand that avgas cost $11.50 a gallon and mogas cost only $6.50 there, but the answer is still NO.
This answer is based on many factors, but the biggest is that ethanol is a polar solvent, which means that it will absorb water — and the answer to the first question tells us where the water comes from. Add the effect of aging from the second question and you have a real problem. In addition, even 5% ethanol blends will attack rubber and metal fuel system components and can cause premature failures.
So remember: Water is present in the fuel system no matter what; FBOs and other fuel systems should practice inventory and proper quality controls to ensure that avgas is sold within a year; and that ethanol is a no-no, even at the 5% level.
You can contact Ben at Visser@GeneralAviationNews.com.
Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985.