Ethanol has been in the news a lot lately and, as the result of phase separation, is believed to have contributed to the failure of a fuel filter installed in a CTSW.

Ethanol absorbs water and blends well with gasoline. However, too much water will overwhelm the ethanol’s ability to remain blended with the gasoline. Because ethanol mixes easier with water than gasoline, it will separate from gasoline and accumulate with the water at the lowest point of a system. The product in the tank will no longer be a homogeneous blend of ethanol and gasoline, but two layers of product; a layer of gasoline on top and an ethanol/water layer on the bottom. This is referred to as “phase separation.”

At this point, the components in the lower part of the fuel system may be exposed to higher concentrations of ethanol than they were originally designed for. The engine may receive almost pure concentrations of alcohol (perhaps as high as 90%). This may lead to failure of fuel system components, vapor lock and engine stoppage.

It is believed that phase separation can occur, not only when water is accidentally introduced into the fuel system, but also when an aircraft is operated in a humid environment and is allowed to sit for long periods.

To avoid ethanol related problems, discontinue using ethanol-blended automotive fuels.

If it can’t be avoided, use a concentration of no more than 5% (considered the maximum acceptable for the Rotax 912S engine). Premium grade blended fuels often contain no more than 5% ethanol. If the percentage is higher, dilute the mix by adding some 100LL to the tanks.


  • Never let fuel sit for too long. If an airplane is going to remain idle for a long time, drain all the fuel from the aircraft through the gascolator, which is the lowest part of the system.
  • Do not operate your aircraft above 10,000 ft while using ethanol blended fuel as vapor lock may occur.
  • Have your mechanic check the inline fuel filter behind the instrument panel often if ethanol-blended fuel is used and the aircraft is flown infrequently. If damage is observed or suspected, replacement fuel filters may be obtained from Flight Design USA.

For comments or questions, email: airworthiness@flightdesignusa

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