Jerry Johnson recently wrote to General Aviation News to ask the following fuel safety question:
Rotax indicates that octane ratings degrade rapidly and significantly with storage. If so, wouldn’t storage for a matter of weeks render the fuel dangerous to use in aircraft?
Ben Visser, an aviation fuels expert who spent 33 years with Shell Oil and who is also a private pilot, provided Jerry with a vary detailed answer to his question. Ben noted that straight hydrocarbon fuel like 100LL or autogas will not lose octane over time but leaded fuels can due to the fact that lead additives can settle. He did add that that the lost itself is not rapid but rather it’s a decrease of a few numbers over a period of a few years. Specifically, he noted that he has tested high lead level avgas samples and his observations showed a loss of several numbers after about two to three years but he added that most 100LL fuels are blended to a level that is above the standard.
Ben concluded that following the guidelines of using 100LL within one year and autogas within six months should not result in any significant loss in the quality of the octane.
However, Ben did point out that ethanol is a different story. He noted that if there is a significant amount of moisture in the fuel, both the ethanol and water can drop out. This would in turn reduce the amount of octane in the finished product by several numbers.
Ben then ended his answer by writing that he believes there is a “total unbridled lack of knowledge and misinformation on octane requirements and quality” out in the aviation marketplace today. He also pointed out that there is even misinformation in articles about aviation fuel that he reads in aviation publications.
Hence and if you have any questions about aviation fuel, reading Ben’s articles on the subject or contacting him directly would be your best bet.