Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985.
Reader Jerry Johnson recently wrote in with a question about fuel storage: “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?”
This is one of those questions with a lot of qualifiers. A straight hydrocarbon fuel like 100LL and autogas do not lose octane with time. For example, iso-octane has a research and motor octane number of 100 and, if stored properly and not contaminated, it will have an octane of 100 for many years. I have tested pure hydrocarbons after being stored for 10 years and did not see any significant loss in octane.
Leaded fuels can lose some octane over time because the lead additive can settle. We are not talking about a rapid loss, but rather a decrease of a few numbers over a few years. I have tested several high lead level avgas samples and observed the loss of several numbers after two to three years. However, most 100LL fuels are blended to above the standard. The ones I have tested that have been stored up to five years still met the 100/130 standards.
Autogas is composed of many different hydrocarbon compounds. Over time some of the light ends, like butane, can evaporate, leaving the octane of the remaining blend slightly lower. However, the data that I’ve seen indicates only a number or so loss over two years storage.
Bottom line is that if you follow the guidelines of using autogas within six months and 100LL within one year, I would not expect any significant loss in octane quality.
Now ethanol fuel can be another story. If there is a significant amount of moisture in the fuel, then the ethanol and water can drop out, which would reduce the octane of the finished product several numbers. Since most ethanol blends are based on 87 R+M/2, I believe that the end product would still meet the octane requirements of most STCs. But most of the autogas STCs exclude fuel containing ethanol in aircraft.
One of the things that fascinates me almost on a daily basis is the total unbridled lack of knowledge and misinformation on octane requirements and quality. I recently read another aviation magazine’s fuel expert expound on the history of avgas. He went on to say that when the industry switched from 100/130 to 100LL it dropped the lean rating and added letters after the rich rating. I don’t know what to say, but I will try.
First, 100LL is actually 100/130 low lead. It meets the same rich and lean octane spec as the old 100/130 high lead level fuels. The only reason that it is called 100LL instead of 100/130LL is that when 100LL was introduced, 100/130 was still very common. Officials in the industry at the time thought that if it was called just 100LL it would be less confusing to the flying public. Boy were they wrong.
Second, the 100 is the lean rating, which is very close to the auto fuels motor method and is still very much in use, and may be the only one used for unleaded fuels. The reason for this is that the rich rating does not work well on many unleaded fuels.
Now the other interesting point is that with leaded fuels, the rich rating correlates with actual field usage much better than the lean rating. So while everyone is running around saying the sky is falling and we will all die unless we have a 100 octane unleaded fuel, a 100 lean rating unleaded fuel may not meet the anti-knock requirements of most of the high octane fleet.
The octane or anti-knock rating of a fuel is one of the most critical — if not the most critical — property of an aviation gasoline. If the octane quality of a fuel does not meet the requirements of an engine, it can lead to a total engine failure.
With that in mind, I have a lot of trouble with some of the articles I read in aviation publications — and then there is the Internet, where misinformation is king. I now understand how old Chris Columbus felt when he was trying to convince the world that the Earth was round.
You can contact Ben at Visser@GeneralAviationNews.com.