The effect of lower octane fuel on your airplane

Last week I was watching TV when I heard a loud clap of thunder and saw a flash. This was followed by the lights going out and the very sickening sound of electronics going bye-bye from the computer desk.

The next morning the lights were back on, but the computer had sacrificed its life to protect a $4 surge protector — and with it all of my files of questions I have received for this column. I am going on my memory as to questions, and we all know that our memories are the second thing to go when we get older. I do not remember the first.

I do remember that several weeks ago I received a question from a couple who operate an FBO. They had a source of 93 R+M/2 premium motor gas that they were selling to LSA and STC’d aircraft. Then their supplier informed them that the octane quality was being reduced to 92, and then some months later, it was reduced to 91. They were wondering what they should do.

This is one of those cases of being between a rock and a hard place. What the oil companies do is introduce a high octane fuel (93) to get a good market share. Then the business types (In a recent column I wrote that the “bean counters” were the cause of all of these money-hungry tactics and several accountants wrote to correct me that it really is the business types who are the problem and not the accountants) realized that if they reduce the octane quality by a number or two, they could greatly increase profits, but not negatively affect sales volume.

This is mainly due to the fact that most automobiles have a knock sensor, so that the reduction in octane will not have a noticeable effect on the performance of maybe 99% of the cars out there — so everyone is happy: The business types make more money and the normal motorist can’t tell any difference in performance.

But, alas, aircraft do not have a knock sensor. So, will the lower octane quality fuel meet the requirements of your aircraft? That is a definite maybe.

I believe that the requirements for all of the STCs for 80/87 avgas aircraft will be met by any regular or premium motor fuel with an R+M/2 rating of 87 or higher. In fact, I believe that Petersen Aviation has tested its STCs using even lower octane fuel to ensure against knock problems, so these STC’d aircraft are not a problem.

However, several of the LSA engines call for a 93 R+M/2 minimum fuel. Can 91 rating fuel be used in these engines? From a technical point, possibly. From a legal standpoint, probably not.

Most aircraft engines have a safety margin between actual minimum octane requirements to operate knock free and what they post as the octane requirement for their engines. But there are a number of factors that affect octane requirements, such as engine deposits, temperature, barometric pressure, humidity, etc. If you are operating right at the limit, it is possible to use up that margin. It would be necessary for each owner to check with the engine manufacturer to see if they allow the use of a 92 or 91 R+M/2 fuel.

The octane requirement for an engine is not a black and white spec — there is a very large gray area. Unfortunately, our legal system is just black and white, and that’s where we must operate.


Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at




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