Panic about unleaded avgas runs rampant

A few weeks ago I visited AirVenture in Oshkosh. It was no surprise the hot topic of discussion was unleaded avgas. You will be happy to know that panic and ignorance was running rampant.

The good news was that the representative from the EPA claimed that there was not a timetable for the removal of lead from avgas. The panic that lead would be removed from avgas by 2017 came from a ruling that each state will need to present a plan to control lead by that date. The EPA representative indicated that the state’s plans probably will not affect avgas. Based on the limited data we have, the best guess as to when lead will be done away with is probably in the 2021 to 2023 time frame.

To give you an idea of the rhetoric at the show, an expert in one forum expounded on how the industry has never had to decrease the performance level of avgas and that this was a completely new and uncharted area. Another went on and on that the industry must have a 100 octane unleaded replacement for 100LL.

To address these two comments, I would like to give you a short history lesson. In the early 1970s, the volume of avgas being sold had declined significantly from the pre-jet age in aviation. It was apparent that the business could not support the individual handling system required to supply two grades of avgas. So the industry came up with the idea of limiting the lead level of 100-130 to 2 grams a gallon and eliminating the 80-87 grade. The ASTM 910 spec was changed to define a grade 100-130 low lead fuel based on a lot of test data.

The biggest concern was that the added toluene, a chemical that acts as an octane booster, would have a negative effect on seals and other rubber fuel system components. When the fuel finally hit the market there were a lot of problems and controversy. The death threats and many of the more serious problems did relate to using 100LL in 80-87 engines, but there were many serious complaints from 100-130 users concerning knock on 100LL.

Now here is where it gets interesting. When the refiners typically made 100-130, they would take aviation alkylate and add lead until they met the 100 lean rating. They would then measure the rich rating, which was almost always above the 130 requirement. When they started making 100LL, they would add 2 grams a gallon lead to aviation alkylate and then add toluene concentrate to bring up the octane. The big difference was that they had to add toluene until they met the 130 rich rating and then the lean rating was almost always above the 100 level. So with the switch to 100LL, the typical lean rating was higher and the rich rating was lower, and the knock complaints in the field went way up.

So what lesson can we learn from this bit of history?

The industry has decreased the performance of avgas before, and the very small change caused a lot of complaints and concerns in the field.

The lean rating does not define the entire anti-knock performance of a fuel. In fact it does not have a good correlation to field performance, especially with non-conventional fuels.

The rich rating does correlate to field anti-knock performance better than the lean rating (but we have found that the rich rating does not work well with unleaded fuels).

The biggest problem with future unleaded fuels is the fear of the unknown. If we had a lot of unforeseen problems with a minor adjustment of the same components, just think of what could happen with all new fuel compositions.

A wise person once said, “a little learning is a dangerous thing, drink deep or taste not that Pierian spring.”

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


  1. says

    Visser’s comments always provide a voice of reason. Our forum in Oshkosh (“Fuels for Sport Aviation”, posted at drew a large crowd interested in using Mogas. 70%-80% of all legacy G.A. aircraft and nearly 100% of all new LSAs run best on 91 octane Mogas, not 100LL. Odd then that most of the forums on aviation fuel focused on a high-octane replacement for 100LL. Given the need also to lower the cost of flying, one would have expected more interest in Mogas, which costs $1-$3 less than 100LL at airfields that carry it, and is even cheaper for those who self-fuel. Plenty of suppliers of ethanol-free Mogas may be found at

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