Most people don’t understand the octane numbers of avgas — and the potential problems with 100 octane unleaded avgas.
The major misunderstanding is that the octane rating of a fuel is not a physical property, it is a rating from a single cylinder test engine. And the problem is the poor correlation with that rating and with actual engines in the real world.
During the early scramble to develop a 100-octane unleaded fuel, there were a number of what we used to call “dumbbell” blends that yielded high octane ratings but knocked significantly in actual aircraft engines. From this information, I believe we must conclude that a fuel that just meets the 100 octane unleaded criteria may not meet the requirements of the entire general aviation fleet.
Part of the problem is that the present fuel is not just 100LL. It is still 100/130 low lead.
When the industry switched from 100/130 high lead to 100/130 low lead, the average lean rating increased in most fuels. But the average rich rating was decreased, with a resulting significant increase in knock complaints from the field.
This indicates to me that the rich rating is more critical in aircraft engines than the lean rating.
But the rich rating is very difficult to run accurately on unleaded fuels.
The other reason that 100LL works well in the present GA fleet is that all of the present fuel suppliers use the same type of base fuel, which is a bottomed alkylate, plus two grams per gallon of Tetraethyl Lead (TEL), and enough toluene concentrate to meet the octane spec.
This leaves us a big question: If suppliers can make a fuel that just meets the lean rating and not the rich rating, and is of unknown composition, will it meet the fuel requirements of the entire GA fleet?
If the composition of the new fuels is known, that — along with a 100 lean rating — would give aircraft and engine manufacturers some indication of how well the new fuels will operate in the field.
To be a little more specific, will new engines manufactured by Continental Aerospace and Lycoming operate safely to full TBO with proper maintenance and without detrimental knocking on these fuels? And what assurance will the manufacturers have that that will be true for all unleaded avgas fuels available?
By implication I am making the assumption that if the fuels work OK in Lycoming and Continental engines, they will probably work OK in the orphans in the fleet.
The real concern here is for the safety of the GA community. But there is also a legal concern for the engine manufacturers. Without an agreed-to specification and knowing the composition of the fuel, the engine manufacturers are operating blind.
For example, say there was a serious accident due to an engine failure. During the trial, a lawyer could ask the company representative what caused the accident. The company rep would report that it was caused by the new fuel having insufficient octane and destroying one or more pistons. The lawyer would then ask if the manufacturer had approved the fuel that the pilot was using. When the rep states that the company had not approved the particular brand of STC’d or non-spec fuel, one would think that would be the end of it, but it’s not.
Unless the manufacturer had publicized widely and insisted that all aircraft with their engines carry a warning that their engine was unsafe when operated on a particular fuel or fuels, the manufacturer would probably still be liable. They would need to then choose to let customers use an unapproved fuel, not have any fuel available at all locations, or be liable for them operating on an unknown quality fuel.
The main problem here is we are asking engine manufacturers to guarantee the safe operation of their engines on a fuel without an agreed-to specification or even an STC and without knowing the complete composition of the fuel in a legal system that can be very adversarial, even on its best days.
The GAMI fuel is proven in actual engines over hundreds of hours. It increases detonation margins. It reduces engine wear. It increases cylinder life. Lead is bad for engines and people despite the nonsensical misbeliefs of some commenters. The only thing holding back unleaded AVGAS is the legacy oil companies not having a product, and not wanting to lose their most profitable fuel product. Get over the nonsensical mythology. Give us the proven unleaded now; Please!
Nervously looking forward to the best possible outcome from the 100LL replacement quest. 🤔
How about going ‘half way’, or a compromise ?
Retain 100LL for those aircraft that require it .
For aircraft certified to use 80/87 octane, have the FAA require all states to provide 91 octane, E0 auto gas; at least one gas station within 25 miles of an airport.?
Then those aircraft can get the EAA auto gas stc.
If needed, have the Fed Gov’t subsidize the fuel.
Time to tell these NO-LEAD FOOLS to take a hike! There has NEVER, EVER, been anyone HARMED by 100LL AvGas… and that includes the users, the watchers, or ‘the children’, or anyone else nearby a general aviation airport. I’ve been flying since 1968 and these Greenies have pissed me off continuously! STOP THE NOISE! Leave our 100LL AvGas alone!
Forget about the greenies, there’s only one company making TEL and they want out of the business. So, 100LL is going away whether we like it or not. The GA industry is going to have to adapt, one way or another.
Your statement about the lack of harm from leaded fuel is absolutely false and frankly ignorant. When lead was removed from auto fuel in the 70’s, the rate of lead poisoning decreased substantially, in the U.S. and worldwide. While aircraft don’t have the same impact due to fewer engines that emit lead, the lead emissions harm human health, especially among children.
Another excellent article Ben!
Creation of Aviation fuel clearly is more complex than the average person (or almost anyone in government) understands.
I noticed when buying fuel for my car the other day a sticker on the gas pump. “WARNING: QuikTrip gasoline is not intended for use in aircraft engines”. Maybe this is on lots of pumps, I just never noticed. Maybe just another item put in by there attorneys.
Anyway, got me to wondering what other limits can fuel retailers and/or manufacturers put on their product.
This brings up the big question of why cars all have knock detectors to guarantee the engines will run properly and safely.
I remember in the 60s when I would get poor gas, we would know it because the car would knock and we would act accordingly by reducing the throttle until it stopped knocking.
In an airplane engine it is hard to impossible to tell if the engine is knocking because of all the noises that drown out the knocking sound.
With the possible changes coming in aviation fuel the need for a knock detector is now becoming an important and needed engine instrument.
I started a search to find one for my aircraft so when I use the new fuels I would be assured that all was well with my engine. Turns out this device is unavailable for an FAA certified aircraft.
I think to move on with what we are doing with the unleaded fuel we are going to need knock detectors. In the future, knock detectors might need to be mandated to improve aviation safety.
Currently, there are some non-certified knock detectors that are used in experimental aircraft, but we really need to advance them to certified aircraft to help with the unleaded fuel venture.