There is still a lot of confusion about the transition to a new unleaded avgas, especially around the differences between fuels approved through the Supplemental Type Certificate (STC) and a specification from the American Society for Testing and Materials (ASTM).
It may help to look at the history of specifications for avgas.
The Wright brothers started this with a straight run fuel in their first flight. The octane number for that type of fuel would probably have been in the 30 to 40 range — and on a 120-foot flight that worked out OK.
Actually, their engine had very poor cooling around the exhaust valve, so they probably had pre-ignition for the whole flight.
As the performance level of aircraft developed, the knock resistance of fuels also improved. Between the Wright brothers’ first flight and World War II, there were a lot of different specifications.
For instance, in the late 1930s, officials in Britain and the US noted the superior performance of German aircraft when compared to Allied aircraft. That led to a very intense program to increase the performance of their aircraft, especially fighter aircraft.
Part of that program was to improve the fuels so that the compression ratios and supercharger settings could be increased for better performance, especially at higher altitudes.
Their first step was to develop a new octane rating system that would correlate with the anti-knock requirements for supercharged aircraft engines as the existing lean rating was not adequate. This led to the birth of the rich or supercharge rating.
The War Department, in cooperation with the major oil companies, worked on the development of a new high-octane fuel, which became known as 100/130 avgas.
The War Department specified that all avgas would need to be a high quality alkylate derived from what is called the bottoming process — an extra distillation where motor alkylate is pumped into a column and all of the product that boils off at 350°F is captured and condensed. This product is called aviation alkylate.
The product that does not boil off is called “heavy ends” or “bottoms” and are sent back for further processing.
Meanwhile lead is added to the aviation alkylate to meet the 100/130 octane specification.
While there were additional requirements, that was the base of the specification, which lasted throughout the war.
Then in 1947, ASTM developed the D-910 specification that we have today for avgas. There have been a few updates to that spec, like the addition of 100/130 low lead, but no major changes.
So, they developed a specification in 1947 that has lasted over 75 years, but we can’t develop a new specification for an unleaded avgas after trying for over 25 years! Were they that much smarter back then?
It’s important to realize that in 1947 avgas was about the only fuel used in aviation, so there was a lot more time and effort dedicated to the production and development of the product.
Now avgas is a very small part of the fuel business. Some estimates put it as small as 3/10ths of 1% of the fuel produced.
And, of course, they also had the blueprint for the specification set by the War Department.
The reason that the specification has withstood the test of time is simple: All of the oil companies that produce avgas have used the same formula set down by the War Department in World War II to produce the product.
For comparison, the present ASTM committee is trying to write a specification for a product that can be made from a variety of components and/or additives. This is an almost impossible task considering the fuels need to be compatible with all of the different aircraft and fuel components that have been made since 1903.
When the present committee started its effort, its members felt that since many companies were testing fuels with a wide array of components, they should start a totally new specification and not just update the D-910 specification.
However, considering the difficulty and legal problems in writing a new specification and the fact that the leading contenders for a new fuel are General Aviation Modifications Inc. (GAMI) and Swift Fuels — both based on a high octane alkylate and a super aromatic component — maybe the committee needs to review that decision.
It might make more sense to just update the D-910 specification, adding an aromatics specification and also a composition limit.
Ben Visser,
Sir, in Brazil auto as well as light aviation piston engines, fuel is derived from sugar cane which is the second most produced crop other than soybeans in that country. This fuel contains ethanol.
Since Viton O rings which are compatible with ethanol, why are we not using autofuel that contains ethanol in our low compression engines. Our fuel senders floats in our fuel tanks are metallic and anything downstream to the engine is either metallic or Viton compatible with ethanol. Fuel containing ethanol does not corrode aluminum in the supply lines in the aircraft.
So in small bore low compression engines such as Continental 0-200 engines, why are we not allowed to run fuel containing ethanol-?
Has any data been derived from testing done with fuel containing ethanol in low compression engines such as Continental 0-200s-?
Your reply will be appreciated by me.
Ben … why does the RVP of mogas go way up during the winter blend season ?? I recently took a webinar where I learned that the summer blend RVP was 9.0 max while winter blend could be 11.5 to 13.5.
From Ben Visser: The higher RVP in the winter is for improved starting in cold weather. Also, Butane is not expensive and has good octane so refineries want to use as much as possible, but too high of an RVP in the summer would increase evaporative loss and air pollution.
Here’s something I learned about this fact, as well, Ben. I’m a snowbird, leaving FL for points north in mid-May through mid-Nov. A few years ago, I left a very low mileage GM pickup truck in my garage and filled it up — like I do my airplane — before leaving. When I got back, I backed the thing out of my garage and left it running while I went into my home. When I came back out, it wasn’t running and threw an engine code. The short of the story is that it was an EVAP code. The truck was fine when I left but not fine when I got back. Ultimately, I troubleshot the problem to a clogged EVAP cannister. After learning about the EVAP ratings of summer fuel, suddenly everything fell into place. And, talking to auto mechanic friends, they told me one should take the fuel cap off and not fill the tank prior to leaving. Now, I do that with all my vehicles. Likely not an issue for airplanes but is for cars/trucks. Thanks.
“There is still a lot of confusion about the transition to a new unleaded avgas” But there is no confusion concerning the use of the best unleaded aviation fuel, mogas, which has been an FAA-approved fuel since the 1980s. It could power the vast majority of legacy piston aircraft, nearly all new piston aircraft with modern engines, and is available at thousands of sellers, see pure-gas.org
Partial solutions like MOGAS (Ethanol free) sound good, but really throws a lot of owners and operators “under the bus”.
We really need ONE FUEL for the future, be it the current 100LL, GAMI fuel, Swift fuel or something else.
I’ve had a mogas STC for 50 years, but cannot use it since all gas in California has ethanol in it.
I’ve emailed my CA legislators that I could use 93 octane gas if they would remove the ethanol from it, and I would not be using 100LL with TEL in it.!!
I get no good replies.
Time for you to move east, Jim 🙂
I did it 25 years ago.
Good answer, Mr. Larry.
I grew up in the East, and moved to CA for an Engineering job. 50 yrs later, I now have a cattle ranch in No.CA, and get to fly all year…and no freezing Wx.!
I like it here, except for the politics and anti-energy legislature.
I can empathize, Jim. I learned to fly at Beale AFB but lived most of my time around the Mojave desert above LA. IF I were ever to go back — NOT likely — it’d be to NoCal. That said, I established a summer operation in a small town near Oshkosh and love the dichotomy of moving back and forth each year to points south. This is year 19 of doing that. I’da never guessed that I’d do that when I was out there but … an engineering position led me to do it, too.
WI premium fuel does NOT have ethanol and they actually advertise that on many pumps.
I got my certificate at the Beale Aeroclub as well.!!
My Cessna is based at Marysville, and I’m a member of the EAA chapter there too.
I live in the foothills east of Beale, under the class C.
I’m anxious to see what unleaded fuel is eventually available here.