Mogas Q&A with the expert

From time to time, your bloggers get inquiries and observations about mogas. A few weeks ago we received an email with some questions and observations. I had seen some of the questions before, and I believe we have covered some of these issues before in our blogs, but maybe it is time to examine them again.

When I received the email, I forwarded it to Todd Petersen at Petersen Aviation which has provided mogas STCs for almost three decades and is an expert on mogas issues. He has granted permission to use his answers.

Question 1: The reason I am writing is because I have found several problems with mogas. Below are the ones that stood out to me, starting with excessive carbon build up. I have heard about exhaust leaving black soot when running mogas.

Todd’s response: “In respect to carbon, we expect the exhaust to have carbon showing when burning auto gas. Everyone is used to the nice gray color of the exhaust when using 100LL, but it will turn black with soot when burning auto gas. This may require that you clean the belly more often but otherwise should have no effect one way or the other in terms of operating your airplane. If you think it is excessive or are concerned with it, then switch brands of fuel. This usually lessens the amount of carbon. The absolute cheapest auto fuel will give more soot than a more expensive premium grade.”

Question 2: This Service Bulletin from Jabiru USA seems to deal with additives besides ethanol causing trouble.

Todd’s response: “Aromatics added to auto gas do not cause problems to any parts specifically that are not subject to the same problem when using 100LL. Before 100LL was made, avgas did not contain aromatics, but 100LL does, and a lot of it. When it was first introduced in the 1970s there were many problems with materials compatibility, which were pretty much all resolved by the OEMs by 1980 or so. Therefore when autofuel was introduced in the 1980s the components had been made compatible with aromatics only a few years before.

Proof today that the basic chemistry of autofuel is compatible comes from Lycoming’s addition of autofuel to its Service Bulletin 1070-S, which it never could have done if there were actual materials compatibility issues. Of course you have to keep alcohol out of the fuel. Fuel with ethanol changes everything.”

“Within one of the links I noted Jabiru’s complaint about fiberglass tanks. Depending on what resin is used, yes they may not tolerate gasoline. However, look at all the boats with fiberglass tanks that have been burning nothing but auto fuel for years and years without any issues until ethanol was added. This is a non-issue today. With the proper resin, and with no ethanol, fiberglass is fine for fuel tanks. Jabiru is trying to limit its liability exposure. I can’t fault them for that.”

Question 3 is about vapor lock discussed in a posting on the Vintage Bonanza blog.

Todd’s response: “Vapor lock will always be a possibility on avgas or auto gas. While it is true that it is more likely on auto gas, it should be noted that in the 1990s the EPA ordered the refiners to reduce volatility, and therefore vapor lock potential, in order to cut down on evaporative emissions.

What this means is that today’s auto gas is more like avgas than ever in respect to vapor lock potential. It can still be had with a RVP higher than avgas, but not by nearly as much as 30 years ago.

The whole point of our flight testing was to weed out airplanes that were likely to give vapor lock. Note that in the article about the Bonanza, the pilot noted low fuel pressure. That is not the same as an engine failure and he most likely could have flown it all day like that without needing to switch tanks. He also could have tried the boost pump before switching tanks. Anyway, it’s possible, but not likely, and not nearly as likely as 30 years ago.”

Question 4: I am sure you have seen the Canadian response to mogas, which seems to limit the altitude to 6,000 feet, from a report from Transport Canada in 1993? 

Todd’s response: “The Canadians do not limit STC’d airplanes to 6,000 feet. They limit airplanes to 6,000 feet that do not have an STC, that have never been through hot fuel flight testing.”

I hope this will clear up some misconceptions about using mogas in aircraft and I want to thank Todd Petersen for taking the time to answer this inquiry.

Contributed by Dean Billing