In defense of autogas

Recently an airport commissioner in North Carolina contacted us regarding his commission’s plans to add autogas as a means to lower the cost of flying and increase overall activity at his airport. A large Shell-branded avgas supplier based in his state refused to provide autogas, but he was able to find a local fuel jobber owned by a fellow pilot who was happy to bring 93 AKI ethanol-free fuel to this small airport, even in small quantities of a few thousand gallons.

At this time, the airport’s avgas supplier provided the commissioner with Shell Aviation Bulletin SAB Q109, which originated in the company’s U.K. office in 2009. An article authored by Shell’s Technology Manager for Aviation Fuel, Rob Midgley, starts with the bold headline “Motor Gasoline — The Dangers in Aviation Use.” The airport commissioner recently sent us SAB Q109 and asked us to comment on its accuracy.

One needs to preface a review of Shell SAB Q109 with the history of autogas STCs in the U.K., as described by Todd Petersen, the owner of Petersen Aviation and most autogas STCs: “Long ago the [British] CAA refused to accept our 91 AKI STCs because they didn’t think their fuel could be counted on to really be 91 AKI.  The fact that our testing was done with 89.5 AKI fuel didn’t matter to them. They rejected our 91 octane STCs and, while they were at it, they also rejected STCs on twins. Apparently the CAA is of the opinion that British autogas is substandard, poor quality, dirty fuel, worse than gas in Sri Lanka, Philippines, Malaysia, Ukraine, and other places that approve our STCs. Great Britain is the only country that has done this.”

One does not find SAB Q109 on Shell’s otherwise extensive list of service bulletins, so one can only assume that it applies solely to aircraft in the UK.

Following are excerpts from Shell’s SAB Q109 (in italics) with our comments appearing below each:

Shell has always taken the firm position that only aviation fuels should be supplied to aviation applications, but this article highlights the potentially high risk game that is played by those prepared to take advantage of the regulatory approvals and highlights the reasons why we never knowingly provide ground fuels to aircraft.

Where is the evidence of “high risk”? These comments imply that Shell does not adhere to ASTM D4814, required for ground vehicles and aircraft. (In the UK, Shell would be dealing with the fuel standard EN228 rather than D-4814.  The two specs were deemed virtually identical in a study by the University of Dresden many years ago.) Other ASTM standards defines aviation-specific fuels. Engine manufacturers and owners of autogas STCs (the EAA and Petersen Aviation) are unlikely to view their enormous investments in time, equipment and money required to gain these STCs as “a game” that takes advantage of regulatory approvals.

Many people are aware of the fact that Supplementary Type Certificates (STCs) are in place to allow some light aircraft/engine combinations to use Motor Gasoline (Mogas) instead of avgas. Even where the use of Mogas is allowed by the aviation authorities, Shell Aviation, other major oil companies and the engine manufacturers have always taken the position that Mogas should not be used in aircraft applications, in particular in the common aircooled Textron Lycoming and Teledyne Continental engines.

“Some light aircraft” is in fact 70%-80% of all legacy piston aircraft and nearly 100% of all latest-generation aircraft engines. Extensive testing is required for any STC; fuels have continually improved in the past three decades; the accident/track record for autogas-powered aircraft has been excellent. Lycoming issued a bulletin nearly two years ago “approving” all the engines for 93 AKI for which Petersen Aviation received STCs in the early 1990s.  Lycoming and Continental are now building a generation of alternative fuel engines that operate on autogas, and some 60,000 STCs cover past engines from these companies. Shell’s contention that Lycoming and Continental have “always” taken a position opposed to mogas is factually incorrect.

In addition to containing different additives Mogas generally contains higher aromatic levels, and will probably soon also contain alcohol.

100LL contains a high amount of aromatics too. 80/87 avgas had zero aromatics, but 100LL is full of them. Aviation autogas STCs do not allow alcohol/ethanol. Some new engines, such as those from market leader Rotax, are approved (TC’d) for E10. The author of SAB Q109 is confusing fuel from the corner gas station with ethanol-free fuel purchased from bulk terminals prior to ethanol’s addition.

Aircraft engines and systems are designed and certified to use avgas and so changing to a different fuel brings with it additional risk that was not considered at the design stage.

This is precisely what is tested in order to gain STC and TC approval. The tests are rigorous. Petersen Aviation even used 89.5 AKI for its 91 AKI STCs to be more conservative. Many engines in use were originally not designed for the high level of lead in avgas, causing serious maintenance issues due to lead fouling with the disappearance of lower octane aviation fuels. The latest generation of engines have in fact being designed for 95-98 RON (89-91 AKI) autogas, not avgas. Many are diesels that will operate on diesel or jet fuel.

Mogas has a wider boiling point range than avgas and this results in fuel being more volatile – being able to form a vapor more easily than avgas.

This is not a major issue in 2012 due to reduced Reid Vapor Pressure (RVP) beginning in the 1990s as directed by the Environmental Protection Agency (EPA) and improved engine controls now coming to aircraft engines, for instance on the Rotax 912iS and all ULPower engines. Lycoming iE2 and similar electronic controls in new Continental engines will permit lower octane and RVP fuels as car engines already do. Note too that vapor lock can and does occur under certain conditions when using avgas.

… this is exactly why carburettor icing tends to be more of a problem with Mogas than when using a lower volatility fuel such as avgas.

This rare problem has been lessened by our having reduced RVPs in recent years.  The important thing is that it is recognized in the same way and cleared in the same way.  You might get icing now, instead of three minutes from now. The icing was no worse than icing with 100LL, it just could occur sooner. Most of the latest aircraft engines are designed for autogas and feature electronic controls and fuel injection that eliminates carburetors and, with it, the problem of icing.

Mogas routinely changes composition and properties between winter and summer.

Reduced RVPs make this almost a moot point.  Pilots have always been told to avoid using winter fuel in summer. Using summer fuel in winter is not a problem. Using winter gas in summer in unapproved airplanes is one thing. However this is what the flight testing required for a new autogas STC was designed to deal with, using hot, winter blend fuel, in hot summer conditions.  If the airplane has an STC, it is because it was flight tested with winter blend fuel in hot summer conditions and passed. Many airplanes did not, and they remain unapproved for mogas.

The specification of avgas is unchanged from season-to-season, country-to-country and is specifically controlled to avoid vapor lock.

Avgas is not immune from vapor lock, will absorb a certain amount of water, and its contents vary widely by producer despite consistent specifications.

Mogas is not designed to be stored for long periods.

According to Ben Visser, an aviation fuel expert who spent 33 years with Shell and a columnist and blogger with General Aviation News, ethanol-free autogas has a shelf life of six months; avgas can be stored for 12 months. Once ethanol is present in fuel, its shelf life is very short, only a few weeks. We do not recommend the use of ethanol in autogas, although some engines have been approved for E5 or E10.

Mogas does not have the same quality and handling restrictions.

This is one of the many myths that surround autogas. Today’s gasoline is of similar quality as avgas, is stored and transported in the same type of equipment and using the same fuel filters. Where problems arise are at poorly-maintained gas stations, and airports, which are just as susceptible to quality issues as the corner gas station.

Avgas quality is guaranteed by the use of dedicated manufacturing and storage vessels, road tankers that are only used for avgas transportation, and dedicated airfield storage.

Avgas is not immune from being contaminated. It seems especially easy to get contamination with JetA in 100LL since  it has happened numerous times. Not all fuel suppliers can afford to transport avgas in dedicated tankers, but use acceptable industry practices to prevent fuel contamination.

Furthermore, water is removed and the fuel tested throughout the delivery system, and together these measures almost totally eliminate the risk of contamination.

Contamination between avgas and jet fuel occurs despite dedicated tanking, due to human error that can never be entirely prevented. Autogas is stored and dispensed at airports using exactly the same equipment as for avgas. It is filtered and checked for water and sediment precisely as one does the same for avgas and jet fuel.

… distributing Mogas large distances by pumping it down multi-product pipelines along which other products, ranging from diesel fuel to heating oil, are also being moved.

The reason that avgas is not pumped down these same pipelines, which reduces the cost of autogas compared to avgas, is that the lead in avgas would contaminate other fuels.

There is one more fundamental difference where avgas out-performs Mogas – that of octane rating.

This is a non-issue. Any engine certified to operate on autogas will be specified with a certain minimum octane or AKI rating. Just like avgas and jet fuel, autogas must meet an ASTM standard (D4814/EN228 for autogas) that guarantees consistent characteristics worldwide. Only fuel that meets the ASTM standard for gasoline and does not exceed the maximum levels of ethanol (usually zero) and RVP will be certified for use in a given engine.  Note also that autogas has 3%-5% more energy by volume (BTUs/gallon) than does avgas.

We recommend that airports obtain autogas from reputable producers, transport and store it in aviation-grade fuel systems, and check their tanks daily just as they would for avgas or jet fuel. Airports should also demand from their suppliers a Material Safety Data Sheet (MSDS) for proof of the fuel’s contents and composition for each load of fuel they purchase, autogas, avgas or jet fuel.

The additional problem of vapor-lock is also a risk with the higher volatility Mogas fuels. The situation can be even worse if the fuel system is of poor design, such as if a fuel delivery line passes close to some hot part of the engine as vapor is then even more likely to form, especially if the fuel flow is low.

In the 30 years since the FAA first approved an autogas STC, this has not proven to be a significant issue. Vapor lock is not unique to autogas and will occur under the right conditions when burning avgas.

One of the most challenging tests required for an engine STC is the hot fuel test. Todd Petersen describes the procedure: “Each model of airplane had to be flight tested. Differences in fuel systems had to be taken into account so that if you tested an early model but the fuel system was altered in any way two or three model years later, that model had to be flight tested again if we expected the STC to include the later models. We are required to take winter blend gas (RVP 12.5 or higher), keep it 52°F or colder until we tested, then put it in the airplane cold, on a day that’s at least 85°F (hotter is better because the tests are more valid), then we would heat it in the airplane to 85°F. As soon as it hit 85°F, we would hop in and fly to 12,500 at Vmc. If it missed, surged or quit, or if the fuel pressure fell below 1 psi, then it was considered a failure. If it passed, we would do it all over again, but this time heating the fuel to 110°F. Once you pass both sets of tests, the FAA runs the same tests all over again and if it passed the FAA tests, then you had the airframe STC.”

Interestingly the STCs to use Mogas are an approval by the local regulatory authorities (Civil or Federal Aviation Authorities); the oil companies, engine manufacturers and airframe manufacturers generally do not endorse the use of Mogas.

Clearly the author of Shell SAB Q109 is using information that is out of date. About 60,000 autogas FAA-issued STCs have been sold by Petersen Aviation and the EAA since 1982 and these are in use in aircraft around the world with great success. During the recent AERO Friedrichshafen, Europe’s largest GA trade show, nearly every aircraft engine on display was placarded as operating on 95-98 RON (89-91 AKI) autogas or jet fuel, in the case of diesel engines. (The exception were a handful of older engines that require 100 octane fuels.) The new engines running on autogas come also from Lycoming and Continental, which used AERO 2012 to debut their new O-200-AF alternative fuel engine. It is very clear that the aircraft industry is preparing for an environment where the primary aviation fuels are 98 RON autogas, jet fuel and diesel fuel.

One aspect of this, namely the ability for Mogas to contain alcohol, is even recognized as a problem within the licensing authorities and Mogas supplies containing alcohol are specifically excluded from the approval of the STCs and therefore the use of alcohol-containing Mogas in aviation is illegal.

The author os SAB Q109 confuses ethanol-blended gasoline one finds at many gas stations with aviation-grade autogas that may be purchased at most bulk fuel terminals. Since ethanol must be blended into ethanol-free fuel blendstocks at terminals before delivery to retail stations, there exists a vast volume of ethanol-free product for aviation, several orders of magnitude greater than for our “boutique” aviation-only fuels. While a number of engines are TC’d for up to E10 (such as the Rotax 912 series), we recommend against the use of any levels of ethanol in aviation due to corrosion, materials compatibility, less range, and water absorption.

In summary, the pilot may see the advantage to Mogas use as being the price, but the true cost is the risk involved in using an unsuitable and uncontrolled product in an unforgiving environment.

The facts from 30-plus years of use of autogas do not support this statement.

Dean Billing, co-author of this blog, had the following observations following his review of Shell’s SAB Q109: “It is clearly written for the British market, since that is the only country mentioned with some arcane limitations on using mogas even with an STC. Why is alcohol even brought up? No STC allows alcohol. It is not an issue at all, ever, since you can’t put auto fuel with ethanol in your STC’d airplane, and if you have any common sense you would never put it in your ultralight, LSA or homebuilt. Everything else in the article clearly ignores all of the testing needed to get an STC and 30 years of mogas use and it ignores the improvements in the last 30 years in mogas quality.”

For reasons we do not understand, the British aviation authorities do not support autogas STCs despite their acceptance in nearly every other country worldwide where pilots have acquired them. The consequence is that autogas is generally not available at airports in the UK, in contrast to Germany, where it is common and, according to a recent EASA report, 50% of all fuel used in light aircraft is autogas. This means that British pilots must pay more to fuel their aircraft than in Germany. Not surprisingly, the number of pilots per million residents in Great Britain is one of the lowest in Europe at 355, while there are 1,012 pilots per million in Germany, one of the highest rates in Europe. (Data from AOPA and GAMA).

British exhibitors were also few and far between at the recent AERO Friedrichshafen show, but countries where autogas is used extensively, for instance Germany, Czech Republic, Austria, Slovenia, Slovakia, Poland, and Ukraine, constituted the vast majority of aircraft manufacturers and other suppliers at AERO.

“A rising tide raises all ships” — affordable choices in fuel for aircraft lead to more flying, more competition, lower costs and the improved health of sport aviation. We urge Shell’s UK office to rethink its position on autogas in light of its overwhelming popularity as the primary lead-free aviation fuel for future piston aircraft.


The GAfuels Blog is written by two private pilots concerned about the future availability of fuels for piston-engine aircraft: Dean Billing, Sisters, Ore., an expert on autogas and ethanol, and Kent Misegades, Cary, N.C., an aerospace engineer, aviation sales rep for U-Fuel, and president of EAA1114.


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  1. Kmisegades says

    Shaner44 – FBOS that sell avgas alongside mogas tell us that their profit margins are the same.  They do not lose money selling mogas.  If anything, offering a lower-cost product increases flying and attracts more business.  Just like taxes – if you lower them, the total tax revenue will eventually increase.

    There are 110,000 gas stations in the US.  Most sell premium fuel, that is a huge amount of fuel compared to avgas.

    • gbin says

      Profit margins for avgas and mogas may be the same for FBOS selling both to consumers, but are they the same for petroleum producers selling both to FBOS?  That’s the question that comes to mind given how hard Shell seems to be trying to suppress the use of mogas in general aviation.  I’m guessing the answer is as Shaner44 suggests:  Petroleum producers make a greater profit margin selling avgas than mogas.

      In any event, excellent rebuttal of Shell’s attempt to misinform that NC airport commissioner, Mr. Misegades!

      • Kmisegades says

         Thank you.   Yes, one does have the impression that Shell UK is trying to prevent competition.   This is futile though.  As avgas production continues to slide, fewer companies make it, lowering supply and driving up prices, reducing consumption, etc. – a downward spiral.    We are seeing France’s TOTAL also attempt to displace mogas they sell at airports with their UL91, little more than rebranded, more expensive mogas.  This might work in the UK where mogas is rare at airports, but it is DOA in Germany, according to people I spoke with.   It is an interesting year, more to come.

  2. Shaner44 says

    The petroleum producers don’t want avgas replaced by mogas because mogas has a lower profit level – pure and simple.

    Problem: Aircraft engines that do use mogas tend to specify 89-91 AKI, usually 91AKI. The problem with that is most people put 87 AKI into their cars, which means  mogas for aviation is pretty much considered premium mogas.

    We still have a long way to go folks, especially on the ethanol issue. But we need to get there, keep comparing fuel prices when you top off your car and when you top off your aircraft. “Both” should be comparable in price.

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