Autogas experts explain why pump gas is mogas

With the release of its Service Instruction 1070Q on July 16, 2010, Lycoming quietly approved autogas as a fuel for some of its engines — or did they? As news of the company’s about-face after years of advising against the fuel became public, company officials have attempted to explain their confusing stance against a backdrop of thousands of pilots who have happily used autogas in Lycoming engines under STCs from the EAA and Petersen Aviation since 1982 when the fuel first gained approval from the FAA.

This past week, AVweb published a three-part series on the subject written by Michael Kraft, General Manager at Lycoming Engines. In Part One, he explains why the company now supports autogas for certain engines. In Part Two, Kraft attempts to redefine autogas, differentiating between what he calls “pump gas” and a tightly-defined specification for autogas that he states is not available at retail gas stations. In Part Three, he implies that autogas users have been lucky not to have fallen out of the sky by virtue of their using a widely-available, FAA-approved fuel that Lycoming still does not formally recognize.

While the authors of GAfuel praise Lycoming for its partial endorsement of autogas, we’d like to comment on a few things that we believe are incorrect or misleading in this three-part series (see also the reader comments to these articles, especially the strong endorsements from those who use autogas).

First, let’s examine the type of autogas that Lycoming’s SI 1070Q allows. In his initial statement found in italics in Part I of the AVweb series, Mr. Kraft makes an interesting distinction about unleaded auto gasoline that doesn’t really exist. Two terms are used repeatedly throughout this series: “pump gas” and “mogas.” “Pump gas” refers specifically to fuel that conforms to the ASTM D4814 or Euronorm 228 specifications for automotive fuel and is available for retail purchase at automobile filling stations. “Mogas,” as he defines it, refers specifically to a fuel that also conforms to the ASTM D4814 or Euronorm 228 specifications, but would be controlled and labeled differently such that it is “fit-for-purpose” for aviation.”  Mogas as defined in this article, as a “labeled” product, does not exist and we doubt that you will be able to convince any gasoline distributor to label it as such at your local gas station.

As the article continues in Section 1, a thesis is developed that only a specific set of specifications for auto gasoline can be used in aviation “mogas”, setting it apart from the gasoline delivered to any service station for use in your car, defined by Kraft as “pump-gas”, and this specific fuel is “fit-for-purpose” for aviation.  So what are these tight specifications that will distinguish “mogas” from the dreaded “pump gas” that you use in your car? Four specifications are spelled out in the article:

1.  93 AKI for detonation margin (hot day OAT and 500F cylinder heads).

2.  Vapor pressure Class A-4 to prevent vapor lock.

3.  No ethanol and maximum 1% oxygenates.

4.  ASTM D4814 Revision 09b and EN228 Revision 2008:E.

We will give you that 93 AKI is a premium specification and it is not commonly available throughout the US.  It is especially rare west of the Rockies but it is available and refineries do make it, so that spec doesn’t separate “mogas” from “pump gas” since there are service stations that sell 93 AKI auto fuel. A quick review of shows that in states that have many retail sellers of ethanol-free fuel, about a quarter of these sell 93+ AKI, so clearly it is available at fuel terminals. So the octane requirement spelled out by Lycoming does not separate “mogas” from “pump gas.”

Vapor pressure Class A-4 specifies auto gasoline with an RVP (Reid Vapor Pressure, see another GAfuels article on the subject) of 9.0 or less, as spelled out in Service Instruction 1070:  “The ASTM D4814­09b maximum vapor pressure limit is 9.0 psi (0.62 kPa) maximum for a Class A rating.”  We have looked through the latest OPIS PADD report that lists the availability of unleaded auto gasoline at practically every fuel terminal in the country and all of them have gasoline with an RVP of 9.0 or less available.  In fact it is the most commonly available RVP for pump gas.  The only other RVP available is Low RVP (LRVP) gasoline with a maximum vapor pressure of 7.8 that is required for certain urban areas during the summer.  So the Class A-4 requirement spelled out by Lycoming does not separate “mogas” from “pump gas”.

No ethanol and maximum 1% oxygenate have always been required for aviation use for STC’d aircraft so that specification also doesn’t separate “mogas” from “pump gas,” as long as you can obtain ethanol-free auto gasoline.  While there is no implicit guarantee that ethanol-free gasoline will be available at your local gas station in the foreseeable future due to the unintended consequences for the RFS mandate in EISA 2007, it should always be available at terminals since ethanol may not be pumped through gasoline pipelines. If anything, however, retail sellers have been reacting to customer demand for it in the past year, and the fuel has made a comeback in parts of the US, especially the Midwest.

The final Lycoming specification is the most ironic.  ASTM D4814 Revision 09B is obsolete and is now a “Historic Standard” according to the ASTM web site.  That suffix number, 09B, means that it is the specification for auto gasoline in 2009.  The most current version of ASTM D4814 is D4814-11.  We doubt that Lycoming is intending that “mogas” must be ordered to an obsolete ASTM specification to be “fit-for-purpose” for aviation, but that is the only conclusion you can draw when, in all probability, all 93 AKI, ethanol-free “pump-gas” meets its “mogas” requirement.

In short, Lycoming’s specification for a “fit-for-aviation” autogas can, in fact, be met at many gas stations around the country, provided the fuel contains no ethanol and has 93 AKI or higher rating. By contrast, autogas STC’s from Petersen Aviation require at most 91 AKI, and the company has a mountain of evidence to show that when used according to their STCs, aircraft burning autogas have enjoyed excellent performance, lower fuel costs and, often, lower maintenance costs due to lack of lead deposits.

While attending EAA AirVenture 2011, GAfuels author Kent Misegades spoke to two Lycoming engineers about their future support for autogas under the bright red Lycoming tent near ConocoPhillips Plaza. They confirmed that the new O-233 LSA engine, while currently categorized as an experimental powerplant, will be submitted for ASTM compliance for use in LSA category aircraft. The naturally-aspirated O-233 will be first, followed by the IO-233 fuel-injected version. Both are expected to be approved for 91 AKI, lead-free, ethanol-free autogas, they said. The company’s more powerful, FADEC-equipped engines currently approved for 93 AKI lead-free, ethanol-free autogas should be capable of running well on 91 AKI, commented one Lycoming engineer who expected approval of the lower-octane fuel in the future. Note that Lycoming’s approval pertains only to the powerplant; certification of an engine-airframe combination is necessary for anything other than E-AB (experimental) category aircraft.

That certification is not easy to come by, as the personnel at the EAA and Petersen Aviation learned in the 1980s while working on the first autogas STCs. For a description from those who were involved, see this GAfuels article.

Todd Petersen, owner of Petersen Aviation, commented recently on Lycoming’s assertion that autogas airworthiness from the FAA is a matter of luck, not by design:

“I must take issue with Michael Kraft’s statement that using pump gas is achieving airworthiness not by design but by luck. The STC’s were not developed through luck. Detonation and endurance testing was required in order to develop the STC’s in the first place, as well as flight testing for airframe STC’s which make the engine STC’s worthwhile. In certificated airplanes engine approval by itself is, without airframe approval, largely irrelevant.

Auto fuel specifications are indeed designed for ground vehicles, which is why testing was done to approve airplanes to use it. Yes these specifications change frequently. If they changed to the point of creating adverse circumstances then Mr. Kraft’s point would be well taken, but with one exception (ethanol) they have changed only for the better, making autogas (without ethanol) more like avgas than it ever was.

Mr. Kraft also stated the following: “93 AKI is “Super Premium” fuel.  That AKI rated octane is what is needed to achieve the exact same power performance on the approved engines as 100LL avgas. 93AKI is produced worldwide, but not necessarily distributed worldwide.”

This statement implies that an engine burning 91 octane doesn’t give the same amount of power as 93 octane.   In fact, octane has to do with improving the detonation margins.  Power is more a function of BTU’s per gallon.  You do not need 93 AKI to achieve the same amount of “power” as on 100LL.  Our detonation testing demonstrated that you get about 3% more power on auto fuel than on 100LL because there are more BTU’s per gallon.  If it were less, the FAA could never have issued the STC’s.

While limiting volatility to class A-4 may reduce the chances of vapor lock it will not eliminate it, and it will not make a Bendix injection equipped Lycoming safe to operate on mogas or pump gas.  I have been personally been  involved with hot fuel flight testing of airplanes using Bendix injection systems.  Regardless of the type of airplane, it failed.  I’ve spoken to owners of Bendix equipped airplane who have it happen frequently on avgas.

Lycoming erred when they approved Bendix injection. I’m sure they feel that it’s an airframe issue and therefore not their problem, and indeed that may be the case — in certificated airplanes. Owners of Experimentals though are using  Lycoming engines without any sort of testing of the airframe fuel system against vapor lock.  Those guys are test pilots whether they know it or not.  Let’s hope that Lycoming makes a change to the injection system which will make their proposed I0-233 safe against vapor lock.  If they fail to do so, then it’s Airworthiness by Luck.

I welcome Lycoming’s revised stance on the use of autogas in general. It’s especially refreshing given the repeated attempts by Lycoming throughout the 1980’s to discredit auto fuel STC’s.

Some of the reader comments to Lycoming’s articles imply that aircraft using autogas have experienced a disproportionate number of fuel-related accidents, although none provided hard evidence of this. When autogas has been used according to STCs and TCs, we are unaware of such problems occurring in a great frequency than for other fuels.

But what about fuel-related problems with 100LL? No fuel is immune to issues, even when it is supposed to be produced according to tight, international standards. Common problems with avgas include:

1. creates lead emissions, a major public relations issue for aviation.

2. leaves lead deposits on engines and airframe, increasing maintenance costs and the risk of stuck valves and subsequent power loss.

3. is subject, as for any fuel, to problems associated with the poor condition of airport fuel storage tanks, water contamination and stability.

4. contains more octane than needed for the majority of aircraft, unduly increasing the cost of flying.

There may actually be a greater risk of stability and quality issues for avgas than for ethanol-free “pump gas,”due to more frequent refilling and storage equipment upgrades at gas stations than at airports, as well as frequent inspections of “pump gas” for proper octane. Contrary to many claims, avgas is not always stored and transported in dedicated equipment; many fuel companies can simply not afford the extra trucks/trailers. As long as tanks are properly flushed when a fuel change is made, this poses no major problems, however. Avgas users, too, are not immune to vapor lock problems under hot & high conditions or when engines are not properly maintained.

As described above, Pump Gas is indeed Mogas, provided it contains no ethanol and has the proper AKI rating specified in the engine/airframe STC or TC. Given this, one wonders what other motives Lycoming might still have in their reluctance to join many of their peers in endorsing the use of quality autogas, regardless the source. Other than the concerns of Lycoming’s legal department, how much of the company’s continued reluctance in endorsing autogas is the result of market pressures? After all, Lycoming really missed the boat on powerplants for the fast-growing LSA sector, where nearly all engines from Rotax, Jabiru, ULPower and others run best on 91+ AKI autogas. Superior Air Parts recently re-introduced its highly-regarded XP-320 and XP-360 engines for experimentals, and they too are approved for 91+ AKI ethanol-free autogas.

If Lycoming insists on a special supply chain for a tightly-defined specification of autogas, why does it not partner with a major avgas supplier to produce and distribute it? The ideal choice would be ConocoPhillips, who by virtue of being the new “owners” of the ConocoPhillips Plaza at AirVenture Oshkosh, appear to be concerned with the needs of sport aviation, the focus of the EAA. ConocoPhillips already produces vast quantities of autogas, and has a distribution network that can probably supply every airport in North America. There is plenty of time between now and AirVenture 2012 for the company to prepare for the launch of an autogas that would meet aviation’s modest needs and satisfy Lycoming.

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.


  1. says

    Lycoming like all typical US businesses stopped innovating a LONG time ago. Their only competition, Continental engines doesn’t bother either. As a duopoly in the aviation engine business neither company needs to do a single thing to continue reaping the benefits of their 60+ year old designs and tooling.

    They exist mealy to continue to generate cash flow for their over paid, ignorant management.

  2. Dennis Reiley says

    Evidently Lycoming doesn’t understand the phrase “cutting one’s own throat.” They can alibi all they want, including citing exceptions and conditions to those exceptions.

    What they really need to do is design an engine and its fuel system to use the same fuel we put in our POV’s. The word “can’t” comes too easily to their lips.

    They and everyone else can complain all they want about the difficulties in doing so. They can go into how altitude and temperature causes problems, etc. But the bottom line is that problems exist to be solved. Either they will do so or their competition will. If they wait until the competition does, Lycoming could be in bankruptcy before they catch up. Which will they prefer?

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