UL91 explained

Last month, Lycoming expanded its support for alternative fuels by announcing that it was seeking EASA approval for operation of many of its engines on the unleaded aviation fuel UL91. The company also announced it was planning to approve its new O-233 LSA engine for autogas, a clear response to engines from Rotax, Jabiru, ULPower, HKS, Hirth, AeroVee and others that have been approved for autogas for years.

Despite tens of thousands of Lycoming engines operating since 1982 under autogas STCs from the EAA and Petersen Aviation, Lycoming has long advocated for an aviation-specific, lower octane, lead-free fuel.

This news of UL91 however has caused more confusion than euphoria, largely as it is unknown and unavailable in the US. The fuel adheres to the obscure ASTM D7547 standard, rushed through the approval process when the US Department of Defense needed a reliable fuel for its Rotax-powered drones. By the time the standard was approved however, DoD had shifted its attention to its “heavy fuel” initiative, with the goal to power everything from Abrahms tanks to helicopters, jet aircraft and diesel engine-powered drones with JP-8, the military’s jet fuel.

News about UL91 reappeared in December 2010 when the ASTM was reported to be considering changing the fuel’s designation to NLA (Non-Leaded Avgas) to avoid confusion with the popular 91/96UL and 91/98UL fuels from Sweden’s Hjelmco Oil. The term NLA however has disappeared, so it appears that the ASTM is sticking with UL91, which should not be confused with 94UL, another ASTM-approved unleaded aviation fuel that is also not available in the US. Confused yet?

As an aside, remember that aviation fuels are usually denoted by their lean MON octane rating (MON = Motor Octane Number). Thus, 100LL = 100 MON and UL91 = 91 MON. Hjelmco still uses the lean/rich MON numbers to denote their fuel, thus Helmco’s 91/96UL would be the same as a 91 MON aviation fuel. To add to the confusion, most countries denote octane ratings for highway vehicles according to their RON (RON = Research Octane Number), except the US, which uses the Anti-Knock Index, AKI (AKI = (RON+MON)/2 ). A fairly accurate rule of thumb is AKI = RON X 0.95; thus European Premium Plus autogas of 98 RON is closely equivalent to 93 AKI in the U.S. There is no simple conversion factor between MON and AKI; RON and MON numbers are determined in laboratory settings under strict, repeatable procedures.

UL91 is an unleaded aviation fuel approved by EASA (the European equivalent of the FAA) and now in production by France’s TOTAL. It is available at a handful of airfields in France, Germany, and the U.K. Given however that Hjelmco’s own unleaded aviation fuel has been around for two decades in Europe, one wonders why anyone would bother going through the effort and expense of approving and producing UL91. Lars Hjelmberg, owner of Hjelmberg Oil, seems to share our confusion in a recent posting on his company’s website:

“The initiative from Lycoming Engines shall be seen in light of a recent launch in France and the UK of the grade UL 91 unleaded avgas by the French avgas producer TOTAL. The approval from EASA is expected within a few months. Hjelmco Oil has proprietary unleaded Aviation Gasolines grade 91/96 and grade 91/98. These fuels have been in constant production since 1991 and were approved by Lycoming already 17 years ago. The performance of the Hjelmco unleaded avgas exceeds the performance of the grade UL 91 unleaded avgas and is thus approved by Lycoming Engines for all their avgas 91/96, AVGAS 80/87 and Motorgasoline rated engines.”

This statement suggests that UL91 is a joint effort between Lycoming and TOTAL to create a new fuel that will directly compete with Hjelmco Oil’s products. That’s what free markets do, but in the end consumers will have the last say. Assuming pilots prefer an unleaded aviation fuel (or are forced to buy it due to environmental concerns over leaded avgas), one can only assume that UL91 is less expensive than the alternatives, which in Europe are 100LL and autogas. While 100LL is generally available in Europe, its use has been in decline for years, with many airports no longer selling it. Autogas availability varies widely, from rare in the U.K. to common in Germany, where one recent EASA report estimated that half of all fuel consumed in piston engine aircraft is lead-free, ethanol-free autogas. Last week I asked some friends in the U.K. and Germany about the cost of these various fuels, and this is what they told me:

From Thomas Stute, CFI in a large flying club based in Friedrichshafen, Germany, and owner of a vintage 1960 Klemm 107C powered by a Lycoming O-320:

“Unfortunately, our fuel costs continue to climb steadily. One liter of 100LL now costs 2.7-3.0 Euro [$13.40 – $14.90 per gallon], depending on the airfield. Lead-free, ethanol-free 96 RON [91 AKI] autogas costs 1.70 Euro/liter [$8.44 per gallon] at local gas stations. UL91 is just now being introduced in Germany, first at the Bonn-Hangelar airport. The price is supposed to be around 2 Euros/liter [$9.93 per gallon]. Whether this is the sensible alternative to 100LL has yet to be determined. We have used autogas in our Klemm for years and have had no problems with it. We’re now also converting our club’s PA-28 Cherokee over to autogas.”

From Paul Hendry-Smith, whose company, TLAC, sells the Sherwood Ranger kitplane in Fakenham, England:

“The UL91 is beginning to take off (pardon the pun); we might stock it here shortly. It is 20p per litre cheaper than 100ll selling at about £1.73 per, plus it is the right fuel for the Rotax engines, zero lead. Mogas has ethanol in it now over here and is variable in volume of ethanol. Rotax and others along with the authorities will not allow ethanol in aircraft because of the vapour lock issues. UL91 hits the sweet spot.”

Based on Paul’s information, the cost of these fuels in his area are as follows:

Avgas: $10.25 – $15.08

UL91: $10.50 – $12.00

Autogas (can be E0 or E10, price at gas stations, not airports): $8.38 – $8.92

Although UL91 is not yet approved by the FAA, and is not available in the U.S., we can make some conclusions from the information here. First, Lycoming appears to no longer expect a drop-in replacement for 100LL and is expanding the number of fuels its supports. Second, availability of UL91 varies widely in Europe. Third, the cost of UL91 is comparable to avgas in the UK. Fourth, autogas, used widely in Germany, costs about 40% less than 100LL, which is similar to the U.S. Fifth, the higher cost of avgas relative to autogas is similar in Europe to the U.S.

There remain many unanswered questions surrounding UL91, for instance:

1. With Hjelmco’s lead-free aviation fuels in production for two decades and approved in Lycoming and other engines, why bother with a new fuel that has no clear advantages?

2. With lead-free autogas widely available at European airports, and engine manufacturers approving most new engines for it, why bother with another expensive boutique aviation fuel that, like autogas and UL94, is not suitable for high compression engines needing an 100 octane fuel?

3. Will Lycoming seek FAA approval for UL91?

4. Will anyone import or produce UL91 in the U.S.?

5. What percentage of the legacy piston-engine fleet can be powered by UL91?

And perhaps the most obvious question — why call it UL91 when all other aviation fuels are denoted by their octane rating first, ie 91UL, 94UL, etc.?

More choices for consumers, however, is always a good idea. Now it’s up to free markets to determine if and where UL91 will find its niche.

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


People who read this article also read articles on airparks, airshow, airshows, avgas, aviation fuel, aviation news, aircraft owner, avionics, buy a plane, FAA, fly-in, flying, general aviation, learn to fly, pilots, Light-Sport Aircraft, LSA, and Sport Pilot.


  1. Mike says

    Great…the fuel debate continues to go
    on. The answer is relatively simple, but there will be a cost to
    modify most of these piston airplane engines to run on it. Since the
    octane rating of 91 is readily available…think refiners that
    process and distribute the fuels, the “pipeline for delivery” is
    already in place. The problem is that most of these fuels contain
    ethanol, which has some corrosive properties that create havoc to the
    fuel systems in most piston powered airplanes.

    With the lawsuits in California over
    leaded aviation fuels, the saying has always been what is happening
    in California, is going to happen in the United States, so fasten
    your seat belts because this is a big issue to the environmentalist.
    Since ethanol seems to be the problem, we need to install parts that
    are not affected by the ethanol, because the engine will run just
    fine and vapor lock isn’t the problem if you engineer the fuel system
    properly. Those familiar with ethanol and aviation development,
    airplanes have flown and run on 100% ethanol, with two side effects:
    1) The engine runs cooler and 2) it puts out a little more
    horsepower due to increase compression ratio’s. I know we can’t
    blame the “mean oil companies” and now have to point our finger
    at the “capitalistic farmers”, but the fact is that ethanol is
    accepted as an alternative clean fuel, so it is what it is.

    The goal has to be to deliver usable
    fuels that at the same time are affordable, and the 91 octane ethanol
    blend is that answer. The more ethanol that is blended, the higher
    the octane rating goes as the flame speed is slower so you won’t have
    detonation issues. Of course, because it isn’t as volatile as
    regular gasoline, the engine will burn more fuel to make horsepower.
    This usually balances out by a cost that is 10% less than regular
    fuels but with a 10% higher fuel burn. Because lead seems to be the
    problem, this is the next best alternative to finding a cheap fuel
    that is readily available that can be adapted to aviation.

    Reading Rod’s comments, I had to agree
    with the statement that aviation is available to anyone and that the
    price of flying is based on a rational business model. I recently
    was looking at a new Audi A5 Quatro Coupe that stickered out at
    $43,600 which is more than I want to spend, so I am left driving a
    Volkswagon Passat…so whose fault is that and who do I blame for my

    – we make sense of General Aviation

  2. Rod Beck says

    Jes, Sorry you don’t “get it”. My point is this: if WE can’t afford “something”,we just  have to do without it – flying, airplanes, $75k cars, and $2M homes. When I took my first “dual” lesson in a “J-3” in 1956, it was only $8/hr! Seems your premise was that by lowing the cost of fuel, this would enable you to fly MORE often – it would also allows others the same benefit who have higher capital (investment) airplanes. There are MANYof us in this society that CAN”T afford a lot of things – does that mean we’re still entitled to them? and yes, I coulln’t agree with you more- we need LESS lawyers in govermnent and MORE businessmen/women who have a real world understanding how the “free market” works!  Am I a little clearer now?

  3. Rod Beck says

    Yes, Jes the FACTS are this: 1 One in 1,400 or so have a NEED or interest in GA – thus a VERY limited market 2. The high “fixed cost” of the product or service – therefore, higher “break-even” points – for the provider. 3. Now, I’ll be careful not to offend MINIMAL users (YOU KNOW WHO YOU ARE) of the AVP “Aviation Retail Provider” a/k/a - FBO, flight school, maintenance/avionics shop,etc – total lack of any degree of volume DEMAND.

    Why is it only GA inparticular, that so many complain WHY they should be provided with a low “cost”product (aircraft) that’s affordable (subjective) and constantly using this “fuel thing” as the REASON and cause so many who want to fly don’t? BULL!

    Do this same folks demand: 1. Lexus, Mercedes, or BMW  build and offer a car for $20K? 2. They are are “entitled” to buy a home in Alpine (NJ) Palm Srings (CA)
    for $250K? Is ANYONE getting this?

    Most ADULTS come to the reality that they can’t have EVERYTHING they want in life – including taking flying lessons, buying an airplane OR owning a highline car or living in one of the more “upscale” communities in this country. Am I “connecting the dots here”?

    So for all those who are still stuck in childhood or pre-adolescence – time to get out of the “sandbox” or playground and grow up!

    • JES says

       Rod, I’m not even sure what it is that you’re trying to say here.   I know that for me, the fuel cost is a substantial portion of my cost to fly, and that saving $1.50 a gallon if 91 octane alcohol free Mogas was more readily available would make a significant difference in my annual flight time.  I can afford to be in aviation because I built my planes and drive a 20+ year old car with 300,000 miles on it.  I guess if the goal is to price out those that can’t afford to fly due to the high costs, of which fuel is a big portion, then we are on the right path.  But once aviation becomes an elitist activity for the rich and famous, which it is quickly becoming, you will see community support for the local airports also continue to wane.  The market will continue to shrink, and the costs will continue to skyrocket.

      My point in my original post was that if the “Free Market” was going to decide, there would be much better support for 91 octane alcohol free fuel.  The market is there, but the lobby groups AOPA/EAA are not willing to fragment their support for the 100LL replacement and the FBOs are unwilling to fragment their sales as long as they can leave the end user with the choice of 100LL or no fuel.  There is no “Free Market” anymore.  That is a myth based on what the United States used to be before the lawyers took over the government. 

  4. JES says

     Free markets will decide???  Unfortunately, the market in aviation is so tiny and the costs so high that it is no longer a free market here in the US.  Most airports will only carry one grade of fuel due to the costs for tanks insurance etc.  Almost all of them choose to carry the one fuel everyone can burn, 100LL.  It doesn’t matter that many of us are being priced out of aviation by the costs.  The fuel companies threaten to cut off the 100LL supply if there is Mogas allowed on the field.  That’s intimidation, not free market.  Those that own the fueling facilities view allowing anything other than 100LL onto the field not as more affordable fuel for their customers, but as a net loss to their 100LL sales.  The lobby groups such as EAA and AOPA keep ignoring Mogas in favor of the fantasy 100LL drop in replacement for the warbirds and big engine guys.  Doesn’t matter than many of their members are dropping out as they can’t afford the fuel.  I don’t see a “Free Market” deciding what we will get for fuel.  I see a market controlled by lobby groups and oil companies.  What the customer wants is not what the customer will get.

    • Kevin Krywko says

      Hear, Hear!

      Free Market my —–. the oil companies will just kill this along with any other alternate (eco friendly) sources of energy so they can continue to fleece there customers for more and more profit.

    • Kent Misegades says

       Jes, one of our problems in the US is the fact that most airports are owned by some government entity, which is where the problems start.  Many airport managers are thus bureaucrats, not passionate pilots.  Same for many airport commissioners.  They are often more interested in protecting FBOs from competition than the needs of their real customers, pilots and airplane owners. It is ironic that in Germany where I fly annually, many sport aviation airfields are owned by flying clubs and offer 2-4 different fuels, despite the fact that weather there precludes flying much of the year.  Privatization of our government airports would help. Competition is a rising tide that raises all ships.  Look to the explosion in telephone technology since deregulation as an example how free markets result in more choices, better quality and lower costs.

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