AVGAS, super petrol, leaded or unleaded?

For the usage of a two-stroke-engine the lead addition is completely unnecessary, as it is used to seal up the valves in older engines. But there are no valves in a two-stroke-engine. The problem is not the engine nor the fuel pipes. The problem might be found in the tank, knowingly made of Epoxi-resin. Unfortunately the stuff used in unleaded petrol for the anti-knock also is a very good dissolver for synthetics.
The easiest and safest recommendation is to use AVGAS in future. In the sense of environmental awareness of the engine it would be a giant step backwards, and very expensive too.
We launched a special investigation, to find out in a laboratory test whether in future it will be possible to use unleaded petrol, such as Euro-Super for example.

The results of a laboratory test show, that you can use the unleaded gasoline, “EURO95”.
The use of leaded gasoline or “Super Plus” is unnecessary.

– w. dirks –


As far as we know a number of our customers wanted to be “good to the motor” and regularly used AVGAS. That is possible in principle, but actually does not give a benefit.

AVGAS causes a harsher combustion than MOGAS, because of its high octane rating. One could see this with the Porsche Aircraft engine some years ago. This increases the vibrations of the engine, and that is critical in retractable motor gliders. This is why we want to recommend to all customers: gas up with EURO Super of lead-free 95.

In other countries this type of fuel is widely available, but may go under another name. For instance, in the USA the octane number is expressed as “ROZ” or “MOZ”, so that regular fuel has an 82-octane rating. Super probably corresponds to our EURO 95, but please inquire first. (As far as I know, it es o.k.!.
A problem is coming up with the Ethanol in the fuel. Please read the article of Jim Herd below!

Again the request, not to fill up out of canisters, but always to use a pump with a filter. It’s hard to believe how many unwanted engine stops are cause by dirt getting into the carburetor.

And that once in a while the tank should be drained would not have to be repeated. One customer with a DG-400 was asked after an unscheduled motor stop if he had drained the tank. “Drained? I never drain the tank” he said.
So he tore his gear out in an emergency out landing. And that really was not necessary.

Two Stroke Oil

Fuel must be mixed with self mixing Super quality two stroke oil – specification JASO FC or FD or higher quality.

The SOLO company recommends the following oil types: CASTROL Actevo 2T or CASTROL Super TwoStroke.

Please avoid to use synthetic oil of other suppliers!

Allan Martini wrote an excellent Explanation – especially for US customers:

There is no difference between octane measurements in the US and Europe but the emphasis in specifing may be different. I will give a summary first and then some details  There are four commonly used ways to measure and state octane values. These are:

RON    Reaserch Octane Number.  This is the value commonly used by DG, Stemme, etc to specify an octane requirement. NOTE  I don’t have my DG manuals anymore and I haven’t been able to find anyone here that has heard of  ROZ, but I have the question still out to some of my friends in Chevron.

MON   Motor Octane Number. This value is determined in a different way. It is often considered to be more sensitve to changes in operating conditions and their effect on the engine than RON.

AKI    Anti-Knock Index.  This is the number that is posted on the gas pump in the USA as “Octane”. It is derived as  (RON + MON)/2  In other words, an average of RON and MON values.

RON is typically 8 to 10 points higher than MON.  Therefor, the average is typically 4 to 5 points above MON and 4 to 5 poins lower than RON.  Thus, a requirement for 95 RON gasoline should be met by 91 “octane” fuel in the US.  This explains Gary Evans information.

RdON  Road Octane Number.  Not generally used but mostly related to fine tuning fuel for racing engines.

Some background:  The term octane for fuel anti-knock qualities was developed, I believe, by General Motors in the early 30’s.  The RON is the percentage mixture of two gasoline components, iso-octane (C8H18) which has very good compression/antiknock qualities and heptane (C7H16) which has poor compression/antiknock qualities. If the mixture contains 80% iso-octane it is deemed to have an RON of 80.

The test fuel is then burned in a single cylinder test engine with variable compression and rigid conditions of rpm, air temp, spark advance, barometric pressure, etc and the antiknock qualities measured.  Any other mixture of hydrocarbons which has the same antiknock qualities will also be considered to have an RON of 80.

MON is measured in the same type of engine using several changes in rpm, temp, throttle position, etc which make the operating conditions much more severe (and possibly more realistic) and the octane numbers are lower.

RdON is measured in multi-cylinder engines, usually at wide open throttle, and usually on an engine dynamometer.  The procedure is used to develope racing fuels.

All that is probably more than you wanted to know, but I hope it helps.

Allan Martini

Fuel pipes

Fuel pipes in our planes are security-relevant parts and have to be changed every few years.

Since some years there exists a new type of fuel pipe made from PU material (polyurethane material) which is supposed to have no run-time limit. We only use this type of fuel pipes in new gliders due to the following reasons:

  • There exists no experience on long term utilization of this kind of pipes yet.
  • Gliders are not suited – as so often – to gain long term experience as there are too few gliders that fly under comparable conditions.
  • It is rather difficult to issue a Technical Note for older gliders because in some types even the fittings had to be changed. The fuel system in a DG-808C, DG-1000T or LS10-st is other than in older gliders.
  • These PU-pipes swell more than rubber hose do, because of the increased content of alcohol in the fuels. Soon there will be a research project launched addressing the utilization of new fuel pipes for aviation. The association of the German glider manufacturers lobby for the inclusion of the PU-pipes within the test program. We have to await the results from this research project!

Here is a very precisely written article from USA:

Piston Aviation Fuel in America

– by Jim Herd

The situation regarding fuel for piston aircraft has been “evolving” in the USA recently. The dust has not settled yet, and it won’t for perhaps a few years, but in the meantime there is growing confusion and concern over what fuel is appropriate for the engines of motorgliders. Each manufacturer must make their own comments & recommendations, but I hope I can offer useful information here that is broadly relevant. And of course, all of soaring is involved with fuel for tow planes. It is a complex and inter-twined story featuring lead, ethanol, and octane – with serious safety implications.

My credentials are that I have studied the subject of fuel for piston aviation over the past few years. I am a mechanical engineer with many years as a high-tech business executive, but outside aviation. I am not a petro-chemical engineer, but from the point of view of a pilot and an aircraft owner, I can offer you a working knowledge. I own a DG-800B with a Solo two-cycle engine, and also a Beech Bonanza with a Continental engine – both of these aircraft are subject to significant issues regarding fuel.

Some basic facts are necessary as a foundation to understand what is going on.

1.      Avgas is generally a consistent and stable formulation across the civilized world, simply because piston aircraft traverse the globe and need a consistent fuel to do so. Some characteristics of avgas are attractive to motorglider pilots. For example, it is guaranteed to be stable for at least one year, whereas mogas is usually only “good” for 3 months. Avgas is also proven at high altitudes, high temperatures and high humidity – so it won’t attract water or lose certain chemicals when very hot or very high.
2.      Mogas varies widely around the world. But not only that, it often varies greatly within a country on the basis of manufacturer, individual batch mixes, seasonal formulations, and practical factors such as raw material cost and availability. Most U.S. mogas now has zero lead and significant ethanol content. Mogas is also more prone to vapor lock in the fuel lines, which can cause the engine to quit!
3.      Piston aircraft vary widely in their requirements for fuel. Octane rating is a primary concern, as is chemical compatibility with every part of the aircraft that comes into contact with fuel.
4.      Octane is measured on different scales in different parts of the world, so this is a source of errors and confusion. In the USA we define octane as “R+M/2” or “AKI”, and this is roughly 4 points lower than the same fuel measured in Europe as “RON” or “ROZ”. For example, 91 in the USA is 95 in Europe.
5.      Octane is a measure of resistance to “detonation” or “knock”, which is a phenomenon by which a spark ignition engine acts more like a diesel engine. In other words, compression and heat alone – no spark – are sufficient to cause an actual explosion in the combustion chamber. Normally, a spark-ignition piston engine never has an explosion – it is a rapid burning process that initiates only with a spark. Detonation in a spark-ignition engine can cause catastrophic failure in a matter of seconds – quite literally, the engine can blow itself apart! And without very sophisticated instrumentation it is not possible to detect detonation from the cockpit! So detonation in piston aircraft is a really big deal and must be avoided.
6.      In a properly running spark-ignition piston engine, detonation is managed with compression ratio, ignition timing, rpm, fuel/air mixture richness, engine temperature, and fuel chemistry. Compression ratios of around 8.5 to 1 or higher generally require lead in the fuel (TEL – tetra-ethyl-lead) to combat detonation, whereas compression ratios of 7.5 to 1 or less usually require no lead. And I am talking here about conventional 4-cycle aviation engines without induction boosting. Almost all “turbo-chargers” or “super-chargers” in aircraft will require high octane and therefore lead. And it is not possible to compare aircraft engines with auto engines.
7.      Most traditional “legacy” piston aircraft are not compatible with ethanol due to chemical breakdown of fuel lines, carburetor diaphragms, sealants in fuel tanks, etc. This includes most Continental and Lycoming engines in aircraft such as Piper, Cessna, Beech, and Cirrus. Ethanol is also “hygroscopic”, so it attracts water – not good – that water can find its was all through the fuel system and it will not necessarily drop to the bottom and exit with a simple drainage of a cupful at the sump valves.
Corrosion, misfire or engine stoppage can result from water in fuel. Ethanol can be a very serious maintenance and safety issue in aircraft not specifically tested and approved for ethanol at some specified percentage level! Even if approved, ethanol demands very careful observation by the
8.      Many smaller and newer aircraft and engines do not require lead for detonation safety, and they may be approved for ethanol to some level, such as 5% or 10%. Example – Rotax. But there is a move to increase ethanol content to 15% in U.S. mogas, so this would be “unauthorized” for aircraft engines that are approved for 5% or 10% ethanol. At your local mogas pumps,   look for a label such as “E-10” for 10% ethanol.
9.      Avgas meets a much greater array of tight specifications than mogas, and it is subject to far tighter restrictions and documentation with its manufacture, distribution, and retail delivery. So you can count on much better consistency and quality and longevity. Obviously, aviation safety is a much more serious business than auto safety.
10.     Avgas usage in the USA is one half of one percent of mogas usage, so avgas is considered as a “boutique fuel”. Also, since it contains lead, it is not allowed to be distributed efficiently and interchangeably with other fuels – it is isolated to special delivery pipes and tanks all the way to the customer. Can you spell – added cost! And remember that the cost of avgas has roughly doubled in the last decade – it represents roughly a third of total cost for most power planes.
11.     For all these reasons, avgas is generally between one and five U.S. dollars per gallon more than mogas. (Typically, a dollar or two more at smaller airports.) And the small & diminishing market has caused only one grade of avgas to be available – 100LL (100 octane low lead) so a single fuel is formulated to serve all piston planes, even though many planes could use a lower octane. Until recently, only a very small number of planes have run on mogas, but interest is growing due to cost.
12.     Environmental extremists are lobbying very hard to remove the lead from avgas. This source of lead now represents about 50% of lead emissions into the environment in the USA. Of course, that must be viewed in context – it is a miniscule quantity! Much good work has been done over the last few decades to remove about 99% of the environmental lead from other so    urces – that’s a really good thing. Removing lead from mogas several decades ago was accomplished with minimal hassle and cost to the consumer. But given the tremendous difficulty and cost of eradicating the last (very small) amount of lead, there is a serious question as to whether this is “reasonable”. This is called the “law of diminishing returns” – one gallon of avgas contains about half of one gram of lead. A dollar bill weighs one gram. But sensible or not, moving to remove lead is the current trajectory, no matter how difficult and costly. And it seems to be gaining political momentum in the USA.
13.     New formulations for unleaded avgas have been under development for a couple of decades already, with no clear and attractive solution because the specification challenges are huge. In the last couple of years some promising chemistries have emerged, but none are yet certified and it is a very long and very expensive   road – similar to certifying a new aircraft model. There is also a broad array of political activity surrounding all of this, with many special interest groups pushing in different directions. The FAA and EPA are also heavily involved, and there is litigation from the environmental extremists. No-one knows if or when any acceptable new unleaded avgas formulations will emerge, and consumer cost might be at least a dollar more per gallon than 100LL, or more. This is considered “unaffordable” by many power plane pilots, and it could conceivably destroy the personal power plane market.
14.     Roughly 70% of the fleet of conventional piston aircraft in the USA does not need the current 100LL because their compression is low and they have no significant risk of detonation. These are also the smaller aircraft that are often used for recreation and are very cost-sensitive, so there is pressure to split away from 100LL (or any future unleaded high octane avgas) and find a cheaper solution such as high octane unleaded mogas.
15.     The 30% of piston planes that must have 100 octane to be safe could potentially be modified to run on lower octane. But this would cost thousands of dollars per plane and would likely involve performance losses.
16.     In the USA, there is already a high octane unleaded mogas fuel that is acceptable to the low compression power planes. It is used by many race cars, race boats, and vintage cars. However, ethanol becomes the problem.
There is a lobby group trying to remove ethanol from high octane mogas, but with little success so far.
17.     Unfortunately, ethanol was touted as a great advance because it can be a biofuel and “renewable”, but it has several bad side effects. Not only are there the chemical compatibility and water absorption issues, but corn diverted to make ethanol has raised global food prices, with starvation as the result in Third World countries. Huge government subsidies for corn ethanol in the USA are now in decline, which questions the economic viability of ethanol.
18.     Standard certified aircraft in the USA must only use “approved” fuel, and the process to approve any fuel for each make & model is extremely costly and protracted. No aircraft manufacturer is eager to invest huge money to test new fuels, because there may be no return for that investment. Experimental aircraft have more freedom.
19.     100VLL – “100 octane Very Low Lead”. This is a very recent fuel spec that simply has 20% lower lead content than 100LL. In fact, most avgas pumps have been dispensing to this new standard for years already, with no adverse results in use, so it really isn’t a technical change of any significance. But it does demonstrate real progress already made by the industry to reduce lead.

So where are we in today’s world in the USA?

Most tow planes have little choice but to use 100LL (or 100VLL) until a new fuel is approved – no matter what the price. Motor gliders usually have the choice of avgas or mogas, depending on what is authorized or recommended by the engine and airframe manufacturer. But as you can see, if the pilot is interested in mogas s/he should also carefully consider other safety factors such as ethanol, octane rating, water absorption, vapor lock, and shelf life. No aircraft manufacturer has tested and approved the vast array of mogas chemistries that are being dispensed around the world. In effect, pilots using mogas may be “test pilots”.
I recently asked DG and Solo for their comments on all of this, because the fuel situation in the USA is markedly different from Europe and elsewhere around the world. So here is a summary of the major points from Herr Emmerich of Solo and Herr Dirks of DG:

1.      Avgas and mogas are both approved for DG/Solo sailplanes in the USA.

2.      The two-cycle Solo engines do not operate at high compression and so they are not prone to detonation, if ignition timing is correct, and it is “fixed” in these engines. Solo has not seen any detonation problems so far with either avgas or mogas, but they have only tested European mogas.

3.      Solo has successfully tested their engines with mogas and 15% ethanol, but that is a short term test and does not include testing the fuel system components of each airframe design over the long haul.

4.      Ethanol attracts water (airborne humidity) so Solo recommends long term mogas fuel storage should be in sealed containers if longer than 4 weeks. Practically speaking, this means removing the fuel from the aircraft at the end of each season.

5.      Solo reports a potential long term ethanol problem with polyurethane hoses. So careful and regular inspection by the pilot and/or mechanic is advised. Solo is also changing to rubber hoses for their non-aviation engines, due to ethanol.

6.      DG has successfully tested 5% ethanol with their fuel components such as fuel hoses and fuel tank. No greater ethanol content is authorized or recommended. And the polyurethane hoses now used by DG and often retrofitted to older gliders should not be used with more than 5% ethanol.

7.      DG uses Polyurethane fuel hoses in the following gliders, and therefore 5% ethanol is definitely the maximum limit: DG-808C, DG-1000T, LS8-t, and LS10-st. However, the new DG1000-M has rubber hoses, so 10% ethanol is probably acceptable.

8.      Minimum acceptable octane rating for DG/Solo is 95 “ROZ” or 91 “R+M/2” in the USA. That is basically “premium mogas” octane and considerably below 100 R+M/2 octane for 100LL.

9.      DG recommends the best fuel is 91 octane (U.S. measurement) mogas with no more than 5% ethanol. However, this is not generally available in the USA.

10.    DG’s alternative recommendation for fuel is a 50/50 mix of mogas and avgas. This may depend on your local fuel chemistries, but the idea is to reduce 10% ethanol in mogas to 5%, and to gain the benefit of high octane in avgas. This also limits the lead content and fuel cost of using 100% 100LL avgas.

11.     The new emerging U.S. fuels are not recommended because no testing has been done. Examples – Swift Fuel and G100UL, but these are not yet commercially available. Solo may conduct special testing if a new fuel looks like it will become commercially available in the USA and widely used.

12.     There is no demonstrated vibration difference between avgas and mogas in a DG/Solo sailplane. However, sailplanes with Rotax engines may be sensitive to the lead in avgas that may cause detonation and vibration.
However, there is no proven evidence of this.

13.     Avgas odor is considerably less than mogas. This may be considered a significant factor where fuel components are close to the cockpit.

And here is my personal opinion:
I have been using 100% avgas for 12 years in my DG-800B. I can not know if it was responsible for such problems as vibration damage and fuel hose degradation in my glider. However, avgas is consistent in terms of performance and quality. I have removed one cylinder head and found only small deposits of lead and carbon after 100 hours of engine time. It is almost impossible, and certainly inconvenient, to find high octane mogas with no ethanol in my area, so I will not use mogas. The extra cost of avgas is not significant for a self-launch sailplane.

I suggest pilots with other engines – rotary, Rotax, Lycoming, Continental, etc – should consider this topic, seek professional advice, and inspect your engine and fuel system very carefully and frequently.

As for my Bonanza, and most tow planes, I think we will need to stick with avgas and stay aware because unpleasant things may be in the works for our fuel cost. For power planes authorized for mogas, it is a viable and less expensive solution. Just beware of ethanol and other issues noted herein!