Autogas - use and personal experience

[Mike Smith]

I used to have a C-140A and used autogas in it. The 170A I bought has an autofuel STC and I was wondering what kinds of experience other 170 people had with autogas. Since the price of autofuel went down it makes it reallllly attractive again. I flew 400 hours on my 140 with it and never had a problem. With the STC everything on the airplane has been brought up to spec.'s for the autofuel. Any other comments from autofuel users out there?

Also, I used to have a autofuel airport guide - but I probably gave it to the guy who bought my 140. Anyone know of anything like that available on the internet or to purchase?

I've looked at the Petersen website and read all those facts and studies. I was just looking for some practical experience/comments from 170 pilots.

Thanks,
Mike Smith
1950 C-170A

[Larry Holtz]

Mike
I have the EAA autogas STC on my '48 170. I use autogas all the time unless on a long crosscountry with none available. I have never had any trouble with using it. I have experimented with putting mogas in one tank and 100LL in the other and switching back and forth during cruise looking for change. None that I can tell. For airports with autogas, go to airnav.com.

[zero.one.victor]

Mike,I burn primarily cargas in my ragwing also. No problems. Most of my flying is local,so I just do the jerrycan thing. On trips I burn 100LL. A nearby airport (Arlington) has a "Mogas" pump,but they charge almost $2 a gallon for it. The 100LL is only 50 or 60 cents more! So I rarely buy cargas at an airport. I think it's probably a good thing to burn some 100LL once in a while anyway,as recommended in the Petersen STC paperwork.

Eric

[Bruce Fenstermacher]

Mike
800hrs in 0-320 Cherokee, 250hrs A-65 Cub, and 300hrs and counting in my c-145 '50 170, all car gas, no problems. I also add Marvel Mystery Oil which I'd due if I ran 100LL. Do a search there are lots of posts pro and con here.

[doug8082a]

Speaking of Mogas - I ran across an article on MSNBC.com which spoke of the introduction of ethanol to mogas. Apparently, the Senate is working on the energy situation and has approved language that could require ethanol to be used in auto fuel by the year 2012. Does anyone how this would affect aircraft mogas users - if at all?

Here's a link to the article. It discusses the ethanol issue and also nuclear energy. The main bit on ethanol is near the bottom.
http://www.msnbc.com/news/921021.asp?0dm=C24AN

[Gooney]

HI Folks, I have a most interesting article, published by the head of Shell Petroleum Aviation, UK all about the implications of using MOGAS, it was published in Proffesional Aviator and is 4 pages long, if anyone would like me to send it on please send me a PM, you will find it very interseting.

[Mike Smith]

Bruce,
Whats the reason for the Marvel Mystery oil? Are you putting it in the engine oil are adding it to the fuel tank? I've heard of Marvel Mystery oil but I'm not sure what the "goal" is when people are adding it to the airplane ... or where they are adding it.
I'm a believer in the autogas use. I just want to get all the information I can about it since I haven't used it since 1995 when I sold my C-140A.

Mike
1950 C-170A

[Bruce Fenstermacher]

I use MMO in the gas as a top lubricant to prevent or slow the process of sticking valves. 4 ounces per 10 gallons.

Again a lot of posts already here about MMO. Lots of people think it does nothing.

All I know is with out it on my A-65 I had 2 stuck valves (cleaned the others) after my first 100 hours. Flew the next 150 hours with no problems. Almost the same thing with my c-145. I try to use it every tank but if I miss a few I don't panic.

Nothing has change with car gas use since you flew it in your 140A.

[Gooney]

Hi Folks

I am having difficulty with the web site so if you need the 4 page article on MOGAS vs AVGAS please e-mail me on ianpentz@emirates.net.ae (I don't like leaving my e-mail but what to do George?)

Ciao

Ian
(Gooney)

[Mike Smith]

Here's the article Gooney spoke of, it's kind of lengthy and definately biased against mogas. Maybe it could be that it's written by an executive of a large oil company? Read through it and let me know what you think. The interesting things to think about for me are the hot/high altitude aspects (flying over 6000 feet).

Mike Smith
1950 C-170A

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Avgas or Mogas? Is there any difference?

I am often asked about the use of Mogas as an alternative fuel for
General Aviation. Is it a cheaper alternative to Avgas 100LL? Surely
Mogas better, as it gets rid of the problem of Lead fouling? Is Mogas
more like the fuels that were available in the 1940, a period when many
of the engines used today have their roots? Most importantly, is it
safe?
Your editor, Philip Whiteman, asked me to write about the differences
between these gasoline products and their suitability for aviation use.

One of the differences that most people know about is that Avgas 100LL
contains Lead, whereas Mogas generally does not. Avgas 100LL contains an
additive, known as Tetra Ethyl Lead (TEL) which is there primarily to
increase the Octane of a fuel. See the separate panel for a fuller
explanation of what an Octane rating is, and how TEL works. Up until
quite recently, TEL was also used in road fuels to boost the octane
rating but, as some of the Lead by-products formed during combustion
have proven to be toxic, TEL was banned from road fuel (Mogas) within
Europe at the end of 1999.

This left Mogas with a problem; what was going to compensate for
reduction in octane performance? In order to offset the lack of TEL in
Mogas, the fuel formulators have had to change the cocktail of
components present in unleaded Mogas base fuel to increasing the octane
to acceptable levels. Unfortunately, like most things in life, there is
a compromise. Some of the chemical species used to boost octane, leave
the fuel weaker in other areas such as energy content, the fuel's
reaction with water, adverse seal swell, carburettor icing and storage
stability. Most of these issues are not a problem if the fuel is
consumed quickly, the rubbers and plastics in the fuel system are
designed to use this type of fuel and it is restricted to ground use -
all common features of automotive gasoline consumption.

With Avgas 100LL the base fuel is optimised for the demands of aviation.
This means that the base fuel is somewhat different (even ignoring the
TEL) from Mogas. All hydrocarbon fuels are a soup of molecule shapes and
sizes, all with varying properties, but Avgas 100LL base fuel is
probably as close as the industry gets to a synthetic fuel. This is
driven by the exacting performance requirements that are demanded by
aviation applications. These encompass a number of key parameters.
. Octane (in practice about 106 lean mixture and 130 rich mixture
ratings to compare with the 85 MON figure for Mogas)
. Energy content (around 5% higher than for Mogas, so 5% more
power)
. Long term storage stability. Prevents gums and lacquers forming
in delicate fuel system components (not a problem in road fuels, as they
tend to be used within a couple of weeks of manufacture)
. Resistance to forming a vapour in fuel lines - especially at the
reduced pressure of altitude
. The fuel composition and performance must not be affected by the
presence of water. Some Mogas blends use alcohol as an octane booster
but, as we all know from mixing water and whisky, alcohol dissolves in
water and so alcohol is removed from the fuel and into any free water
present in the fuel tank. The net result that the octane performance of
the fuel disappears with the water drain sample.
. The boiling point range of the fuel is limited. Low boiling
point fractions contribute to volatility problems such as vapour lock
and excessive carburettor icing. High boiling point fractions take a
long time to vaporise and can result in excessive wet fuel washing the
lubricant off the cylinder wall.

Because of these demands, not only is the composition of the base fuel
limited, but also basic properties, such as the boiling point range, are
restricted. Mogas has a typical boiling point range of 30 deg C to 210
deg C, whereas Avgas 100LL has a range of 70 deg C to 170 deg C. This
helps Avgas 100LL overcome all the volatility related problems, and the
restriction of the heavier fuel fractions helps prevent lubricant
cylinder washing, and poor fuel distribution. Unfortunately, and here
comes the compromise again, it also means that the amount of Avgas 100LL
that the fuel manufacturer can produce out of a barrel of crude oil is
about half that of the Mogas yield.

I recognise that some engine manufacturers (notably Rotax) recommend the
use of unleaded fuels, as their engines tend to suffer with Lead
deposits in the combustion chamber, but in most engines, Lead fouling
can be avoided (see separate panel). The fact is Avgas 100LL has the
properties it does for good safety reasons, and so the oil companies
continue to produce it to meet these rigorous standards. If the refiners
felt that they could use common Mogas refining streams, then they would,
as it would greatly reduce manufacturing costs. The fact that this does
not happen should tell you that there is good reason not to do so.

It seems that the practice of using Mogas as substitute for aviation
fuel is most common in the PFA or experimental categories, often because
many pilots fly from grass strips, which have no Avgas fuelling
facilities. For convenience, it is common for these pilots to drive down
to their nearest road forecourt and fill up a few jerry cans with
unleaded Mogas. After all, experience tells you that this fuel has
worked in your car for as long as memory serves, without any problems,
right? Well perhaps so - but this does not mean that it is ideal for use
in the average light aircraft.

First of all, let's look at who can use Unleaded Mogas for use in their
aircraft. All C of A aircraft and most experimental / Permit to Fly
aircraft are certified to use Avgas 100LL as their primary fuel. Some
aircraft have subsequently been granted Supplementary Type Certificates
(STC's), which allow them to use unleaded Mogas (typically, in Europe,
this is the normal 95 RON unleaded Mogas to EN228). These STC's are
awarded by either the CAA, or the PFA, in the UK. An STC is only granted
after an assessment is made of fuel system for the airframe and engine
combination concerned. The primary concern here is to consider the
potential for the fuel system to vapour lock.

Note that the granting of an STC is not necessarily an approval by
either the airframe or the engine manufacturer to use Mogas, rather it
is granted by the airworthiness authority for the type. Most engine
manufacturers - with the notable exception of Rotax - do not approve the
use of Mogas, so be aware of potential engine warranty invalidation.

So now you have your STC, what other restrictions are placed upon you?

In addition to the STC, the CAA Airworthiness Notice 98 covers the use
of Mogas in aircraft applications. This document places several
restrictions on the use of Mogas. These restrictions cover some fairly
mundane procedural items such as the need to placard the cockpit,
aircraft handbook and fuel filler that Mogas is in use, but there are
also some significant operational restrictions too.

Remember that boiling point range for Mogas? Well that significantly
contributes to an increase in Vapour Pressure of Mogas. A high vapour
pressure fuel will form a stable vapour at higher pressure (meaning
lower altitude) or a lower temperature than a low vapour pressure fuel.
So what you might ask? Well if a vapour bubble forms in a fuel line,
then fuel flow past the bubble is restricted or, in the worst case,
prevented.

Vapour pressure of the fuel is a more significant consideration for
aircraft applications rather than cars, as most of us spend the majority
of our time flying above ground level. This means reduced ambient
pressure, which increases the risk of vapour lock. To put some numbers
on this, the Ried Vapour Pressure of Avgas 100LL is limited to a maximum
49 kPa , Summer production Mogas is 70 kPa, and Winter production Mogas
is up to 100kPa. Yes Mogas does change its composition between Winter
and Summer too, so you can never be too sure what you are buying. By the
way, the International Standard Atmosphere sea level pressure is 101.3
kPa - very close to the Winter Mogas RVP.

I hear many people saying that if they have an STC, then their aircraft
has been checked and so will not suffer from vapour lock. Arguments are
that gravity feed tanks, or boost pumps in the tanks - which give
positive pressure in the fuel lines, makes them immune from vapour lock.
Well not so. Even with an STC, Airworthiness Notice 98 also puts some
restrictions on your flying when using Mogas.

There is a maximum pressure altitude of 6,000 feet and a maximum fuel
temperature limitation of 20 deg C for operating with Mogas. Notice that
this is a fuel temperature limitation and not an air temperature
limitation. The tank temperature can be significantly above 20 deg C if
an aircraft has been sitting in direct sunlight - especially if it is
painted a dark colour.

Picture this. It is a beautiful sunny day and you have just flown to
your favourite airfield to enjoy a nice tea and a slice. The air
temperature may be in the mid teens, but your aircraft has been sat in
the sun for a couple of hours. What is the temperature of the fuel? Have
you ever even considered it? Assuming you do, and you suspect that it is
over 20 deg C, what are you going to do next? A.N. 98 says you should
not fly, but what are your options? Are you going to stay overnight to
let things cool down, drain the tank and fill with Avgas 100LL, or are
you going to try to get home? I bet I know what most people would do.

Now you need to taxi to take off, so the engine compartment is warm and
the fuel flow is low, meaning that the already warm fuel is heating up
even more in the fuel lines under the cowling. There is just enough fuel
in the carburettor to get to take off speed before the vapour lock in
the fuel line shows itself and the engine dies.

All sounds a bit far fetched? Don't you believe it - it has happened and
it has killed people.

One final thing I did not mention is that an STC to run Mogas is invalid
for public category aircraft, only private and Permit to Fly aircraft
are allowed to use it. Ask yourself why this might be? Because the CAA
feel that to expose the paying public to flying on Mogas is too great a
risk.

I know that many people tell me that they have used Mogas for years
without any problems; I feel that they have taken a chance and been
lucky. I am happy for them to make the decision to use Mogas if they
wish, but it really should be an informed decision and one where the
restrictions are also observed.

Fuel Quality and Handling
You may think that taking fuel from a busy forecourt would result in a
better product quality than taking Avgas 100LL from an airfield with
perhaps 5% of the turnover. Surely there is less chance of dirt or water
contamination? Not so.
There are very few restrictions in place over the quality and handling
of bulk road fuels, in terms of testing, handling or filtration.
Aviation fuel installations are obliged to comply with a number of
auditable regulations, which control the quality standards for handling
aviation fuels on airports. Areas covered are not only standards for the
fixed equipment, tanks, hoses, pumps themselves, but also routine
inspection procedures for the equipment. From the product quality side,
there are also testing and filtration requirements, which include not
only the stipulation to check airfield bulk fuel tanks every day for
sediment and water but also to check incoming fuel supplies for
contamination.

On top of the mandatory regulations, Shell has decided that there should
be additional safety standards for all sites, which they either run
directly or supply. Firstly Shell operates dedicated fuel delivery
vehicles for aviation fuels. So if you see a Shell Avgas tanker on the
road, this tanker has always carried Avgas, so there is no risk of cross
contamination. Furthermore, multi compartment vehicles carry the same
fuel in each compartment - so Jet A-1 or diesel cannot be carried in a
compartment adjacent to Avgas - thus further reducing the risk of
contamination across a bulkhead. As a final check at the airfield, the
fuel is checked prior to discharge for fluid contamination, particles
and water. If the fuel fails any of these checks the load is rejected.
On the airfield, Shell aslo ensures that the final filter before
aircraft loading is of a special type, so that if all the other testing
and sampling fails, the fuel delivered will still be clean and dry. This
"filter monitor", as it is known, is a fail safe device which chemically
absorbs water as well as filtering out dirt, so that the fuel supplied
to the aircraft is filtered to1 micron and contains less than 15 parts
per million of water.
To give you some idea of this level of filtration, the human eye can see
particles down to about 15-20 microns in size, and red blood cells are
about 5 microns in diameter. With the audited quality chain in place,
the aviation fuel customer can be confident that the fuel supplied is on
grade, not contaminated, clean and dry.
None of these safety features are included in the Mogas supply chain.
Finally, the pump and aircraft are electrically bonded prior to
refuelling to ensure that the static charge on the aircraft (which can
be considerable) is the same as the fuel installation. If this is not
done, then a spark from the hose end can easily cause a fuel fire. How
may Mogas users take such precautions and how many aggravate the problem
by using plastic fuel containers and funnels?

Octane Rating
Octane rating is a measure of how resistant a fuel is to pre-detonation
or 'pinking'; the higher the Octane rating, the more the fuel/air
mixture can be compressed without detonation happening. To make this
clear, octane rating is not a measure of the amount energy in the fuel,
but is a measure of its resistance to detonation.

The way the scale is calculated is by comparing the resistance to
detonation of the fuel is to a percentage mix of a substance with a very
a known very high resistance to detonation - octane - and one with a
poor resistance - normal heptane. So a 85 MON octane fuel gives the same
performance in the MON test as 85% octane and 15% normal heptane. For
octane numbers above 100, the number is referred to as a Performance
Number and is compared to a scale of 100% octane with varying levels of
TEL.

The advantage or higher octane fuels is that a higher compression ratio
or supercharging ratio can be used, which then leads to a higher engine
cycle efficiency, and in turn means more power output for a given fuel
burn.

If you managed that, then I am going to now confuse things further.
There are four principal ways to measure Octane rating, Research Octane
Number (RON), Motor Octane Rating (MON), Aviation Lean Mixture and Rich
Mixture ratings. The difference between these methods centres on
different test engines, the speed the test engine is run, the load the
engine is put under and the fuel / air ratio used in the test. All of
these things make the test engine behave in different ways and so
varying these values produces different octane values.
Road fuels tend to be measured on a RON scale, for which unleaded fuels
(such as premium unleaded Mogas EN228 and super unleaded Mogas EN7800)
tend to be 95-98 RON but are only 85-87 MON respectively.
Avgas 100LL is measured using the Aviation Lean Mixture test (which is
broadly similar results to the MON method), but Avgas 100LL also has a
Rich Mixture Performance Number. The Lean Mixture rating for Avgas
100LL is a minimum of 100 octane (15 octane higher than the comparable
85 MON for unleaded EN228 Mogas), but Avgas also has a Rich Mixture
Performance Number of 130 which allows higher supercharger boost
pressures to be used without detonation occurring. This is particularly
a problem when using high power settings at low altitude, for example,
during take-off.
This increase in detonation performance at rich mixture settings comes
about because of a number of different factors. TEL tends to be more
effective at detonation suppression when high inlet temperatures are
encountered, such as in supercharged or turbocharged air-cooled engines,
but also the abnormally rich mixture cools the gas in the cylinder so
that it has less tendency to ignite spontaneously - the cause of
detonation.

Preventing Lead Fouling

Lead was previously believed by many to work by slowing the flame front,
thus leading to a slower pressure rise in the cylinder. While general
flame-front propagation speed does affect octane ratings, TEL does not
significantly affect it. Tetra Ethyl Lead works by reacting in front of
the flame to form Lead Oxides, which shield the end gas from the heat of
the flame front. Also, according to research, Lead effects the pre-flame
reactions by retarding branching sequences, which decompose the unburned
fuel into more reactive species, which would encourage them to ignite
spontaneously. TEL is a very effective additive in increasing Octane
rating and nothing has been discovered to date which has the same
potency.

Now we're back to our old friend the compromise again. Leaded fuels will
always burn with more deposits than unleaded fuels.

Lead Oxide (which you remember is formed ahead of the flame front) is a
solid up to about 900 deg C - well within the wall temperatures inside a
piston engine. In order to prevent these deposits from causing a
problem, a Lead scavenging compound is added to Avgas 100LL - this
compound being Ethylene Dibromide. This scavenger is designed to react
with the Lead oxide to form Lead Bromide, which is more volatile -
becoming a gas at around 200 - 250 deg C. This is a low enough
temperature to ensure that the Lead is removed from the engine as a gas,
and it subsequently goes back to the solid phase as the exhaust gas
cools in the atmosphere. As a point of interest the pale ash / tan
coloured staining that is often seen leading from the exhausts of high
powered engines, such as those found on the Warbirds, is in fact Lead
Bromide.

The problem is, the Ethylene Dibromide scavenger is not very effective
at reacting with the Lead at low temps. This means that if the
combustion temperature is too low, then the scavenger will not react
with the Lead and solid Lead Oxide deposits will form on spark plugs and
valve stems.

It is clear that temperature is a key factor in preventing Lead fouling
and with this in mind a "lean burn shutdown" should be carried out to
help prevent Lead deposits.

Engines that have been involved with long, low power descents, or have
taxied for some distance, can have quite low cylinder temperatures and
this - as we have seen - can lead to lead fouling. To remedy this, once
on the aircraft is on the stand, the engine speed should be kept between
1000 and 1200 rpm until the engine temperatures have stabilised (or at
least 60 - 90 seconds). Once the temperatures are stable, the engine
speed should be increased to 1800 rpm for a period of 15 to 20 seconds,
which should generate enough temperature to burn off any deposits. Once
this period is past, the engine speed should be reduced to 1000 - 1200
rpm once again and then immediately shut down using the mixture control
without bringing the power back to idle.

Similarly, when the engine is started, the warm up should be conducted
at 1000 - 1200 rpm to try to get the temperature of combustion high
enough to prevent Lead fouling.

Plugs

Are the plugs of the correct type? Champion have plugs such as the
REM-37-BY plug, which is supposed to reduce the effects of Lead fouling,
as this is a "hot" rating plug and has a bigger void around the
electrode. If Lead fouling is a continual problem in your engine then
seek out an approved "hot" rating plug, or fine wire spark plug, to
reduce the risk of fouling.

Whilst manufacturers like Rotax recommend the use of unleaded fuels in
their engines, Lead fouling can be helped even here with the selection
of a hotter spark plug. For example if you are running a Rotax 912,
using the recommended NGK DCPR8E plug and you find it is fouling, then
try a hotter plug like the DCPR7E (also allowed by Rotax). This should
help as the plug tip will run hotter and discourage the Lead Oxide
deposits from forming.

Finally, are the plugs rotated regularly? It is common knowledge that
the lower plugs in a dual ignition engine will tend to run dirtier than
the top plugs, even in normal operation - so rotating plugs every 25 to
50 hours is recommended by the engine manufacturers. This should result
in a self cleaning action, but cleaning the pugs every 100 hours should
also help. Note that the plugs should always be swapped in magneto sets
as the plugs themselves can herald clues about the magneto condition and
swapping the plugs randomly loses this information.

[N1478D]

It is so hard to tell if an article is propaganda, based on half-truths, or is accurate.

[GAHorn]

well, a clue is, ...does the author have any particular bias by which he might profit?
In this case, I don't see any. I do not recall any aircraft accident where 100ll was shown to be the direct cause of engine failure. There have been several that blamed autogas useage. In fairness, I believe that in most cases it was due to contaminants in the fuel, or failed fuel system components due to ageing/deterioration.

[Gooney]

Hi Folks

I have never used MOGAS in my 25 years as a proffesional aviator, so I have no basis on which to place any value to the use of MOGAS, but when a 4 page article filled with factual info like this comes out, you would be ignoring it at your own peril, please take something out of it at least.

It has some obvious factual info especially with regards to altitude usage and life usage of MOGAS that I think are valid. Don't forget this guy, is in test and development in fuel, he works with fuel evey working day.

There is an old saying in aviation and those that have been flying for twice as long as me will back me up says, when there is doubt there is no doubt, why not take that doubt out by simply paying a extra few cents for quality controlled fuel?

Just my 6 cents worth, but as I said earlier if it goes about the fact that you are trying to save on money, you are been penny rich pounds foolish!

Those with no alternatives well that I can understand, don't let that fuel stand in your tanks over long periods. Folks lets look after our endangered speci!

Ciao

Ian
(Gooney)

[Dave Clark]

For most aircraft owners who fly 100 hours per year or so the cost of fuel is a relatively small part of the overall cost of flying each year. I decided a long time ago to not get stressed each time I get nicked to fuel the plane but just accept it. It makes flying a whole lot more fun plus I never don't fly because of the cost of the gas so I fly more. I think the article has some merits. From personal experience the auto fuel burned far better and cleaner in the O-470 than it did in the C-145 I don't know why. Perhaps running temperatures. But if I still had the C-145 I would not use auto fuel very much. For those who do great, and I won't flame them.

[170C]

I had Gooney send me the 4 page article that Royal Dutch Shell apparently had someone in the company write. As a person who has burned a lot of mogas in the past 20 yrs, but almost none in the past 3.5 yrs, I found the data pretty interesting. I had heard a lot of the data from other sources previously. For example the fact that local fuel co's cannot haul 100 LL in their tankers without first purging them and maybe going a bit furthur in the attempt to make sure the fuel isn't contaminated. However, when hauling auto fuel (all 3 grades of unleaded) they can have just delivered Diesel or JP- 4, etc and don't have to go to any effort to purge the tank as the minor amount of the other fuel is so diluted there is apparently no problem for automobiles. Although I never had any problems with auto fuel other than some O-Ring swelling, I have to admit the article sure makes me think. Sure would be nice to be able to read an article discussing the subject where the company didn't have a potential vested interest in selling the more expensive, to refine, fuel. Wonder if EAA & Petersen's STC's have been updated to deal with the so called formulated auto fuels we now have?

Ole Pokey