Mixing Mogas and Avgas

[Blue Albatross]

This might sound like a moronic question, so forgive me in advance.

My plane is certified to fly on both Avgas and Mogas. I am wondering if it is safe or unsafe to mix fuel types in the tanks.

For instance, I fill up on Mogas, stop and refuel with Avgas (because it's all that's available on the field).

Can this mixing cause problems either with the tanks, tubing or within the engine itself?

Thanks

[S-Works]

Nope, it's fine.

[Pilot DAR]

Perfectly fine, and depending upon which engine, and how you lean it, mixing could actually be good for it. If you read deep into the paperwork which came with your STC's you will probably find the reference to mixing the fuels being approved.

[Dan Winterland]

It's OK. Used to do it with my RF4 all the time.

[BossEyed]

I'd have thought you'd use JP-5 in that, Dan. Did the pictures come out OK? :ok:





(Agree - mixing no problem if aircraft OK for MOGAS)

[BackPacker]

I would think that there's one caveat though and that's probably that up till about a 90/10 mix you will be limited by the worst properties of both fuels.

I have no specific experience with mogas/avgas, but the Diamond I fly is certified for both Jet-A and Diesel fuel. However, when it comes to cold starting/cold weather operating, even with a 90% Jet-A, 10% Diesel mixture you've got to apply the temperature limits as if you are flying 100% Diesel.

I would guess you would find more or less the same thing with, for instance, vapour lock. If there are specific (altitude?) limits when using mogas because of vapour lock, you'll probably find that they apply as well to a 75-90% avgas, 25-10% mogas mixture, or something along those lines.

[Rod1]

Experimenting on the ground with my MCR, you can get it to suffer vapour lock fairly consistently if you know how. Run the same test with 30% Avgas 70% Mogas and it will not play. In practice I have never had a problem with vapour lock in 150+ hours of operation. I do switch to Avgas in very hot weather.

Rod1

[Pilot DAR]

Operate your aircraft in accordance with the Mogas STC. If there are limits, follow them. I would be suprised to hear that there are still altitude or temperature limitations associated with vapour lock, but I don't know all the wording of the STC's. In the very early days a few approvals did have altitude and temperature limits, but I believe these are long gone. We did a lot of testing to confirm that (for many Cessnas anyway) such limits were not required.

That said, I fly 100% Mogas all the time, unless I'm away, and have to refuel with Avgas. I have done flown Mogas in my plane for 23 years, and 2600 hours. Add to that, another 500 or so in other Mogas powered aircraft. In the 182, I'd fly out on pure Mogas 5 hours, and back on a fill up of Avgas. Some days were 70F at 11,500 feet. I never had a problem.

You will not notice any operating difference, other than the cost.

First: Follow STC limitations and instructions,
Second: Use clean, fresh Mogas, obtained, transported, and handled safely,
Third: Don't worry!

[CanAmdelta1]

As PD alluded to, the handling of MoGas for automobiles is not as carefully done as AvGas, meaning higher chance of contamination by water, particulate matter, dealership cross tankage contamination etc.
Also the issue with high % Ethanol in some MoGas.

Reid Vapor Pressure is mostly steady on AvGas 5.5-7 but MoGas RVP can vary widely depending on season and location, say for instance early summer in Canada vs early summer in Florida. I believe this may have been how vapor lock and altitude concerns started.
Also the difference in octane level from Avgas to Mogas can cause delta temperatures in engine.

As previous threads above mention, follow STC and be aware of variables.

[Big Pistons Forever]

Operate your aircraft in accordance with the Mogas STC. If there are limits, follow them. I would be suprised to hear that there are still altitude or temperature limitations associated with vapour lock, but I don't know all the wording of the STC's. In the very early days a few approvals did have altitude and temperature limits, but I believe these are long gone. We did a lot of testing to confirm that (for many Cessnas anyway) such limits were not required.


I think you have to be carefull with generalizations. I have a MOGAS STC for my Grumman AA1B which I basically use as an emergency contingency if I get stuck at an airport with no AVGAS. The STC paperwork clearly states that MOGAS use above 24 deg C is prohibited. This is obviously very restrictive during the summer months. Maybe Cessna's are less restrictive due to their gravity feed systems but I would advise anyone flying a low wing , or non Cessna high wing aircraft on MOGAS to make sure there are no temperature restrictions.

[Pilot DAR]

use above 24 deg C is prohibited

Thanks, that's a new one for me, I will adjust my thinking!

I used to fly both an AA1 and AA5, the latter which the owner occasionally fueled with Mogas during the winter. Though I should have familiarized myself with the legallity, I was not asking as hard as I should have. He asked me to ferry it to it's new owner upon it's sale. He told me it had lots of gas, but it did not. So I filled it to the brim with Avgas, on his account.

The new owner and his check pilot ran it out of tht same gas later the next day, and wrecked it. They got out only embarrassed (tough plane).

The seller phoned me in a panic that night, asking what kind of fuel I'd put in... "Full tanks of Avgas, on your account!". "Oh thank goodness, they crashed it today, and I did not want the investigators finding Mogas in it!". My plan to p1ss him off had failed - he was pleased with my fuel choice!

Problem was the new owner had no license, so suddenly the check pilot was the pilot of record (which he had not expected), and guilty of running out of fuel. Thus he was subject to the offense of running out of gas, and violated accordingly.

[Dan Winterland]

''I'd have thought you'd use JP-5 in that, Dan. Did the pictures come out OK?''

Ha ha. Fournier RF4 - but I'm sure you knew that. But one CFI checking my logbook in the States for a BFR didn't. He saw an entry which said Denver to Boston, RF4, 30 minutes! He was very impressed. I didn't let on that it was the original Denver, (Norfolk, UK) to the original Boston (Lincolnshire, UK) a distance of about 20nm - easily attainanble in a motor glider!

I was around when the first trials of MOGAS were being done in the UK in the eighties. It was always envisaged that the two would be mixed.

[CanAmdelta1]

Blue Albatross

In the spirit of info sharing so you can make the best decision for your situation and location let me say the following.

My previous post along with this one is based on North America, mainly US manufactured fuel. The Major Oil Companies formulate and operate their refining differently in each country, even if they share a common name.
Shell USA vs Shell elsewhere, BP England vs BP USA

Avgas is manufactured using olefins and isobutane in the presence of sulfuric acid or hydroflouric acid. It is mostly stable as far as gumming and generating unwanted carbon deposits. Also not usually harmful to silver components in engines.

Mogas is made from a molecular cracked feedstock. This makes it necessary to add antioxidents, caustic, butane etc. It is less stable as far as gumming etc. Throw in oxygenates, Reformulated gasolenes, Aromatics and Mogas can have extremely different compositions from the same Brand in different countries. Not to mention independent dealers.

Mogas and Avgas can have vastly different vapour pressures, freezing points and octanes

Also Avgas does not contain Aromatics. Mogas does. Aromatics are destructive to elastomers.

On jet vs diesel...spec. on jet (non combat) is -40C freeze. Diesel is the point at which it gets hazy, called cloud point 10F.

Only you can determine how any of this applies or does'nt apply to your situation, geographic location and your individual plane's STC.
I would hope all of the above is considered by Aero Companies and Regulatory Agencies prior to STC approval. That way if you flyover here on your Mogas you can get back on ours!!:ok:

[Pilot DAR]

I would hope all of the above is considered by Aero Companies and Regulatory Agencies prior to STC approval

The design requirements which must be shown are the same for the aircraft, and the engine respectively, regardless of what type of fuel is being approved.

Back in the day, 100/130 (green Avgas) was an alternate fuel for those aircraft designed an approved for 80/87, it was certainly not choice one for those low compression engines. When the two of those fuels were discontinued for the single 100LL (for the Avgas suppliers convenience I expect) somehow 100 octane leaed Avgas became the de facto fuel for the low compression engines overnight ( I don't recall seeing any STC's for that, and to this day, wonder about the showning of design compliance for 100LL!). Yes, it has half the lead, but is still very troubling for those engines, and causes maintenance difficulties (and forced landings) which did not occur with 80/87 (lead fouled plugs). No flight manual supplement was issued for low compression engined aircraft which were now expected to use 100LL, to explain how to operate the engine to achieve the best relibility.

So, yes, Mogas is a very different fuel to Avgas. But testing has been down in amazing amounts, and Mogas which meets the prevailing American and Canadian (I cannot speak for other nations) has been shown to meet all of the resign requirements for the STC approved aircraft and engines (with some limitations, in some cases, as noted). 30 years of excellent service history are very reassuring too!

[IO540]

On a peripheral topic, what would happen if I put say 10USG of avtur in one tank and then topped it off with avgas - say a 1:5 avtur-avgas mixture?

Detonation might be an issue, perhaps?

How far can this be pushed?

I would have thought that avtur would be soluble in avgas; most things seem to be (except water).

[Big Pistons Forever]

On a peripheral topic, what would happen if I put say 10USG of avtur in one tank and then topped it off with avgas - say a 1:5 avtur-avgas mixture?

Detonation might be an issue, perhaps?

How far can this be pushed?

I would have thought that avtur would be soluble in avgas; most things seem to be (except water).

If you are lucky it will have been a while since the AVTUR was pumped and since AVTUR is heavier than AVGAS the tank will settle out so the 10 Gals of AVTUR is sitting on the bottom of the tank, the engine will then get a 100% dose of AVTUR , suffer explosive detonation and be utterly distroyed shortly after starting but while you are safely on the ground, If you are unlucky you will try to takeoff while the AVTUR is still settling therefore allowing the aircraft to actually get airborne before the engine gets a slug of solid AVTUR and destroys itself....

The Lycoming website has a section whcih deals with the issue of AVGAS contaminated with AVTUR. It basically says that any engine run with fuel that has any amount of AVTUR must be torn down and completely inspected before further flight

[Rod1]

Operate your aircraft in accordance with the Mogas STC.

I have no Mogas STC.

Rod1

[Pilot DAR]

I have no Mogas STC.

The lack of an STC is an indication that either your aircraft type has been found by someone to not be able to accept Mogas for some engine or airframe reason, and thus not approved. Or very simply, it is an aircraft type which has not been shown to comply with the applicable certification standards with Mogas.

That would mean that if you want to operate it on Mogas, you have to do the work to demonstrate design compliance, or demonstrate it is safe and airworthy, as the case may be.

For non certified aircraft, a good clue would be: Is there an STC available for that model of engine? If not, the effort of Mogas trials would be an uphill challange.

[Rod1]

It is approved via the LAA permit system, no STC in sight.

Rod1

[Pilot DAR]

I am not familiar with an LAA permit. In Canada, a "permit" aircraft is very likely not a type certified model, and thus not eligible to be modified in accordance with an STC. I can issue a Canadian STC only to a type certified aircraft, with a C of A. Mogas STC's require a lot of testing, and are disproportionately expensive, relative to the type of aircraft, and operation. Those who have achieved STC approval of Mogas have make expensive investments, and that's why the STC's have a noticable cost.

[Maoraigh1]

STC is an FAA, and I assume Transport Canada concept. Not an EASA one. Temperature limit under EASA/CAA/LAA is, if I remember, 21C. It's placarded.

[Rod1]

Oh, I see, an STC is an American thing!

UK rules;

Unleaded Mogas fuel is restricted by CAP 747, Appendix 8, General Concessions 4 and 5 to operation with a fuel not exceeding 20° C and an altitude not exceeding 6000 ft.

But then you all knew that, right :suspect:

Rod1

[Sam Rutherford]

I have a mogas stc for my Maule - involved paperwork and also changing the fuel pump and some piping.

Many STCs though (still costing USD1000 or so) are nothing but a paperwork exercise - nobody goes near the plane at any stage. So, know the rules, do your best, but if you're stuck (and the rebels are running towards you with guns blazing) then your plane will fly fine with mogas (STC or no STC).

As someone said earlier, a mogas/avgvas blend is often really good for your engine.

Safe flights, Sam.

[Pilot DAR]

(and the rebels are running towards you with guns blazing) then your plane will fly fine with mogas (STC or no STC).

Well, if your plane is a Piper Tomahawk, Cessna 185, or 206 (for three examples of aircraft with which I have do Mogas experience back in the testing days), the rebels might catch up to you a mile or so off the end of the runway.

Many aircraft fly fine with Mogas, a few not so well. Don't experiment, unless doing so is your profession....

[EDMJ]

The lack of an STC is an indication that either your aircraft type has been found by someone to not be able to accept Mogas for some engine or airframe reason, and thus not approved. Or very simply, it is an aircraft type which has not been shown to comply with the applicable certification standards with Mogas.


He mentioned earlier that he flies an MCR, which I suspect is powered by a Rotax 912 which in turn (and according to Rotax themselves) should actually be fed with MOGAS rather than AVGAS (they apparently don't like the lead), hence no need for an STC.

[Pilot DAR]

(and according to Rotax themselves) should actually be fed with MOGAS rather than AVGAS

I have zero knowledge of Rotax engines on Mogas. "Should" won't be an aspect though. If the engine is type certified, and installed in a type certified aircraft, the approved fuels will be stated in the TCDS, an STC, or a government policy letter which specifically mentions Mogas use in that engine. If it's not listed in one of those places. it is not approved.

If the aircraft is not type certified, the answer, and degree of certainty, are much less clear - no "approval" is available...

[popobowa]

A P28-235 (O) has a MOGAS STC, I know that A Beech Debonair IO470 has one...Rotax engines run better on MOGas (they stay much cleaner!). I ran one (Rotax)in Africa until 2000 (TBO) sometimes on 100LL but mostly on (African) MoGAs (which is of very varying Oct and Quality). Running on 100LL showed fouling and heat signs(small molten metal spheres on PLug cathodes).
Running on Af moGas showe dark to black caulifowe-like growth but the engine made 2000tbo time and was still a good runner.
I am tempted to buy a Beech A36 with IO550 (non turbo) and would like to know if I can risc to run it (55-60% cruise) on such MoGas if 100LL isnt availiable (which happens frequently>

[ExSp33db1rd]

I was advised to run the VW in my Turbulent on a mixture of Avgas and Mogas, to replace the lead that used to be in Mogas, but is now unleaded, never had any problem. There was a specific ratio which I can't just quote, but about 25% Avgas if I recall.

I initially ran it on Avgas only - after all, it's an aeroplane, isn't it ? - but then realised that the engine was a cooking standard 1200 cc Beetle, designed to run on the cheapest petrol, so why waste money, but eventually settled on premium petrol with a dash of Avgas.

[Desert185]

Quote:
(and the rebels are running towards you with guns blazing) then your plane will fly fine with mogas (STC or no STC).
Well, if your plane is a Piper Tomahawk, Cessna 185, or 206 (for three examples of aircraft with which I have do Mogas experience back in the testing days), the rebels might catch up to you a mile or so off the end of the runway.

Many aircraft fly fine with Mogas, a few not so well. Don't experiment, unless doing so is your profession....

Only add mogas to one tank. Takeoff with the avgas tank and cruise with the mogas tank while thumbing nose at the rebels. :ok:

[Croqueteer]

With the 0-200 and C90 I have experienced sticking valve problems with mogas. I think some on some overhauls the valve guides are done to too tight a clearance.

[Sam Rutherford]

Just to clarify, Mogas and petrol (as found in the motorway service station) are NOT the same thing!

I don't think that there is any Mogas anywhere in Africa, but could be wrong.

Cheers, Sam.



PS of course, you could always go diesel/JET! :cool:

[Mariner9]

Sorry Sam, not correct, the terms Mogas, petrol and gasoline are used interchangeably in the oil industry. Indeed, the Tanzanian gasoline specification describes the fuel as "Mogas".

I should know - I helped the Tanzanian Petroleum Import committee draw up their latest set of Fuel import test procedures ;)

Most gasoline in Tanzania is imported into Dar es Salaam from India or the UAE, and the specification was drawn up with the expected Tanzanian climactic conditions in mind (i.e. hot).

Temperature limitations imposed by the CAA/FAA on Mogas use in aircraft are not really relevant to Tanzanian Mogas as the distillation criteria and vapour pressure of Tanzanian Mogas differs from US/European fuels.

[Sam Rutherford]

Oil industry I don't know, but in aviation - petrol is not Mogas (most importantly because Mogas cannot contain ethanol, though there are other differences as well).

I'm just trying to make the point that Mogas certified aircraft are not flying legally if they have taken petrol from the petrol station - something a lot of people don't realise.

Cheers, Sam.

[DaveW]

Sam, CAA Safety Sense Leaflet 4 "Use of MOGAS" (http://www.caa.co.uk/docs/33/20120117SSL04.pdf) is relevant here, and doesn't support that.

It quotes CAP 747 (http://www.caa.co.uk/docs/33/CAP747.pdf), and specifically the General Concessions (GC) contained there.

One GC does indeed say:

e) GC No. 2 permits leaded motor
gasoline (Leaded MOGAS) to be
used with certain engine/aircraft
combinations provided that the fuel
is obtained from an aerodrome
aviation fuel installation in full
compliance with the applicable
requirements of the ANO (equivalent
to the storage and quality control
procedures applied to AVGAS).
Therefore the permissions granted
under GC No. 2 exclude the use of
fuel obtained from a filling station/
garage forecourt.

But the SSL/CAP747 goes on also to say:

f) GC No. 3 provides a partial
exemption from the relevant Article of
the ANO to allow certain light aircraft
to use Leaded MOGAS obtained from
garage forecourts subject to the
conditions contained in the Notice.

g) GC No. 4 provides a partial
exemption from the relevant Article of
the ANO to allow microlight
aeroplanes to use Unleaded MOGAS
obtained from garage forecourts
subject to the conditions
contained in the Notice.

h) GC No. 5 provides a partial
exemption from the relevant Article of
the ANO to allow certain light aircraft
to use Unleaded MOGAS obtained
from garage forecourts subject to
the conditions contained in the
Notice.

So it isn't accurate to state that MOGAS Certified aircraft are flying illegally with petrol station fuel - there's more to it than that.

There's lots more interesting and useful stuff in that SSL for anybody who wishes to operate using MOGAS, including the following relevant to the OP:

If you use a mixture
AVGAS/MOGAS with more than 25%
MOGAS, it will be assumed that your
aircraft is using MOGAS.

[ExSp33db1rd]

With the 0-200 and C90 I have experienced sticking valve problems with mogas.That's why I was advised to use a cocktail of Mogas and Avgas, the lead has some lubrication properties for the valve guides - I'm told.

(they apparently don't like the lead) Rotax 912, that is.

Not so much that they don't like it, but after about 10 hours the oil looks like yoghurt. Apparently that's not a problem, but it does look prettty doubtful, and the oil/filter/plug change interval is half the time if run on Mogas. We can't get Mogas, so Premium Petrol from the local filling station on the way to the airport is all we can do, but NZ is threatening to add Ethanol, so then we will be screwed. NZ did something along those lines a few years back - can't remember precisely what - and half the cars on the roads came to a stop with soggy fuel lines, so they had to back-track for "further tests".

"When in doubt, do nowt"

[Sam Rutherford]

Okay, thanks - very interesting. Particulalry as they don't talk about the ethanol 'issue' - which as far as I know is specified in all the STCs (ie the 'Mogas' cannot contain ethanol).

I'm also surprised that NZ fuel is still ethanol free - I thought it was pretty much global to have ethonol there now (in forecourt petrol).

Very interesting thread, thank you for the corrections!

Cheers, Sam.

[cockney steve]

First, the "contamination" bogey.

This is more a figment of imagination, than reality
Yes there was a Supermarket chain that had a batch of"contaminated" fuel.....but that's the only one I know of in over 50 years....I'd venture that deliveries to filling stations will probably be of the order of 100:1 compared to GA._ in terms of volume, it's probably vastly bigger.

Have you bought a fuel-dispensing pump recently?....No, thought not;)
An operator has to have the pump regularly checked and calibrated -woe betide anyone caught giving wrong measure (the tolerance allowed is ~a teaspoonful per Imp. Gallon) So, nobody will willingly compromise their expensive dispense equipment, which has built in filtration and water-seperators, to protect the metering-head..

UK road-fuel is mandated by law , to have (IIRC) 5% Ethanol...this is the maximum that legacy fuel-systems will reliably tolerate. As with the Lead-free furore, there would be major problems converting the nation's cars to accept higher Ethanol content and given that MPG suffers, any advantage is political,rather than practical.
I'm sure tha vast majority of UK GA activities could quite happily continue on standard Pump Petrol with no percieved difference....Aerobatting on a hot day, might be an exception...that, I don't know. Certain designs, with valve-seats cut straight into cast-iron heads (and some valve-guides too!) relied on the Lead, to coat and lubricate the working surfaces...Aeroengines with hardened seats do not have this problem.. Once you get a petrol-engine warm, it'll run on paraffin.....smokey and lower power, but it'll run Morris Vedette marine engine was built that way, as were many petrol/TVO tractors

That shows that petrol-engines are very robust and very tolerant of fuel-quality PROVIDED THEY ARE MANAGED CORRECTLY.

[Mariner9]

First, the "contamination" bogey.

This is more a figment of imagination, than reality
Yes there was a Supermarket chain that had a batch of"contaminated" fuel.

Agree Steve that fuel contaminations at the pumps is rare, but thankfully for me, (my job depends upon fuel contaminations), they are far from rare in the fuel production/distribution chain. My company investigates and resolves around 2000 cases of fuel quality problems per annum world wide, including the aforesaid supermarket incident :ok:.

However, I usually deal with batches of 10's of thousands of tons; fuel contamination at the forecourt pumps is extremely rare - we nearly always catch it before it reaches anywhere near the forecourt - quality control of motor fuels is far better than most lay people realise. If its any consolation to the Mogas users, we investigate far more contaminations to Jet A-1 than we do to Mogas (about 5:1) but that is similarly identified and resolved before it gets near the end users.

UK road-fuel is mandated by law , to have (IIRC) 5% Ethanol.

Not strictly correct, it is mandated by law to comply with EN228, which in fact allows from zero % up to a maximum of 10% ethanol. However, the current RTFO limits set by UK Government are met with Mogas at up to 5% ethanol, and the UK fuels industry has no plans at the moment to increase ethanol content beyond current (max 5%) levels, and the Government say they have no plans to mandate them to do so.

[Maoraigh1]

With the 0-200 and C90 I have experienced sticking valve problems with mogas.

We've used unleaded EN228 on an O200 since it was allowed after 4* went out. We occasionally fill with avgas when away from home, but don't deliberately mix.
However after a top-end overhaul, we were advised to run on Avgas, to build up lead, and did so. I think it was 25 hours on avgas.
We test the fuel for alcohol with the water/fuel technique as specified, and have seldom detected it.

[mayte]

nobody say nothing about magnetos? if you fly with MO GAS magneto timming has to be on 28 BTC , and if you fly with AVGAS magnetos timming has be changed at 24 BTC ,at least CONTINENTAL O200 engines , in the other way if you operate with MO GAS , the restriction about altitud is because MO GAS has alcohol and alcohol contains water if you fly more than 6000 fts of altitud you have the risk the worst thing for pilots , ICE ON CARBURETTOR :ok:

[9 lives]

The mag timing of the O-200 is related to an AD, which was associated with certain cylinders. I know that my O-200, with it's new cylinders, is not longer subject to that AD< and as such, it is timed to 28 BTC, and I run both Mogas, and occasionally Avgas (so certainly mixtures of both).

Mogas containing alcohol is prohibited in Cessnas, and unwise to run.

My experience (about 3400 hours flying Mogas in many models of Cessnas) has demonstrated not greater propensity to carb ice on Mogas, than on Avgas. I recall the specs for Mogas suggesting a slightly greater susceptibility to carb ice on Mogas, but in the practical world, that has not been my experience. I do have a carb air temp indicator installed though.

I did high altitude testing of Mogas in a 182 and 180; Up to 13,000 feet, and 19C, and have never had a problem with Mogas.

I think you should expect non alcohol Mogas, and Avgas to be operationally the same. Aside from a black exhaust residue for Mogas, I think that otherwise the differences between the gasolines would be indistinguishable to the pilot in an approved Continetal......

[phiggsbroadband]

Just to complicate the issue, some dozen airfields in UK also stock UL91... Whatever that is.

[Mach Jump]

...UL91... Whatever that is.

UL91 is unleaded AVGAS, without the disadvantages and uncertainties of MOGAS/Forecourt petrol.

Check that your aircraft is approved to use it though, as approval for MOGAS doesn't automatically include UL91.


MJ:ok:

[ChickenHouse]

@Blue Albatross: Did you read the papers, which came with your certification?

I only know STC's with a big pile of paper, some filler cap sticker which answer the question already, and a paper to keep at the POH in the plane stating the same. On mine it says certified for Autofuel/UL91/100LL in any mixtures, both in the extended documentation and on the sticker. BTW - it also says Autofuel STC, not "Mogas" STC ...

Speaking of UL91, this is simply Avgas without the lead. This brew is much closer to the specs the old engine were built for, compared to 100LL - for which the old O-200/O-300 were never certified to run, though ;-).

"Mogas" usually is standard car gasoline with special overlooking to avoid ethanol in it. Even though some state the standard tap gasoline is not appropriate for aviation, I have my doubts that this is more protection of UL91 and Mogas business. Such as Total (selling Mogas) states there are additions for modern car engines in their premium plus gasoline, it may or may not have an effect on us. I was running an old O-300 on Aral Ultimate for a long time, which is ethanol free because they replaced it with ether and therefore covered by the autofuel STC, which was fine except for two thing: there is some yellow stuff not burning totally and spread over the place and the engine runs a bit hotter compared to Avgas, so you have to keep an eye on EGT/CHT.

One word to the ice on carb - yes, there is a physical evidence for something called Mpemba effect, which may cause Mogas run carb engines to encounter earlier ice pickup. The effect is more prominent at hot liquids, so this may also count for the temperature limits of using Mogas. Personally, I have run Autofuel (of various kind and sometimes dubious origin) in hot environments and never encountered that effect - though some bubbling trouble at hot&high.

[9 lives]

Did you read the papers, which came with your certification?

This is the key. The required (approved) fuels will most likely be specified by the standard with which they are to comply. For this reasons, an STC from one authority might not be what you need in another nation, as the standard specified for the fuel approved might be a standard which is not applicable in the other nation, and therefore, you cannot obtain approved fuel in that nation. The actual effect on the engine and airframe will probably be negligible (other than for the possible presence of ethanol).

"Mogas", "Autofuel" and "Autogas" are more colloquial terms, and might be used somewhat interchangeably. It doesn't really matter if you're using gasoline which meets the specified standard.

engine runs a bit hotter compared to Avgas, so you have to keep an eye on EGT/CHT.

Some Mogas has a very slightly greater energy content than Avgas, so this might be a small factor, but I would be surprised to find that the effect was noticeable on EGT/CHT. I don't worry about it.

[ChickenHouse]

I agree, it is a crucial point to get to know what we put in our tanks and "Mogas" requires quite some studies to do it right.

For example, I am running my O300 on a Petersen Autofuel STC - fuel allowed from gasoline taps for cars, with max. 1% Ethanol content and min ROZ 98 (EN228, ASTM D-439/D-4814, (ROZ+MOZ)/2 >87 octane). When I started to study the issue, I was amazed how few people knew ROZ/MOZ/Octane settings. In central Europe many times you can easily get "Super Plus/Premium" with ROZ 98, but you have to have a look at additives and ethanol (EN228 was changed and now has up to 5% ethanol, they tossed the E Zero signs ...).

At airfields, I am always a bit suspicious about the Mogas they give you - many times they do not follow the tap labelling requirements of car stations. I remember my last flight in Germany, where we found out by surprise, that they sold us ROZ 95 E5 as "Mogas", which is not what I would expect in a civilized country. Ok, may be disputable with civilization now there ... I did not really notice a difference in burning that in an old C172, except for some higher EGT.

I had a look at EGT/CHT and indeed, the O300 runs noticeably hotter on Mogas, with ROZ 102-98-85 in hotter running series. I hot summer I was running the engine very LOP, but this I can do with 100LL and 102, but with ROZ 98 I even get hot coloring in the painting of the back of the exhaust when going all the way LOP.