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?





(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]

Originally Posted by 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.

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!!

[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]

Originally Posted by 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).

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.