Autogas STC

[bytheseatofmy]

One of my students has his own 1960 C172,last month we had problems with the airplane that resembled carb ice but turned out to be a worn carb and
krud in the float chamber.
Well here's a question for you all.
Does installing a new carburetor
negate the "autogas" STC?
Thanks guys.

 

[erj-145mech]

No it doesn't, plain and simple.

Running auto fuel is a false economy. It will cost more in the long run. You can't control how much alcohol is in the car gas, and that is not compatable with some of the aircraft component items.

The STC says that the gas must meet the ASTM specs, how do you know that the gas meets those specs? Most states don't require the fuel to be tested or meet spec to be sold in that state.

Caveat emptor...

 

[bytheseatofmy]

we have absolutely no thoughts about using "autogas".It was just one of those "I wonder" questions ... Thanks for the post!!

 

[Gorilla]

My O235-C2C loved MOGAS. It ran better on 93 octane than it did on 100LL, as there was no leading issues. Three years of MOGAS, not one episode of vapor lock or abnormal wear. YMMV, it was just my experience.

 

[erj-145mech]

Thats great that you had a positive experience with the car gas.

If you get your carburetor overhauled, your warranty will be void if you run car gas thru it.

I had two customers, one with an R-985 and one with a R-1340, had cylinder head separations in flight, caused by the use of auto fuel. The octane rating was raised by the distiller by using methanol in the blend. Neither operator was using full throttle take offs either. They were limiting the manifold pressures to 33" on both engines.

Another customer with a PA-28 had vapor lock issues at altitude. Granted it was in Texas in August, but operators running av-gas in like equipment didn't have the same problems.

Auto fuel is blended regionally as well as seasonally, where av-gas is blended the same year round as well as nation wide. The av-gas that you buy in Houston is going to meet the same specs as the av-gas that you buy in Palm Springs, but the MOGAS that you buy in Dallas will be different than the MOGAS that you buy in Fargo. And the MOGAS that you buy in Dallas in January, will also be different than the MOGAS that you receive in June.

Also the quality of av-gas storage is stricter than the quality that MOGAS receives. Av-gas is filtered each time that it is transferred, where car gas isn't. Av-gas moisture and sediment is monitored on a regular basis in bulk storage, whereas car gas isn't.

It the variables that you cannot control in car gas is what concerns me about using it in aircraft. Chances are that you will not experience ill effects in aircraft that may fall into a sport aviation category, but as the compexity increases, that window will narrow considerably.

 

[USMCmech]

Well put, ERJmech

IF one decides to run Mogas that person is responsible for making sure that it is clean, proper octane, ect. Think of it like building and flying in a homebuilt, It's up to you to make sure you are safe.

I can't think of any radial engines that have Mogas STCs, so anybody useing it in those planes was illeagle as well as downright stupid. You MUST get the STC If you want to use Mogas. These are limited to small low compression engines that were origionally designed for low octane anyway. 100LL is overkill in a O-200 in a C-152.

Proceed with caution.

 

[avbug]

Mogas is not the same as Avgas...not just composition, but testing, lot to lot, and by standard. 100 octane avgas is not the same as 100 octane auto fuel, and even among auto fuel of the same octane ratings, the fuel doesn't test out the same.

Autofuel, despite all the mythbusting wannabes who will tell you diferently, isn't stable. It doesn't last. It gums up, it goes rancid, and it's octane rating and properties change as it does so. If you're running fresh autofuel regularly through your low compression engine, you're probably okay...most of the time.

Autofuel lacks the aeromatics that preserve your fuel cells. Autofuel lacks a lot, really; it's only advantage in an aircraft is it's lower cost, and it can be false economy. Most mechanics are very much against the use of autofuel in aircraft...especially any mechanic that's had the privilege of gumming out carburetors and fuel systems that have been contaminated with auto fuel.

Replacing the carburetor with the same carburetor and jetting does not invalidate the STC, as it's an aircraft supplemental type certificate, not a carburetor STC. If the STC is issued inclusive of the same type carburetor, then changing for the same part number doesn't change the nature of efficacy of the STC.

You should check AD's for your aircraft and ensure that your carburetor does have the latest applicable float type (seems to be a revolving issue the last few years) before you install it, however.

 

[aucfi]

Does it do any harm to switch between MOGAS and AVGAS on an irregular basis? I run AVGAS in my Cub roughly 80% of the time and run MOGAS when I cannot get AVGAS.

The TCDS lists the minimum octane for the C-65 engine to be 78 octane if I remember correctly. I cannot tell a difference between AV or MO gas performance wise.

au

 

[USMCmech]

Does it do any harm to switch between MOGAS and AVGAS on an irregular basis? I run AVGAS in my Cub roughly 80% of the time and run MOGAS when I cannot get AVGAS.

Probably not.

I knew a guy who ran Mogas in his PA-28-141 and filled up with AVgas when at other airports, and Mogas from his own tank at home. He never reported any problems.

Again, you MUST get the STC before you fill up from the Mogas pump. Just because the TCDS says 78 octane was OK dosen't nessecerally mean anything today. By getting the STC you are paying for a good bit of research that didn't indicate problems. If you don't you are ventureing into the complete unkown.

 

[aucfi]

Yeah I have the STC. Everything I have heard about MOGAS and aircraft engines has been bad and I hate running it but sometimes there isnt much choice.

Thanks

au

 

[avbug]

The TCDS is referring to a different standard and method of determining octane...your TCDS octane value doesn't necessarily correspond to the octane rating you see at the pump for MOGAS.

You're better off filling up with Avgas if the airplane is going to sit, and no, mixing avgas and auto fuel isn't going to hurt...other than making the fuel mixture less stable for long term storage..and putting your system at risk of gumming up. (varnish).

Getting an STC doesn't do anything other than give you a bit of paper that says you're legal. That's all. You still need it, but don't think for a minute you're buying more than legality, because you're not.

 

[USMCmech]

A well written article examining the pros and cons of Mogas

http://www.avweb.com/news/maint/187232-1.html

The few pilots I have met who used Mogas reported few problems, but I have hear a lot of other people say that it is terrible.

 

[aucfi]

Very good article; answered a ton of questions that I would have never thought of or considered. Thanks for sharing.

 

[FlyingToIST]

The previous owner of my PA28-151 put a MOGAS STC on it. This actually allows you to mix the fuels as well. You can put 10 gals of avgas on top of the 7 MOGAS you had and vice versa. The thought of running something cheaper was really tempting at first but my mechanic and few others i talked to discouraged me from doing so..

 

[sleddriver77]

Again, you MUST get the STC before you fill up from the Mogas pump.

Must you? For most of the small, low-compression piston engines that are covered by this STC (which is held by EAA) the extent of the changes which must be made to the engine in order to comply with the STC is: Placing a band around a pushrod tube to indicate that that aircraft has been STC'd. (In other words; that you sent a check to EAA to pay for a copy of the STC, and helped reimburse EAA the cost of driving around in their "research" Skyhawk full of car gas for awhile, and now you have the paperwork to prove it.)

IS car gas a good alternative? Probably not. Can it cause detonation, vapor lock, and other problems that AvGas would not? Sometimes. Are you 100% sure of what you're buying at the Car Gas pump? Absouletely not. State and local emissions requirements, and therefore percentages of additives, vary- A LOT. Will running Car Gas eventually eat the seals in your carburetor, fuel tanks and hoses? You Betcha. Will most small piston singles fall out of the sky if you burn car gas now and again. Probably not. If you're gonna run car gas now and again, SHOULD you get the STC? Yup. All it takes is some Fed with an AutoGas hard-on to see you dumping a gallon of unleaded into your airplane and come over and ask if you have the STC.

My $0.02

sleddriver; A&P/IA, EAA member, ATP, and poor dumb sumbitch who, on more than one occasion, has landed on a road for car gas out of necessity. (Alaska and Canada)