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MoGas STC Question

RogerH

RogerH

Well Known Member
Totally confused, I routinely fuel my cars with ethanol-free 91 octane unleaded at a local station. Small cost difference but that's it.

Been looking into the possibility of a Rotax powered aircraft and seeing mention on the Web of paying for an STC to have "permission" to do the same with aircraft????

I'm hoping someone can explain why an STC piece of paper is required, what benefits or protection's does one provide or is this just another bureaucratic FAA paper chase? Sorry if I'm letting my slip show.

Thanks for any explanations.
 
STC is to the best of my knowledge, only for certified aircraft to burn auto fuel. Many various STC out there for certified planes, fuel is only one of them.

Supplemental Type Certificate. Our RVs are not type certificated but experimental.

More than just "permission" as the STC sometimes required changing "O" rings or other parts that were not compatible with auto fuel. Other STC sometimes had a long list of required changes/mods to keep stuff safe.

Pretty deep subject on some STC, others just paperwork only.
 
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Mike S said:
STC is to the best of my knowledge, only for certified aircraft to burn auto fuel.

Supplemental Type Certificate. Our RVs are not type certificated but experimental.
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May have been dated sites I was looking at but there was reference to sourcing the STC from EAA.
 
Yes, the EAA has for years sold STCs (paperwork) to legally allow certified aircraft run car gas. But it’s really more than paperwork. The EAA had to show each fuel system was compatible with car gas, which can have a higher vapor pressure than AvGas. And each engine was okay with the specified car gas. Remember, 91 octane car gas has a Motor Octane (AvGas spec) of about 87.
 
BobTurner said:
Yes, the EAA has for years sold STCs (paperwork) to legally allow certified aircraft run car gas. But it’s really more than paperwork. The EAA had to show each fuel system was compatible with car gas, which can have a higher vapor pressure than AvGas. And each engine was okay with the specified car gas. Remember, 91 octane car gas has a Motor Octane (AvGas spec) of about 87.
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Thanks for that clarification, I saw EAA as the source and hence my confusion. All clear now, glad to have moved to the dark side away from “certified” several years ago, definitely not looking back!
 
We ran 91 no ethanol in my wife's Rans S7/ Rotax 912ULS, until we sold it 700 or so hours later. No problems. I still run it in my RV7, PV Lycoming 180HP IO-360, 8.5 to 1 CR. I did retard the timing -2 degrees to 23, for cooler CHT's and better detonation margins. I have run it this way for over 900 hours. About 1/3 of the hours are on fill ups during trips with 100LL. I have friends who run the same engines at 25 degrees advance timing With no problems (I'm maybe a bit cautious).

I do not run winter blend and carry it into the warmer weather in the spring. It is ok to run summer blend into cooler fall and winter weather.

No STC needed for experimentals , and I doubt you need one on factory builts because Rotax; the engine manufacturer has documented recommendations saying Non ethanol, high octane is best, then E93 is next preferred and 100LL come in last.
 
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Warning, there is no such thing as an STC for a certified airplane to run mogas on an injected lycoming. The reason is because of vapor lock on the return less fuel system design and possibility of the engine mounted fuel pump heating the fuel.

Just because Boyd above gets away with it, doesn’t mean you will. In order to run mogas on an injected lycoming, many switch to electronic fuel injection which removes the engine driven pump and pressurizes the fuel system, while others swap to entirely electric pumps for mechanical injection, while others still bleed fuel past the mechanical pump and cool it with blast air.

Obviously the first two options make the engine electrically dependent, and all options are screwing with the fuel system.

Tread lightly if you are going to use mogas with a mechanical fuel pump.
 
akschu said:
Warning, there is no such thing as an STC for a certified airplane to run mogas on an injected lycoming. The reason is because of vapor lock on the return less fuel system design and possibility of the engine mounted fuel pump heating the fuel.

Just because Boyd above gets away with it, doesn’t mean you will. In order to run mogas on an injected lycoming, many switch to electronic fuel injection which removes the engine driven pump and pressurizes the fuel system, while others swap to entirely electric pumps for mechanical injection, while others still bleed fuel past the mechanical pump and cool it with blast air.

Obviously the first two options make the engine electrically dependent, and all options are screwing with the fuel system.

Tread lightly if you are going to use mogas with a mechanical fuel pump.
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I have run 100s of hours on mogas in my RV7 with an IO-360 and engine driven pump with no problems whatsoever with just the standard fuel injection system and electronic ignition.
 
RogerH said:
Totally confused, I routinely fuel my cars with ethanol-free 91 octane unleaded at a local station. Small cost difference but that's it.

Been looking into the possibility of a Rotax powered aircraft and seeing mention on the Web of paying for an STC to have "permission" to do the same with aircraft????

I'm hoping someone can explain why an STC piece of paper is required, what benefits or protection's does one provide or is this just another bureaucratic FAA paper chase? Sorry if I'm letting my slip show.

Thanks for any explanations.
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For certified aircraft they have a type certificate TC. Every aircraft manufactured must conform to the TC, and continue to conform for its entire lifetime. The TC if you read one is a wealth of information. Airspeed markings ie white, yellow arc, redline that all came from the TC, required placards…you got it TC. (Yes I know ADs can require placards) Approved engine and prop configuration…yup TC. Anything that deviates from the TC makes the aircraft unairworthy. BTW a certified engine has its own TC.
What fuel you can use is on the TC.



The S in STC is for Supplemental. After much testing, proving, and gnashing of teeth the FAA grants the applicant of the aircraft modifications an STC. They now own the STC. BTW it ain’t a cheap process. The sale of STC is hardly a money making scheme, it’s generally a recoup of costs. There are some that are lucrative. It’s hardly a get rich quick scheme.

That STC allows what is essentially an approved addendum to the TC. It’s not a blanket addendum however. The STC owner can keep it all to themselves or they can share it with other aircraft. Usually means selling it to you for a specific aircraft.

You must be in possession of the STC and the installation must conform to the STC. If it doesn’t conform…you guessed it it’s not airworthy.

STC is about an approved modification and the aircraft’s airworthiness compliance.

It’s all about airworthiness.

To answer your question, no it’s not bureaucratic paperwork.
 
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bjdecker said:
Ok, low compression pistons -- what about the spark advance? and what kind of IO-360? -Axxx or -Bxxx/-Mxxx (Angle Valve or Parallel Valve)
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I have no idea about the spark advance it is whatever Lightspeed has set in is system for the rpm and mp. It really doesn't matter about the engine model vapor lock occurs in advance of the fuel being injected. I can't remember the model # of the engine as I sold it two years ago and can't look it up.
 
Norman CYYJ said:
I have run 100s of hours on mogas in my RV7 with an IO-360 and engine driven pump with no problems whatsoever with just the standard fuel injection system and electronic ignition.
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Yup, and it just takes one time on a hot day and poor quality fuel for the fan to quit.

If you feel comfortable doing what the certified world wouldn't certify, then great, but I think it's wise to put an additional data point out there for others to consider.
 
Norman CYYJ said:
I have no idea about the spark advance it is whatever Lightspeed has set in is system for the rpm and mp. It really doesn't matter about the engine model vapor lock occurs in advance of the fuel being injected. I can't remember the model # of the engine as I sold it two years ago and can't look it up.
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Understood - but for the record it’s not just vapor lock that’s the cause for concern with mogas. The spark advance, compression ratio, and combustion chamber shape will contribute to the detonation (pre-ignition) margin available with a corresponding change in octane (100 vs 91).
 
bjdecker said:
Understood - but for the record it’s not just vapor lock that’s the cause for concern with mogas. The spark advance, compression ratio, and combustion chamber shape will contribute to the detonation (pre-ignition) margin available with a corresponding change in octane (100 vs 91).
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Well before I started using mogas I ran lot of tests and could find no operational differences in cylinder head temps at low rpm or wide open throttle at 1500 ft and various altitudes up to 17000ft. The biggest difference i found was that the plugs stayed clean and the cylinders looked clean. Each rpm and altitude change was operated on 1000ll for 15 minutes and all temps were recorded then the same tests were done with mogas for 15 minutes. There are a lot of people using mogas in their 360s and mogas 100ll mixes. I haven't heard of an engine being destroyed using mogas. But I am not a scientist or a chemical engineer I can only go by what I experienced and have seen in the last 55 years of flying.
 
Norman CYYJ said:
Well before I started using mogas I ran lot of tests and could find no operational differences in cylinder head temps at low rpm or wide open throttle at 1500 ft and various altitudes up to 17000ft. The biggest difference i found was that the plugs stayed clean and the cylinders looked clean. Each rpm and altitude change was operated on 1000ll for 15 minutes and all temps were recorded then the same tests were done with mogas for 15 minutes. There are a lot of people using mogas in their 360s and mogas 100ll mixes. I haven't heard of an engine being destroyed using mogas. But I am not a scientist or a chemical engineer I can only go by what I experienced and have seen in the last 55 years of flying.
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Perfectly valid points and data — was just trying to tease out some of the other contributing factors so people can better understand the ins/outs of using mogas.
 
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