STC for MoGas in Franklins

[jeffw]

Attached is a photo scan of STC Number SE2127CE, issued June 25, 1985 by the FAA. I've had it for some time and misplaced it; just came across it today.

It does specify a couple of qualifications that speak to a Petersen Drawing List document F97210, June 23, 1985, and requiring specific approval for each model aircraft to insure compatibility with its fuel system.

My understanding from a few sources over the years is that the Franklin 6A4 series of engines were based on a design that was completed before there was added lead in fuel to boost its octane, prior to needing it in WWII. So these engines do not do well when you run 100LL through them except when you don't have any other choice. This is what I have experienced in the 28 years owning my Cruisair with a 6A4-165.

This is as close as I have come to finding some sort of 'doc' for a nominal 'approval' for running MoGas. Or just don't broadcast your operational preferences.

I did a few searches for the STC Number and the Petersen Drawing List number with no luck, maybe someone else who has the means for searching these docs could do better.

And, the scan is blurry. The original I have is likely a copy of a copy or so, it is blurry itself and the scan as printed is the same quality as my 'original'. It obviously is depicted very small. Try downloading it and enlarging it, or email me with your email and I will send you a .jpg, a .pdf, or a .tiff as you prefer.

UPDATE - I found that after I posted this, I just double clicked the image and it opened up the /url/ full size.

imge .jpg

 

[Larry Lowenkron]

I talked to Petersen about this years ago. The issuance of a mogas STC requires 2 stc's. One for the engine and one for the airframe. Petersen told me that there were not enough Cruisairs to make it cost effective to get the STC. You may notice that Stinson 108 (w the Franklin) are approved. There are a few more (!) of them.
After talking with Petersen, I called Earl Lawrence while he was at EAA (and before he went to the dark side, though I must say he's done a great job there). He was a Cruisair owner at that time too. Same result - there weren't enough of us to make it worthwhile.
I have used mogas in the past (without EtOH) and all I noticed was a SLIGHTLY higher EGT and CHT...not enough to be concerned about...and a thicker wallet after refueling.

 

[Larry Lowenkron]

BTW, there is now an STC for 91 and 94 octane fuel - STC SA01757WI on the FAA STC site for Cruisairs. Checking with Petersen again he said it is NOT for mogas but some of the "designer" fuels (Swift fuel or ??)
Petersen also says on his site to occasionally mix 100LL so as to protect the valve seats on Franklin engines. The Franklin guru on the Stinson Club web site says that it is not necessary considering that Franklin seats are Stellite.

 

[jeffw]

The Aircraft Specification No. A-733 (9/10/1956) for fuel specifies 80 minimum octane aviation gasoline for the 6A4-150, and on line 110 the same for the 6A4-165.

ARGUMENT for consideration: MoGas is sold at some airports at the gas pumps, that makes MoGas an aviation gasoline; airport MoGas is identical to 91 octane (or 94 EO) ethynol free (EO) you get at the Quick Mart.

Therefore, an STC isn't required if you are complying with the Aircraft Specification and using 80 minimum octane aviation gasoline.

Other information to consider:
https://generalaviationnews.com/2011/03/16/10-mogas-myths/

From cited read,
Myth #1 Gasoline used in airplanes is unsafe. Fact – International standard ASTM D4814 is used for both fuel production and engine TC/STC certification. Mogas has been an FAA-approved aviation fuel since the 1980s and has had an excellent safety record.

 

[leadsledfan]

Don't worry!
The FAA will finally choose a winner in the lead-free fuel contest in 2017!
I mean 2018!
oh wait. 2020?
-Adam

 

[leadsledfan]

If your field has UL91 or UL94 or Swift Fuel you can legally run them in a Franklin.
I was lucky enough to have Swift in California.
-Adam

 

[edwardherold]

Are there any downsides beside higher cost to running 100LL?
I have the higher compression Franklin 220hp anyway.

 

[Dan Cullman]

You must use 100 octane or better in the 220, period.

 

[jeffw]

Note that my suggestion was only for the Franklin 6A4-150 and 165.

The Aircraft Specification for those models only allow for min. 80 octane aviation gasoline. The Franklin 220 has to use what that engine's specification calls for, as Dan says - use 100LL for the 220.

When I first got the Cruisair I mostly used 100LL. My plugs accumulated lead deposits too quickly. Then someone weighed in with their knowledge/opinion about using alcohol free auto gas (MoGas). I have mostly used MoGas for the past 25 years and have not had any problems and my REJ38 plugs do not lead foul much any more. I still have to use 100LL on cross country trips because MoGas is far and few between and I add a lead scavenger chemical that probably helps a little.