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Minimum octane to PA-23-250 is 91 octane as mentioned in the POH.
Is there any option to add 95 unleaded fuel to my Aztec?

DeltaLima
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Segev Dagan
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  • I am not sure which 95 unleaded fuel you are asking about. Is it 95 unleaded Avgas, or 95 unleaded Automotive fuel? Also, are you asking because you want to use unleaded fuel, or are you looking for an alternative to Avgas? – Mike Sowsun Aug 02 '22 at 18:17

4 Answers4

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If you are asking about 95 unleaded automotive fuel(Mogas), then No, you can not use 95 unleaded in your Aztec because there is no STC available anywhere.

The POH for the PA-23-250 Aztec specifies a minimum of 91/96 octane. 91/96 was a military spec fuel and was never really used in General Aviation. Due to the unavailability of 91/96, you will find that 100/130 or 100LL was generally used instead. In 1991 a Swedish company started producing 91/96UL (unleaded), but I am not sure about it’s availability. I believe it’s available in France, Germany, UK, and Sweden.

There are three different ways to measure Octane Rating. RON, MON, and AKI which is actually (RON+MON)/2.

UK and EU auto gas use RON

USA and Canada auto gas use AKI

Avgas uses MON

MON and is rated much lower than RON. 95 octane RON is equivalent to 91 AKI, which is equivalent to 85 octane MON. This roughly equates to 80/87 Avgas.

The low equivalent octane ratings of Mogas might be why there is no Mogas STC available for the Piper PA-23-250 Aztec. I don’t believe there is an Mogas STC for any fuel injected aircraft that normally requires 100/130 or 100L.

If you run 91 octane AKI Mogas(North America) or 95 octane RON Mogas(EU), you might get away with it for awhile, but I wouldn’t risk it because it is only equivalent to 85 octane MON Avgas.

One risk is possible damage to your engines. The other risk is the possibility your insurance company may not cover any accident claim you may have.

Mike Sowsun
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The 91 octane requirement just means you can't use 80/87 avgas, and require, in the old days, 100 octane avgas (green) or nowadays 100 Low Lead avgas (blue). Or any other avgas grade of 91 octane or better, had it existed.

You can use alcohol free unleaded automotive fuel also, of 91 octane or better, if you have the Petersen mogas STC. So the answer is yes, if you incorporate the STC (which is paperwork and placards), and in general the engines will be way happier on unleaded autofuel than leaded avgas (subject to any operating limitations in the STC).

One downside is your plane will now stink like a car, instead of an airplane, but otherwise, the engines will thank you, because except for a benefit related to exhaust valve seat lubrication effects, the lead is bad news. You also may want to stick with avgas if the airplane's going to sit for extended periods, say, 6 months at a time or more, with fuel in it, since avgas is much more stable.

John K
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  • Thanks - I have a mental image of all the planes on the hard stand, clustering away from the one that stinks like a car. – Criggie Aug 03 '22 at 19:32
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    I use car gas in my plane and I hate the smell. You know how smells trigger memories better than anything? When I smell avgas, which is rarely, it transports me back to 1975. – John K Aug 03 '22 at 19:56
  • There is no Mogas STC for the PA-23-250 Aztec. There probably never will be due to octane requirements and also Vapor lock problems. – Mike Sowsun Aug 04 '22 at 17:47
  • Ah thanks. Must have been vapour lock problems that couldn't be overcome. Octane is not a problem even for 100 octane engines unless they are turbocharged and have a high comp ratio. There is an STC for the supercharged R-985, which runs at 36" of MP at max power. The air service I flew for in the early 90s, the owner switched to mogas for his DHC Beaver, fixed his rear plug fouling headaches. – John K Aug 04 '22 at 19:11
  • The DHC-2 Beaver runs on 80/87 so Mogas is fine. The Aztec needs 91/96 minimum, so Mogas doesn’t have a high enough Octane rating. I don’t think there are many 91/96 or 100/130 engines that have a Mogas STC. There is a $10,000 Mogas STC for some 100/130 engines, but it requires the installation of an water-methanol anti-detonant injection (ADI) system. – Mike Sowsun Aug 05 '22 at 01:36
  • Some 100 octane engines run fine on 94 octane mogas, like the O-360. My old glider club had 1000s of hours on O-360 powered (STCd) Super Cubs running premium unleaded. – John K Aug 05 '22 at 02:30
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The question regards the use of unleaded fuel, and the presumed use of 95 octane unleaded fuel. Unleaded 100UL aviation fuel is available that can be used without restriction in aircraft piston engines. In this answer, here, the following was noted regarding unleaded 100UL aviation fuel:

Within a broad perspective, the overall most important issue in purchasing a plane generally seems to involve the cost of operation. The dilemma that was mentioned should be viewed from the perspective of G100UL aviation fuel, details of which can be viewed here. In July of 2021 the FAA approved Supplemental Type Certificates (STCs) authorizing use of G100UL high-octane unleaded avgas in aircraft piston engines, without modification. At least this is something to consider, in the foreseeable future piston-engine aircraft are not likely to be rendered unserviceable due to the unavailability of leaded avgas, or the need to modify engines to use unleaded G100UL fuel.

Thomas Perry
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  • That's very interesting. I thought that project was still ongoing. Can you actually find that fuel anywhere now? – John K Aug 02 '22 at 03:13
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In the U.S., PA-23-250 with the following engine models are approved by FAA to use UL94 from Swift Fuel:

  • O-540-A3D5
  • O-540-A1D5
  • O-540-A1B5
  • IO-540-C4B5
  • IO-540-C1B5

https://www.swiftfuelsavgas.com/stc/Piper/PA-23-250/any/any/any

Mike Sowsun
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igorinov
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