Unleaded Avgas Update

[Clostermann]

Some of you you may be wondering what's going on in this arena. I'll give you my two cents from the inside looking out. So you all heard the noise coming out of Oshkosh about a new UL100 fuel that was supposed to be the answer to everyone's wishes. Everyone got very excited including the pilots associations. Well, Lucy may have pulled the football away from Charlie yet again.

Here's the deal. FAA came out in November saying that the STC route will not lead to "fleet wide approval" of any fuel. What does that mean and why should you care? Well, even though the vast majority of aircraft don't need 100 octane fuel, the vast majority of the fuel consumed is by aircraft that require it. So you know where you are on the totem pole.

This new fuel is unlikely to get "fleet wide approval" from the FAA, unless and until it meets an "industry standard" meaning, until the OEMs and FAA confirm that it is suitable for fleet-wide use and it conforms to an industry standard such as an ASTM fuel specification. Problem is they haven't and likely won't, for three main reasons: 1) the owner won't let them look at it 2) the fuel would not likely pass muster, and 3) the owner has so far refused to go through the ASTM standardization process.

Why not? For several reason including:

1) It may not have have enough detonation resistance to meet the most demanding requirements pf the fleet.
2) its density is higher than 100LL
3) its cost is higher than 100LL
4) the owner has so far refused to submit the fuel for additional testing.

So where does that leave us? The Santa Clara county has announced a ban of leaded fuel starting January 1 2022, likely in the hope they can close their airports and develop the land more profitably. UL94 may work for a while if they don't have any significant traffic from turbocharged aircraft. But make no mistake; this is likely less about public health than real estate development.

The only solution is to develop an unleaded AVGAS that works for the entire fleet. Difficult? yes. Impossible, no. Today we took an important step in that direction with an unleaded fuel that meets detonation resistance in the most demanding engine in the fleet, not a turbo-normalized or inter-cooled engine, and under the most demanding POH conditions.

But it is just a step. There is a lot more testing to go. This is what is required for "fleet wide" approval and for nationwide deployment. The rest is Kabuki theater. Fortunately, your associations finally understand this.

[rv6ejguy]

Good info as always. I am slightly surprised that GAMI won't submit to the full process but I mentioned the specific gravity aspect a couple years ago now being a big deal that can only be got around using STCs which is what Swift and GAMI did. Not surprised at the FAA's stance.

Since GAMI was approved for the R2800 under their STC, apparently at 100LL MAP limits, it seems that it would be capable of meeting the detonation margins for GA turbo engines which run less manifold pressure and have similar specific outputs and similar BMEPs.

Can I assume you are talking about a big petroleum company based in Texas here when you say "we"? Seems that if your fuel is approved for drop in, GAMI and Swift fuels will take a big hit in the market unless much cheaper and they install their own tanks at airports.

[Clostermann]

Originally Posted by rv6ejguy

Since GAMI was approved for the R2800 under their STC, apparently at 100LL MAP limits, it seems that it would be capable of meeting the detonation margins for GA turbo engines which run less manifold pressure and have similar specific outputs and similar BMEPs.

That in itself is surprising since the highest MAP I have heard him quote was tested on his fuel was 42" in the inter-cooled TSIO-550-K. The 2800 runs at much higher MAPs (>50 inches). Makes you wonder what data the ACO relied on to add this engine to the AML-STC....Yet another example of the apparent lack of consistency between STC and PAFI testing.

[David Z]

There are factors other than manifold pressure that affect detonation and fuel octane rating requirements of an engine. The Continental has a compression ratio of 7.5, the R2800 is ~6.7 (depending on which version). Cylinder size, intake air temp, cylinder temp all will have an effect. I'm sure there's more too.

[swjohnsey]

It would be interesting to study how the military on both sides dealt with varying octane in aircraft during WWII. Octane ranged from 87 for the Germans to 150 for the allies.

[airguy]

Originally Posted by swjohnsey

It would be interesting to study how the military on both sides dealt with varying octane in aircraft during WWII. Octane ranged from 87 for the Germans to 150 for the allies.

Well, mostly they replaced a lot of engines.

[Kyle Boatright]

Originally Posted by airguy

Well, mostly they replaced a lot of engines.

They had different performance charts, by engine, for different fuels. Still do for the big recips, although unless you go to South/Central America or Reno, you're pretty much stuck with 100 LL.

[rv6ejguy]

Originally Posted by Clostermann

That in itself is surprising since the highest MAP I have heard him quote was tested on his fuel was 42" in the inter-cooled TSIO-550-K. The 2800 runs at much higher MAPs (>50 inches). Makes you wonder what data the ACO relied on to add this engine to the AML-STC....Yet another example of the apparent lack of consistency between STC and PAFI testing.

This wasn't GAMI but Swift: https://www.purdue.edu/newsroom/rese...iftRadial.html The main point- "Test results showed that 100SF produced a higher detonation onset threshold than 100LL. The engine was operated at 115-145 octane takeoff power settings and there were no indications of engine knock." This was a decade ago.

Seems the R2800s are being limited to around 52-54 inches on 100LL. Similarly on R3350s. I knew a pilot/ engineer on the Martin Mars water bombers in BC.

I suspect there is more testing going on than what is common knowledge. Purdue in particular has strong links to industry and some of the fuel companies going back to before WW2.

[terrye]

Originally Posted by swjohnsey

It would be interesting to study how the military on both sides dealt with varying octane in aircraft during WWII. Octane ranged from 87 for the Germans to 150 for the allies.

You should read Callum Douglas' "The Secret Horsepower Race-Western Front Fighter Engine Development". There was much more than just the fuel octane ratings going on. Design philosophy, materials, politics...

https://www.amazon.com/Secret-Horsep...s%2C136&sr=1-1

[skylor]

Originally Posted by rv6ejguy

Good info as always. I am slightly surprised that GAMI won't submit to the full process but I mentioned the specific gravity aspect a couple years ago now being a big deal that can only be got around using STCs which is what Swift and GAMI did. Not surprised at the FAA's stance.

I recall that George Braly stated years ago that the problem with the ASTM spec for 100LL was that GAMI’s own testing showed that existing supply of 100LL does not meet the ASTM spec. and that the FAA needs to change it because it is unfair to require unleaded fuels to meet requirements that existing leaded fuels apparently don’t meet.

Skylor