It does not meet the composition standard of 100LL, which is why you need to buy an STC (Supplemental Type Certificate) from GAMI to use it legally in an aircraft. (It is a drop in replacement from a performance point of view, but does not meet the original type certificate fuel standard.)
Can't (realistically) be done. The specification for most airplane engines say "100LL" and the specification for 100LL is a composition specification (ASTM D 910), not a performance specification.
If your fuel meets ASTM D 910, it contains lead. If it doesn't contain lead, it doesn't meet ASTM D 910 and therefore definitionally isn't 100LL, so if your engine type certificate data sheet calls for 100LL, it unavoidably calls for lead.
If you are flying an aircraft that is thus-certified, flying it with a different fuel is, per the FAA definition in 14CFR3.5, flying an unairworthy airplane.
"Airworthy means the aircraft conforms to its type design and is in a condition for safe operation."
That's the legal reasoning that the Gami STC is required: to change the TCDS of the engine and airframe to permit a fuel other than 100LL and still be considered airworthy. (The STC process is totally workable and, IMO, doesn't need to be "fixed" and Gami has basically done the work you're asking to be done AFAICT.)
Approval of specific airframes and engines through STCs (Supplementary Type Certificates) is slightly misleading. In the same way, regular unleaded gasoline is also approved through STCs for many airframes and engines. This isn’t the silver 1:1 bullet that replaces 100LL tomorrow. Nevertheless, I’m all for the 100LL alternatives efforts. If I was buying an aircraft, however, I would be more interested in a Jet-A burning piston (basically diesel). Unfortunately that market is currently limited to Diamond aircraft, which are fantastic if not for their tight grip on their service network.
Last summer GAMA received a supplemental type certificate for a 100LL replacement, called G100UL. The STC covers certain Lycoming O-320, O-360, and IO-360 piston engines, which are commonly used in light aircraft.
Based on the type certificates for the common airplanes and engines I looked at, the type certificates were re-issued with 100LL as an/the approved fuel under the type design. (This would be done after testing as required by FAA.)
That’s what the STC process does legally, just being done by the engine/airframe manufacturer instead of third party (presumably because they wanted to continue selling engines/airframes).
What's the FAA incentive to change the type certification (and STC) rules here? There's a perfectly workable STC path, that's not even that expensive [almost rounds to $0 in the scope of private aircraft ownership expenses).
The TC says you must run 100LL. The STC says you can freely mix G100UL in any ratio. The legal/certification problem is solved.
The beauty of G100UL is you just need an STC - just paperwork - to be legal with GA aircraft. All gasoline powered aircraft and engines in the FAA’s type certificate database are covered by the STC for G100UL... which is amazing.
I'll be doing it, as soon as our home base carries it for my 56 o200.
Yes, but it's not currently legal to use in anything.
You still have to apply for and buy an STC for your specific airframe's serial number and engine's serial number. Then you have to add some paperwork to the logbook and POH and add placards to the fuel fillers and cockpit.
In theory you should be able to automatically get it for anything which was approved from the factory to use 100LL, but the STC application form (here: https://stc.g100ul.com/aircraft/) does not allow you to select most mid-century large radials (which were usually originally certified on 130) or the airplanes they were installed on.
To be clear, this is a great thing, but you can't legally just fill G100UL in any random airplane.
This is awesome. However, since it appears every certified airplane needs an STC to be able to use it, it doesn't seem likely that I'll be able to use it in our experimental aircraft any time soon. I can't imagine that our small FBO will switch from 100LL to G100UL as long as there are any airplanes around that can't use it.
While I believe G100UL is an improvement over 100LL in every important dimension except cost (where the delta is small enough to be accepted) and a small density penalty (a gallon is 5% heavier than 100LL, but contains slightly more energy as an offsetting factor, but “full tanks” will now be heavier and if the fuel has been mixed, it’ll be heavier by an unknown amount), it still needs an amount of test cell and flight testing to demonstrate that it’s equal or better.
Right now, the law says “these airplanes are allowed to use these engines”, “these engines are allowed to use 100LL”, and “100LL has the following composition”. To be legal, you need paperwork that breaks one or more of those constraints, which is what GAMI is in the process of pursuing. To get that paperwork, they have to demonstrate functional equivalence and convince the FAA to issue a supplemental type certificate (the legal paperwork that would alter the second condition above). It’s not unreasonable for the FAA to need proof. Whether they need more proof than they have is self-evidently a point of disagreement between GAMI and their supporters (I count myself among this set) and the FAA.
Legally (the subject that requires FAA approval) is that the type-certificate (the legal basis for certification and authority to operate) for these engines specifies the use of 100LL and the standard for 100LL is a composition-based standard not a performance-based standard. If it doesn't have the prescribed amount of TEL, it's definitionally not 100LL, even if it would meet the performance requirement of the engine.
Then, from an engineering standpoint (the subject that companies are working on in order to propose fuels to the FAA via PAFI or GAMI seeking multi-model Supplemental Type Certificate), aircraft engines are stone-age in complexity and are overwhelmingly air-cooled, fixed timing [except during starting], and operate for hours on end at 75%+ of rated power. Your auto is water-cooled, has variable timing, and typically runs at 8-15% of rated power. An aircraft engine has to run reliably with cylinder head temperatures of 460ºF or more (your car's cylinders are much more tightly controlled as a result of the water cooling).
Why can't airplanes use a different engine? Airplanes are (legally) certified to use a specific engine. Those engines are (legally) certified to use a specific fuel [or list of fuels]. Automobile engines have a horrible track record when attempted to be converted to aviation usage. Small, normally aspirated engines are able to run on low-octane, ethanol-free pump gas. The aircraft with high-powered turbo-charged engines represent around 25% of the engines. Because of their higher utilization, those engines burn around 75% of the avgas purchased in a year. (These are the CapeAir fleet, skydive operators, etc.)
It’s UL94, so lower octane (94 octane, as the name suggests). However, this is still fine for lower performance aircraft that make up the majority of the GA fleet.
There’s been a lot of interest in it, especially in light of the recent discussions to close RHV in San Jose. I know one of the flight schools here just switched all their aircraft over to UL94.
Higher performance aircraft will need UL100, which is still not available, but is expected soon. There’s been significant progress in getting it approved over the last year.
Swift Fuels sells the supplemental type certificate aircraft owners need to use UL94. They are offering a free upgrade to the UL100 STC once it’s offered, so aircraft owners don’t have to pay twice to start using UL94 today.
Swift also has 94UL which I believe all the Cirrus aircraft are certified for and I imagine most of the other high performance turbos could get an STC for. There's also the mogas STCs if we could drop the stupid ethanol corn subsidy. Personally I'm currently working on an EAB that will run E10 without issue because I'm so sick of the 100LL shit. There's a LOT of lead in 100LL (despite the name) and aircraft operations naturally expose you to a lot of it.
> The FAA signed on September 1 supplemental type certificates that allow General Aviation Modifications Inc.’s 100-octane unleaded fuel (G100UL) to be used in every general aviation spark-ignition engine and every airframe powered by those engines.
All piston engines and aircraft are certified for G100UL.
There's already an STC (Supplemental Type Certificate; permission from the authorities to modify or operate an aeroplane in a way that is different from when it was manufactured) which allows the use of regular gas (literally, from a gas station) in many different planes - https://www.autofuelstc.com/.
It does exist, it has been approved by the FAA, and you can go buy the STC right now if you want[0]. The incentive is that 100LL has been banned by at least one county[1] with states[2] trying to follow (although the AOPA successfully neutered that one[3]).
> "For many (maybe even most, but I don't know the exact numbers) certified aircraft there is no legal path to get it to run on any other type of fuel."
Until recently this was true. But since September 2022, G100UL is legal and certified for "every spark ignition piston engine and every airframe using a spark ignition piston engine in the FAA’s Type Certificate database.”
No, G100UL is a full drop in replacement that meets the standards off AVGAS which is what is required by the certificate. Fully legal on all engines with no change to the certificate.
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