Unleaded Aviation Fuel

The elephant in the room is now beating at the gates (with apologies for the mixed metaphor): The Tetra Ethyl Lead (TEL) in aviation fuel. The search for a fuel to replace the currently ubiquitous (and only fuel that can be universally used by piston aircraft) 100LL AvGas has been under way for some time. Here, I’ve attempted to summarize the current state of things, and cover the frustrating difficulties that prevent an overnight switch to unleaded fuels.

Why Aviation Fuel is Leaded: Octane

The first thing that must be understood when discussing replacement fuels is the octane rating. (Also, note different numbers mean different things, and it can get confusing; see below.) The main issue is detonation.

 

Aircraft engines with magnetos (i.e., virtually all piston-powered aircraft currently flying) are not computer controlled and have static timing that does not advance with increased RPM; additionally, mixture is manually controlled by the pilot. Automotive engines, liquid-cooled, have computer controlled timing and mixture, and knock sensors that will trigger on-board electronics to retard timing and enrichen the mixture automatically the instant detonation is detected. (Why isn’t this decades-old technology in airplane engines? Cost of developing and certifying such a system for aviation use, small (~170,000 airplanes in the United States) market, etc. (Consider: The ubiquitous (I)O-320 and -360 Lycoming engines date back to the early 1950s.) This interesting video provides (amidst a larger discussion) a succinct explanation of premature detonation in higher compression engines.)

 

Shell has a document that explains the situation quite well:

Octane rating is a measure of how resistant a fuel is to detonation or “pinking”; the higher the Octane rating, the more the fuel / air mixture can be compressed without detonation happening. To make this clear, octane rating is not a measure of the amount energy in the fuel, but is a measure of its resistance to detonation.

The advantage [of] higher octane fuels is that a higher compression ratio or supercharging ratio can be used, which then leads to a higher engine cycle efficiency, which in turn means more power output for a given fuel burn. [...]

 

Avgas is measured on Lean Mixture (similar to MON) but also has a Rich Mixture Octane rating.

The Lean Mixture rating is 100 octane (15 octane higher than the comparable 85 MON for unleaded Mogas) but Avgas also has a Rich Mixture rating of 130 which allows higher supercharger boost pressures to be used without detonation occurring. This is particularly a problem when using high power settings at low altitude, for example during take off.

[...] TEL in Avgas makes a significant difference to the octane rating and without it Octane ratings would be back down to 80 - 85 Lean Mixture - the level for road fuels - instead of 100 / 130. This is not a problem for most typical modern normally aspirated engines as their compression ratios are quite modest and detonation would not be a problem with 80 - 85 Lean Mixture Octane fuel.

However, for those aeroplanes with supercharged or turbocharged engines the use of low octane unleaded fuels would not be suitable. The only way to operate these turbo engines on current unleaded technology fuels would be to significantly reduce the boost pressure of the supercharging and massively de-rate the engines. This de-rating would be so severe that many of the engines would no longer be powerful enough for the aeroplane in question.

(The Shell document also discusses how “Lead compounds from TEL form a protective layer on the valve seat and prevents the soft valve seats from eroding,” but as engines rebuilt or manufactured within the last couple of decades generally have cylinders with hardened valve seats (and cylinders can be replaced without splitting the engine case, relatively inexpensively and non-invasively), this is essentially no longer a factor. Paul Bertorelli agrees. Though it may not be a fully solved problem?)

Benefits of Unleaded Fuel

In addition to the environmental impact benefits (which, despite poorly conducted (i.e., uncontrolled) studies and scaremongering, are actually negligible, but not zero) and avoiding the negative “optics” of running leaded fuel, practical advantages exist for the aircraft operator. E.g., Lycoming has issued Service Letter L270 doubling most maintenance intervals for aircraft operated exclusively with unleaded fuel, for engines that can be so operated (identified in Service Instruction No. 1070AB). Similarly, Rotax calls for more frequent maintenance when leaded fuel is used at least 30% of the time. With unleaded fuel, plug fouling (where lead deposits on spark plugs) and lead sediments in the oil system are a thing of the past. Etc.

TEL is also a single-source item (Cheshire, UK-based firm Innospec Ltd is believed to be the last remaining manufacturer), a precarious thread to hang the entire piston GA fleet from.

AvGas Today

The readily available and universally usable AvGas is known as 100LL (100 octane low lead). Aviation fuels are rated using the MON method, as opposed to what American consumers see at an automotive gas pump, which is the Anti-Knock Index, determined using (RON+MON) / 2. E.g., AvGas at 100 MON is roughly equivalent to 105 octane using the AKI method:

Regular Gasoline

Known as “MoGas” in aviation circles, in California this typically tops out at 91 octane (roughly equivalent to 85 MON). Many aircraft will run on MoGas, though some modification and/or paperwork may be required. The “legacy” engines that can run on MoGas, like the Lycoming O-320s and O-360s, are typically lower compression, lower horsepower versions, and typically carbureted. The higher performance, higher compression engines (over about 8.5:1 compression) cannot run on MoGas due to the missing octane (i.e., MoGas tops out around 91-93 AKI depending on the region, which is < 90 MON; engines like the 200 horsepower fuel injected Lycoming IO-360 used in airplanes like the Mooney M20 and Piper Arrow, run at 8.7:1 compression and require 100 MON minimum).

 

Common issues preventing MoGas from being a universal drop-in replacement, like vapor lock and pneumatic lock - detailed in the Petersen MoGas STC FAQ). Also, even where the engine itself can run on MoGas, issues can arise with particular airframes that prevent them from receiving an STC. For instance:

 

The Mooney and Comanche both experienced vapor lock problems when they were tested. We solved the vapor lock problem but could not overcome pneumatic lock. Pneumatic lock takes place when the fuel boils as it enters the carburetor. The engine then dies due to an over rich mixture. This is just the opposite of a vapor lock where the engine quits or runs poorly due to a lean mixture. The better an airplane performs, the more difficult it is to get it through the flight test program. (https://www.autofuelstc.com/approved_engines_airfames.phtml (emphasis added))

Ethanol

Most MoGas contains various amounts of ethanol. Airplanes using a Petersen Aviation Supplemental Type Certificate (“STC”) cannot run on MoGas containing ethanol; others, like the Rotax engines, can run with a limited amount; the Pilot’s Operating Handbook for the SportCruiser / PiperSport (generically, a “POH”) specifies no more than 5% ethanol, but Rotax in Service Instruction SI-912-016 R2 has approved up to E10 MoGas successfully, and suggests owners contact the manufacturer for approval to run such fuel. For Special Light Sport Aircraft (SLSA) (Light Sport airplanes that are factory-built to FAA-accepted industry standards) running Rotax (and other) engines, there is no STC, rather, the manufacturer must approve deviation from the POH for each owner or operator, for each aircraft. (This does not apply to Experimental Light Sport Aircraft.)

 

There may also be an issue with ethanol’s specific gravity being very close to water: it can hold water in suspension. When temperature drops at altitude, the water can accumulate as ice on fuel screens, etc., which can cause an engine to stop running.

 

Finally, ethanol is not compatible with some seal materials and possibly fuel bladders and “wet wing” fuel tanks (most airplane wings are made out of aluminum): “high percentage ethanol blends such as E85 can corrode aluminum and certain grades of plastic and rubber.” (source)

As the Federal Aviation Administration (FAA) has summarized: Aircraft fuel system components are not made to withstand the corrosive nature of the added ethanol. Also, since some aircraft have been flying for many years, the natural rubber components that are designed for robust, ethanol free, hydrocarbon-based fuels are unable to withstand the abrasive action the ethanol imparts on them. These problems lead to reduced fuel system performance and eventually failure. https://iopscience.iop.org/article/10.1088/1757-899X/370/1/012009/pdf

Fuel Injection

As Petersen discovered in flight testing, only certain fuel injection systems are compatible with MoGas:

We do not have Auto Fuel STCs for engines using Bendix fuel injection because it failed our flight testing. Continental fuel injection did not have that problem and is therefore approved – I0-470-J or -K (225hp). The 260hp I0-470 and 285hp I0-520 are also approved. These last two require Anti Detonation Injection (ADI).

Unleaded Aviation Fuel

PAFI Fuels

The Federal Aviation Administration (“FAA”) began the piston aviation fuels initiative (“PAFI”) in 2014, “to evaluate candidate unleaded replacement fuels and identify those best able to technically satisfy the needs of the existing aircraft fleet, while also considering the production, distribution, cost, availability [and] environmental and health impacts of those fuels.” The FAA selected four fuels, two from Swift, two from Shell, for the initiative. Updates haven’t been encouraging, and the FAA has been silent for over a year as of this writing.

• Swift pulled out in 2018, but continues to work on its UL102.
  
• The FAA “temporarily suspended” testing PAFI fuels, also in 2018.
• Shell continues to work on a universal drop-in unleaded 100LL replacement.
  

Swift UL94

Currently relatively widely available, Swift’s 94UL (which is, as I understand it, essentially 100LL minus the lead; it has an AKI rating of 99 octane) can be run on many engines and airframes, but nowhere near all. Still, for the bulk of, e.g., the training fleet (which is almost universally Cessna 172s and carburetor-equipped Piper PA28s), it’s a viable replacement. Swift claims that its UL94 “satisfies the minimum octane requirements of over 125,000 aircraft (66% of the US piston fleet) as a ‘Drop-in Ready’ Unleaded Avgas.”

GAMI G100UL

This is the option I’m excited about. It exploded onto the scene at EAA AirVenture, when the initial STC was announced on July 27, 2021. The original STC covers a limited number of Cessna 172 models with specific Lycoming engines, but GAMI has tested the fuel on “turbocharged and normally aspirated Continental IO-550s and Lycoming IO-390s and IO-540s […] engines [which] are used in Cirrus SR22Ts and Piper Mirages, Malibus, and Navajos among others.” They projected fleet-wide availability within a year, and just three months later, added an additional 611 engines to the power plant Approved Model List (“AML”). “Airframes approved for pairing with the engines will be identified separately on a list expected soon from the FAA.”
 
The October announcement also notes that “GAMI and fuels distributor Avfuel have outlined their work on a distribution strategy for GAMI’s G100UL avgas, and [AOPA consultant and aviation fuels expert Paul] Millner added that the companies are ‘accelerating its introduction in areas where 100LL is being taken away.’”

(Avfuel is the distributor for G100UL.)

The fuel is expected to weigh about .2 lbs/gallon more than 100LL, and to be priced about $.85/gal more. It can be mixed with 100LL in tanks and in aircraft.

Swift’s 94UL will never provide the anti-knock parameters necessary for higher compression aviation engines like, e.g., the 200 HP IO-360 used in Piper Arrows and Mooney M20E, M20F, and M20J models; G100UL will, and they’re aggressively and rapidly expanding the aircraft it will work in.

 

Update: On September 1st, 2022, “the FAA … signed 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.” (https://www.aopa.org/news-and-media/all-news/2022/september/01/closer-to-an-unleaded-future) The future is already here, it’s just not evenly distributed yet. (With apologies to William S. Gibson.)

The Future

Meanwhile, I’m still holding out hope for electric aircraft. The energy density and endurance aren’t there yet, but battery technology seems to be improving linearly ...