SAF reduces the carbon footprint associated with turbine-powered aircraft operations.
By Shannon Forrest
President, Turbine Mentor ATP/CFII.
Challenger 604/605, Gulfstream IV, MU2B
The average line pilot cares little about aviation fuel outside of the basic tenets taught in flight school. The 3 essentials are the number of gallons that can be uploaded, how much each gallon weighs, and the available flight duration given a specific burn rate.
These principles of fuel management are critical to every aviation operation, but there’s another concept worthy of our attention – acquisition. Acquisition is not simply telling the lineman at the FBO to add 100 gallons of Jet-A per side, or “top her off.” It’s about whether fuel will be available in the desired quantity, in what form, or at all. Fuel management is all about the science.
But fuel acquisition is a combination of science, politics, regulatory policies, agendas, marketing, and diametrically opposed beliefs espoused by competing interests.
A possible precedent
The battle over 100LL avgas in California provides a glimpse into the future of acquisition. Beginning on January 1, 2022, the Santa Clara County government banned RHV (Reid-Hillview, San Jose CA) from selling 100LL aviation fuel.
The County Board of Supervisors justified the decision based on an airborne lead study report that they claim shows local children have increased lead levels in their blood. Opponents, including the National Business Aviation Association (NBAA), General Aviation Manufacturers Association (GAMA), Aircraft Owners and Pilots Association (AOPA), and Experimental Aircraft Association (EAA), pointed out flaws with the study.
The most salient point was that structures in the surrounding area were built with lead paint and contained lead pipes, and that this presented a confounding variable that might not have been accounted for. Nonetheless, authorities shut down the 100LL fuel pump at RHV – an airfield that tallies more than 500 operations a day, with 40% being transient traffic.
Reducing environmental impact
Banning 100LL won’t affect turbine aircraft operations, but it’s the proverbial shot across the bow for anyone operating a machine that has a real or perceived effect on an issue considered important to others. The concept of reducing the environmental impact of turbine aircraft began years ago in the form of carbon offset credits.
The basic idea is that an operator can offset some carbon-based turbine engine emissions by doing something else positive for the environment. As an example, a carbon offset credit is a fee, investment, or tax (preferred term depends on one’s personal belief on the matter) used to balance (offset) the effects of turbine engine exhaust on the environment.
How the credit is used is a function of which entity is collecting the money and their belief systems. Sometimes the “follow the money” principle is obvious. At other times, the recipients’ identity isn’t so clear, as the revenue finds its wayinto a general fund or governmental coffer.
The offset scheme stems from the fact that no viable technology exists that can recapture 100% of jet engine exhaust and make it completely carbon neutral. Given that fact, a carbon offset credit is closer to a penalty fee than an altruistic attempt at environmental consciousness.
United States airlines have committed to reducing carbon emissions by 50% of 2005 numbers by the year 2050. The best chance of doing so seems to lie with developing jet fuel with lower carbon content.
Sustainable aviation fuel
Sustainable aviation fuel (SAF) is the ICAO answer to lowering the carbon footprint associated with traditional Jet A. The term “sustainable” can be interpreted in different ways, but ICAO defines SAFs as “alternative fuels that achieve net greenhouse gas emissions reduction on a life cycle basis … respect the areas of high importance for biodiversity, conservation, and benefits for people from ecosystems, in accordance with international and national regulations … and contribute to local and social and economic development.”
It’s easy to misconstrue from this that “sustainable” means completely renewable. This is not stated explicitly in the ICAO definition, but, if one reads between the lines, it does imply a degree of sustainability. The sustainability aspect becomes important when looking at the global demand for jet fuel. In 2019, US domestic jet fuel consumption was approximately 26 billion gallons.
Worldwide, the demand was 106 billion gallons. And forecasters predict that, by 2050, global annual consumption of jet fuel will exceed 230 billion gallons. A little basic chemistry provides insight into the role that SAF is expected to play in the marketplace.
A compound that contains only hydrogen and carbon is known collectively as a hydrocarbon. The most common naturally occurring hydrocarbon is crude oil, which consists of decomposed organic matter from the remains of plants and animals.
Since the majority of hydrocarbons used for fuel date back to prehistoric times, the moniker “fossil fuel” is applied. It’s also the reason why jet pilots sometimes refer to high-powered takeoffs as “turning dinosaurs into decibels.”
Traditional fuel extraction
Crude oil and other hydrocarbons form in liquid or gaseous states in underground pools and reservoirs. When extracted and transported to a refinery, crude oil is transformed into more usable products through distillation, cracking, treating, or reforming. The crude that produces the best derivates with the least amount of effort is light crude low in sulfur content.
Distillation is the most common methodology. It consists of heating a liquid to a gaseous state, and then allowing it to cool back into a liquid. Each hydrocarbon product encompassed within the crude has a different boiling point which is used for separation and collection. Lighter products like propane and butane condense at the top, whereas heavier oils accumulate near the base of the tower.
Jet fuel, which ranges from 8–16 carbon atoms, pools near the middle, right below gasoline (4–12 carbon atoms) and above diesel (8–23 carbon atoms).
The fact that jet fuel and diesel have overlapping boiling points means that refineries can switch production quickly from jet fuel to diesel and vice versa, depending on which is garnering the most profit in the marketplace.
There’s also a degree of overlap between the boiling point of gasoline and jet fuel, which allows a refinery to steal volume from the distillate slated for gasoline when jet fuel prices are high. The industry term for this is swing fuel.
Production is swung from one product to another, based on demand and return on investment. The standard input metric used when crude enters a refinery is 42 gallons. In terms of volume (based on data from the US Energy Information Administration), a 42-gallon barrel of crude yields about 3.5 gallons of jet fuel. The rest is gasoline and other fuels and waxes.
If the refinery opts to swing the fuel, more jet fuel can be produced, but increasing global demand can outpace production. Jet A (or Jet 1/1A) consists of 4 different families of molecules – n-alkanes, iso-alkanes, cyclo-alkanes, and aromatics.
Regulatory requirements are defined in ASTM D1655-21c, Standard Specification for Aviation Turbine Fuels (astm.org/d1655-21c.html). The specific ratios of these molecules determine the final characteristics of the product.
Variables like specific energy properties, flash point, and freeze point can be manipulated by altering the formula. Each molecule adds functionality. The complex interaction of all the chemical components makes replacing traditional jet fuel challenging.
How SAF works
A main criterion of SAF is that it’s a “drop-in” solution, meaning it can be used without changing or modifying existing hardware or infrastructure. It’s not only the engine that’s a concern. Jet fuel is transported directly by pipelines from refineries to major airports.
As an example, the jet fuel supply for DFW (Intl, Dallas–Fort Worth, TX) is delivered from a pipeline that begins some 400 miles away in Corpus Christi TX. That same pipeline supplies SAT (San Antonio TX) and AUS (Austin TX) as well. In fact, the 10 largest commercial airports in the US are supplied in this manner, so SAF that’s not compatible with the existing delivery system would be untenable.
The current direction of SAFs is to replace the 4 fossil fuel-based molecules in jet fuel with the same hydrocarbon molecules extracted from other sources. In lieu of using crude oil, the focus is on hydrocarbons that are relatively easy to obtain and can be employed relatively quickly and with less environmental impact. New sources include synthetic gas, fats, oils, greases, sugars, alcohols, and even algae.
Basically, anything organic that decays is fair game. Who’s working with SAF? Honeywell is engaged in partnerships that combine refining technology with bioengineering to produce SAFs made from a variety of feedstocks. In 2021, Honeywell supported 2 of the world’s first commercial jet flights to use algae oil SAF.
These took place on a Boeing 787 and an Airbus A350. Other companies heavily invested in SAFs are Neste, World Energy, Alder Fuels, and SkyNRG. Moreover, airlines such as Alaska, American, Delta, and United have publicly announced support for SAFs.
Avfuel has partnered with Neste to deliver SAF to corporate jets. The feedstock comes from used cooking oils, grease, and rendered animal fats, and is available primarily in northern California. Continuous supply is available at ACI Jet SNA (Santa Ana CA), Atlantic Aviation ASE (Aspen CO), Million Air BUR (Burbank CA), and Sonoma Jet Center STS (Santa Rosa CA), as well as TEX (Telluride CO) and TRK (Truckee–Tahoe CA). Although Phillips 66 doesn’t directly produce SAF, it allows customers to participate in a carbon offset program.
Sometimes, press releases and sound bites can be misleading in terms of how much SAF is being consumed on a given flight. Contrary to what the public thinks, the aircraft fuel tanks are not being topped off with the used fryer grease from fast food restaurants. In nearly all cases, a percentage of SAF (usually 20–30%) is blended with traditional jet fuel, or one engine is being fed with SAF while the other(s) consume Jet A.
In theory, SAFs are a good idea. If one operates under the assumption that fossil fuels are a finite resource, eventually the demand for jet fuel will outpace production. However, SAFs are currently facing some challenges. Right now, SAFs are 4 times the cost of jet fuel.
A corporate operator with a goal of being environmentally conscious might be able to stomach the additional expense, but airlines are notoriously cost conscious, especially when it comes to fuel, so converting exclusively to SAF would sink the balance sheet. Secondly, there just aren’t enough alternative feedstocks available to replace the current fossil fuel demand with SAFs.
Those that are available are being used largely to make “green” alternatives to traditional diesel fuels. The chemical similarity between diesel and jet fuel is a negative in this regard. Lastly, there’s the question of tradeoffs.
An algae farm may not sound too invasive, but what about farmlands being converted to growing feedstocks to replace fossil fuels? This is currently happening in the case of corn being converted to ethanol, but at what point does the ecosystem become more oriented toward sustainable fuel production at the expense of food production?
These are deep philosophical questions that are not likely priorities for the average line pilot taxiing up to the fuel pump. Nonetheless, these topics are being discussed by those with the ability to enact regulations that govern the airspace. It’s something to watch. In the future, using SAFs might be a case of “comply, or don’t fly.”