Making The Case For Aviation’s Sustainability Options

aircraft rendering

The Boeing-led Cascade model aims to provide a means to assess the effects of different sustainability choices for aviation.

Credit: Boeing

Biofuels and e-fuels, electricity and hydrogen, mandates and tax credits—these and other options on the table risk overwhelming policymakers being pressed to provide government support to help aviation become sustainable.

  • Cascade model underlines importance of SAF to aviation sustainability
  • Life-cycle emissions of electricity and hydrogen must be considered

What role should an aircraft manufacturer play in that decision-making melee? For Boeing, one key effort is to model the effects of different options on aviation emissions in a way that helps policymakers and customers assess the outcomes of their choices.

Boeing’s model is called Cascade, and the initial results of beta testing are reinforcing what the company is already arguing: Fleet renewal and sustainable aviation fuel (SAF) are the most powerful levers that can be used to improve airline sustainability.

Cascade is suggesting that electric and hydrogen propulsion can help reduce emissions, but perhaps not as much as some hope. The reason is not just the limitations of technology but also the cleanliness of the electrical grid used to recharge batteries or produce green hydrogen.

“There’s almost this belief that, when you say ‘electricity’ or ‘hydrogen,’ those are already green,” says Chris Raymond, Boeing’s chief sustainability officer, a position created in 2020. “Whereas if you say ‘sustainable aviation fuel,’ people say: ‘Yes, but there’s life-cycle emissions.’”

The concept of total life-cycle emissions is understood for liquid fuels, he says, but not for electricity and hydrogen. “One point we try to illustrate with Cascade is that, if you think electricity or hydrogen is your fuel of the future, you need to take into account the same total life-cycle emissions as we do with liquid fuels.”

Take the example of an electric regional aircraft. Assuming a 700-nm-range aircraft that replaces 100% of the current fleet and a 100% clean, renewable-energy grid, Cascade predicts global emissions will drop just 2.5%. “Not a lot, but it helps,” Raymond says.


Now assume a hydrogen-powered regional. With today’s gray hydrogen, emissions get worse. “What if the hydrogen were blue? There’s a crossover point. As it gets closer to cleanly produced green hydrogen, it starts to help,” he says.

Initially, Cascade models the global scheduled air traffic system for the yearlong period from February 2021 through February 2022—more than 32 million flights that generated as estimated 952 million metric tons of CO2.

“One observation is that about 50% of the flights are less than 1,000 km [540 nm], but that’s not where the fuel usage is—50% of the fuel is on flights over 2,800 km, but that’s only about 20% of the flights,” Raymond says. “So we’ve got to solve this long-range problem eventually.”

In a demonstration at Boeing’s Washington office, Raymond used Cascade to show that replacing all existing narrowbodies with new Airbus A320neos and Boeing 737 MAXs “at the snap of a finger” would reduce global emissions 17%. Operational efficiencies such as winglet retrofits and air traffic management improvements would buy another 6%. But a 50% SAF blend would provide a 26% benefit.

“SAF and fleet renewal are the ways the industry can get to net-zero by 2050,” he says. “Our view is you’re going to need SAF no matter what. So we’ve developed this expression ‘SAF and’ because our view is it’s going to take SAF and whatever electricity and hydrogen can do. It’s not an ‘or.’

“We also don’t think we should sit around and wait for the perfect SAF. We’re going to have to embrace a lot of different pathways for SAF. What is anathema in Europe is acceptable in the U.S. or Brazil—ethanol, for example,” Raymond says. “We just need to have some level of sustainability criteria around those feedstocks.”

Boeing sees SAF coming in three phases. The first is existing pathways, primarily HEFA (hydroprocessed esters and fatty acids) from fats, oils and greases. “It starts with waste- and biomass-based SAF, but over time people are going to add renewable electricity or green hydrogen to that equation, and those pathways can be made cleaner,” Raymond says. “We call that power-and-biomass-to-liquid.”

The end state, in Boeing’s view, is power-to-liquid SAF, also called synthetic kerosene or e-fuel. This is produced from green hydrogen and captured CO2 using renewable energy and is more popular in Europe, where it is to be included as part of the EU’s upcoming SAF mandate.


“My only dilemma with that is it’s going to mean choosing to spend a huge amount of available renewable energy making a fuel,” Raymond says. “And if today’s SAFs are 2-4 times more expensive [than fossil jet fuel], wait till we see what that might pencil out at. We’re working with the Massachusetts Institute of Technology on what we have to solve as we migrate to power-to-liquid.”

SAF today is limited to a 50% blend—and in reality to a much lower percentage due to availability—so that the blended fuel can be considered a drop-in replacement for conventional jet fuel. Boeing and Airbus are working to have aircraft compatible with 100% SAF by 2030.

ASTM International has a task group developing a specification for drop-in SAF that can completely replace fossil jet fuel without requiring aircraft or engine modifications. The standards body also has a second task group working on a non-drop-in 100% SAF—a specification for a new, and hopefully better, fuel that each airframe/engine combination would need to be certified to use.

Airbus is chairing the non-drop-in 100% SAF group, but Boeing is also involved, Raymond says. “What we’ll be seeing is how far you can push the chemistry of a SAF and still have it considered drop-in. And what would you come up with as a clean-sheet fuel chemistry if you didn’t care about making airplanes already in the system adapt to it? And how close can those two things get to a convergence point?”

In effect, industry is working left to right to see how far it can push drop-in SAF chemistry and from right to left to develop a new fuel optimized to reduce all emissions, including soot particles, nitrogen oxides, carbon and contrail formation.

The question that teams on both sides of the work are trying to answer, Raymond says, is: “If I wanted to optimize for all those things, what chemistry would I come up with, and how far away is that from a drop-in chemistry? We need to be figuring that out because we might find an ‘aha’ in there. I don’t know if those two will ever coalesce into one, but I think it’s good practice to come at it from both directions.”

Meanwhile, Boeing plans to use Cascade to engage government policy-makers and encourage them to start with policies and incentives “where technologies are mature and the impact can be felt sooner while we’re working on tougher things like electric and hydrogen,” he says. The model can be used to show them why policies that will help scale SAF production or incentivize investment can make a difference.

In addition to its SAF subject matter team, Boeing has a dedicated global policy team. “We’re trying to share what we are seeing in other jurisdictions,” Raymond says. While the EU plans to use the stick of a SAF mandate, the U.S. has enacted the carrot of a blender’s tax credit. “What we’ve said in Europe is it’s not for Boeing to say whether someone wants to use a mandate or not, but you pair it with some incentives,” he adds.

Boeing has also learned that the book-and-claim system, which allows customers to buy SAF for delivery elsewhere and still get the carbon credit, is important as the industry scales up. The EU does not yet recognize book-and-claim. “That is one thing we’ve tried to suggest to the EU alongside our European customers,” he says. “You don’t want people doing stupid things like tankering or road-transporting fuel.”

Boeing and its collaborators are now working on a dynamic mode for Cascade to go from modeling a year in the life of the aviation system to projecting the effects of various choices out to 2050. Users of the tool will be able to enter assumptions for traffic growth, fleet renewal percentage and the types of aircraft that enter the system as well as the rate at which their production ramps up to see how close to net-zero emissions they can get. 

Graham Warwick

Graham leads Aviation Week's coverage of technology, focusing on engineering and technology across the aerospace industry, with a special focus on identifying technologies of strategic importance to aviation, aerospace and defense.