Jumpstarting RD&D for Clean Aviation

Clean Aviation Header Graphic


Emissions from aviation have been accelerating over the past few years, a trend that is likely to continue once the industry comes back from the COVID-19 slowdown. While the aviation industry is expected to continue struggling financially through the early 2020s, we need to put the right policies and investments in place now to ensure the industry can come back stronger and more sustainable than it was before.

Reducing emissions from aviation will require a significant increase in the use of sustainable aviation fuels (SAF), as well as adoption of new aircraft technologies to reduce fuel burn. Federal investment is critical to help research, develop, and deploy the clean fuels and technologies needed to put aviation on a lower-carbon trajectory.

This memo—the first in a series of papers focused on clean aviation—outlines the federal programs that have been most successful in researching and developing new SAF and clean aircraft technologies. Increased funding for these programs and, in some cases, additional direction from Congress will put aviation on a faster path to decarbonization.


In December 2020, the U.S. Environmental Protection Agency (EPA) finalized a new rule which will, for the first time, set carbon emissions standards for airplanes manufactured in the U.S.1 The new rule would help U.S. airlines comply with the International Civil Aviation Organization’s (ICAO) international carbon emissions standards, and ensure aircraft manufactured in the U.S. can be certified here and sold for use in international flights.

This is a step in the right direction, but much more needs to be done to reduce carbon pollution and other environmental and climate impacts from aviation. Aviation is currently responsible for nine percent of U.S. transportation emissions and 2.5 percent of overall emissions.2 And while air travel has declined sharply this year as a result of COVID-19, emissions from aviation have generally increased in recent years due to increasing commercial air traffic.3 Any plan to decarbonize transportation will have to stop the growth in emissions and make much deeper cuts than what EPA’s new rule requires.

Cleaning up aviation will require reducing fuel use with new technologies to make aircraft as fuel-efficient as possible, and making the fuel supply cleaner by developing and scaling up sustainable aviation fuels (SAF). We need to fund the right programs to drive the research, development, and demonstration (RD&D) work necessary to scale up those technologies and fuels. We also need to put the right policies in place to provide long-term market certainty to drive innovation in the private sector and get more of these technologies and fuels into commercial use quickly.

Now is the right time to make these investments. Airlines are expected to continue struggling financially for several years,4 and this could impact their ability to make their own planned investments in clean aviation RD&D.5 Increased federal funding will allow these efforts to continue during the economic downturn, ensuring our airlines can come back cleaner than they were before. It will also have strategic payoffs in the coming years, enabling U.S. airlines to remain competitive with their European counterparts which are investing heavily in SAF.6

This memo outlines policies for jumpstarting our RD&D efforts around SAF and clean aircraft technologies. Future memos will address policies to reduce emissions at airports and policies to ensure rapid adoption of low-emission aircraft technologies and fuels.

Sustainable Aviation Fuels

The majority of the emissions reduction we’ll be able to achieve in aviation will come from blending more SAF into the fuel supply, according to International Coalition for Sustainable Aviation (ICSA) analysis.7 Several airlines and carriers like Southwest and FedEx have signed individual offtake agreements with SAF producers. However, these agreements are for limited amounts of fuel, and only two U.S. airports—Los Angeles International and San Francisco Airport—are blending SAF into their fuel supplies for use by all airlines.89

When it comes to decarbonizing aviation, the top priority for federal RD&D investments should be to get SAF produced at-scale and at a price point where it’s competitive with conventional fuels. Closing the price gap between SAF and conventional jet fuel is particularly challenging now given the low price of regular jet fuel, but it is key to helping airlines transition to a cleaner fuel supply.

SAF can be made in a variety of forms and with a number of unique technologies. This includes fuels made from bio-based feedstocks, those that originate from waste streams, and power-to-liquid synthetic fuels (such as by making hydrocarbons using carbon from direct air capture and hydrogen produced from clean electricity). Federal RD&D policy should be technology and fuel neutral, so long as the fuel meets robust standards for sustainability.  In July 2020, the House of Representatives passed the Moving Forward Act, H.R. 2, which included a clean aviation title. The bill required that SAF producers measure a fuel’s emissions on a life-cycle basis, including the emissions that come from producing and transporting it.10 This should serve as a model for any additional legislation on SAF to ensure we don’t use feedstocks that threaten mass deforestation or are otherwise unsustainable.

There are several federal programs that have had success in helping the private sector develop and deploy SAF. Congress should increase funding for the most successful programs, outlined below, to help identify promising new processes and technologies for producing fuels and get them into commercialization quickly.

Researching and Developing New Pathways for SAF

A fuel pathway is any combination of a feedstock (such as renewable biomass), a fuel production process, and a fuel type (such as liquid or gaseous fuel, or electricity). There are currently seven federally certified pathways for SAF, ranging from synthetic oil derived from municipal waste to bio-jet fuel made from algae.11 A top priority for getting more SAF into the jet fuel supply is continuing RD&D efforts to identify and develop new pathways for SAF—such as through the development of new, renewable feedstocks--and then getting them through the certification process and into commercialization. There are several programs across multiple federal agencies that provide funding for researching and developing new ways to make SAF.

CLEEN: The FAA’s Continuous Lower Energy, Emissions and Noise (CLEEN) Program allows the FAA to enter into cooperative agreements with companies to develop and test sustainable aviation fuels. Under the first two funding phases of CLEEN (which ran from 2010 to 2015 and 2015 to 2020, respectively), companies like Honeywell and Pratt & Whitney completed testing for several different SAFs that have now been certified for use in aircraft.12

Congress should increase funding for CLEEN to enable more SAF projects to move forward. It should also raise the federal cost-share (currently no more than 50% of total project funding) to ensure companies that are struggling because of COVID-19 and the economic downturn can continue participating. As part of the recently-passed COVID relief and government funding package, Congress included a 7.8% increase in funding for FAA’s NextGen Environmental Research Aircraft Technologies and Fuels account, which funds CLEEN as well as ASCENT and CAAFI (see the next two programs in this section). This is a commendable first step that will ensure these programs can scale up quickly and fund more RD&D projects.

Congress can be even more ambitious than this. The House-passed Moving Forward Act of 2020 created a new five-year, $1 billion grant program within the U.S. Department of Transportation for projects to develop, demonstrate, and deploy SAF and low-emission aircraft technologies.13 The bill also provided FAA with an additional $150 million for the study and development of SAF. Taken together, these programs would have provided over $200 million per year for clean aviation RD&D, much of which would go towards developing and demonstrating new SAF pathways. While this bill did not become law, this is the kind of down payment on the clean aviation future we need and would significantly help jumpstart our RD&D efforts. As Congress considers clean aviation legislation in the future, they should funnel this additional funding through existing programs such as CLEEN to get the money awarded as quickly as possible.

 ASCENT: The FAA Center of Excellence for Alternative Jet Fuels & Environment (ASCENT) is a university cooperative that does much of the analytical work to enable development of low-carbon aviation fuels. ASCENT’s work has included characterizing new SAF to help with certification, testing and evaluating new fuels to determine how they affect emissions and engine performance, and creating lifecycle assessment tools used by other programs like CLEEN.14

ASCENT also runs two programs that help producers navigate the often long, onerous, and expensive testing and evaluation processes leading to certification of a new fuel. This includes the D4054 Clearinghouse, which serves as a one-stop shop to help new fuel producers navigate the testing and evaluation process15 and the National Jet Fuels Combustion Program, which helps producers perform more time- and cost-efficient screenings for new fuels before starting the federal certification process.

Congress provided a funding increase for ASCENT in the recent omnibus spending bill, which will help it ramp up its work evaluating new fuels and getting them ready for certification. The Moving Forward Act would have provided an even larger, 33% increase in funding per year for five years. This is an appropriate amount that would lead to the establishment of new pathways for SAF.

CAAFI: The Commercial Aviation Alternative Fuels Initiative (CAAFI), established in 2006, is a coalition of aviation industry stakeholders, energy producers, and government agencies promoting the development and deployment of SAF. CAAFI serves as the voice of the private sector in many of the federal government’s clean aviation efforts, including advising ASCENT on research direction, engaging with USDA on new feedstock development projects, and managing the now-expired Farm-to-Fly Initiative (the latter two are explained in more detail in the next section of this paper). CAAFI also develops tools for producers such as the Feedstock Readiness Level evaluations repository, which helps researchers, fuel producers, and federal agencies better understand the maturity of different feedstocks.16

Congress should increase funding for CAAFI so it can continue coordinating private sector involvement in federally-driven RD&D efforts. It will be particularly important that CAAFI is well-resourced if Congress increases funding for SAF research.

BETO: The Bioenergy Technologies Office (BETO) within the U.S. Department of Energy forms cost-shared, public-private partnerships to develop cost-competitive biofuels from non-food biomass. Its portfolio includes projects to develop new fuel blends that require less fossil fuel to produce, increase the energy density of aviation fuels so less fuel is needed, and produce SAF from wood and sugars.17 In 2018, BETO managed a partnership between the Pacific Northwest National Laboratory and LanzaTech that resulted in the first commercial flight using SAF made from recycled waste carbon gas.18

BETO has funded many lab-scale research projects on SAF, but over the past few years has provided less funding for demonstration and deployment of fuels that are closer to commercialization. The omnibus spending package reduced funding for BETO slightly; Congress should instead increase funding for BETO and direct the office to use some of that funding for SAF demonstration and deployment projects. This will enable more SAF projects that have already gone through early-stage R&D to continue getting federal assistance as they scale up towards commercialization. Congress should also consider funneling any new money for SAF RD&D through BETO.

DOD: As part of the 2020 National Defense Authorization Act, Congress directed the Department of Defense (DOD) to develop technologies to capture carbon dioxide from the air (direct air capture, or DAC) and oceans and convert it into fuel.19 Once deployed at scale, DAC will be a source of ultra-low carbon jet fuel with an essentially infinite feedstock.20 But these DAC-to-fuels are still early in development, and military demand for these technologies will help spur innovation in the industry needed to bring them into commercialization.21

Congress appropriated $8 million for the Direct Air Capture and Blue Carbon Removal Technology Program in its first year, following that with $7.5 million in the subsequent FY21 appropriations cycle. Congress should monitor the program’s progress and, if it demonstrates success, be prepared to provide $10 million in FY22 as stipulated in the SEA FUEL Act,22 so that these DAC-to-fuel technologies can be commercialized more quickly.

Developing Robust Supply Chains

As new pathways for SAF are developed, we need to make sure supply chains exist to harvest the feedstocks, process them into SAF, and blend them into the jet fuel supply. This will involve identifying and scaling up region-specific feedstocks, constructing manufacturing facilities, and optimizing the storage and transportation of feedstocks and fuels. While there are several programs across multiple federal agencies that provide federal support for these activities, the ones listed below have been the most successful and would benefit most from additional funding.

NIFA: USDA’s National Institute of Food and Agriculture (NIFA) has worked with universities and companies in the fuel supply chain to develop regional industries to produce advanced biofuels and other biobased products. NIFA currently has nine of these Regional Bioenergy System Coordinated Agricultural Projects, six of which have ongoing aviation fuel projects.23 One of these, the Northwest Advanced Renewables Alliance (NARA), recently ran a $40 million, five-year research project that led to the first commercial flight powered in part by renewable fuel made from forest wood waste.24

Congress should increase funding for NARA and NIFA’s other regional programs including Sustainable Bioeconomy for Arid Regions (SBAR), Southeast Partnership for Advanced Renewables from Carinata (SPARC), and Integrated Pennycress Research Enabling Farm & Energy Resilience Project (IPREFER), with a goal of bringing more feedstocks for SAF into development, production, and delivery. Congress should also extend the Coordinate Agricultural Projects program, as some of the work of these regional programs has been delayed by COVID-19 related shutdowns and many of the grants expire in 2021 and 2022. Congress should also continue oversight efforts to ensure NIFA is sufficiently staffed to manage these programs, since its workforce fell by two-thirds when the Trump Administration moved NIFA’s headquarters from Washington, DC to Kansas in May of 2020.25

Biomass Crop Assistance Program: This USDA program provides financial assistance to owners and operators of agricultural and non-industrial private forest land to produce and deliver biomass feedstocks. The program has helped more than 800 farmers and landowners establish and maintain bioenergy feedstocks and connect with energy companies that can put these feedstocks to use.26 However, the effectiveness of the program was stymied by the fact that the program only provides 50% of the costs to establish a new crop; it can take one to three years to establish a new crop and there is always the risk of establishment failure, so farmers who are less risk tolerant have shied away from using the program.27 Congress has not provided funding for this program since 2017; Congress should provide funding for the program in future years and raise the matching funds for crop establishment so that more farmers can get assistance planting crops that can feed into the SAF supply chain.

ASCENT: In addition to the RD&D efforts mentioned above, ASCENT also performs economic modeling that can help develop regional supply chains for SAF.28 This includes determining what types of fuels are viable for development in a particular region, what feedstocks are needed, how much of those feedstocks could be developed sustainably, and where facilities and other infrastructure should be constructed to reduce transportation costs and emissions. The aforementioned funding increase included in the Moving Forward Act would have also helped ASCENT continue its SAF supply chain development work.29

Farm-to-Fly: The Farm-to-Fly Initiative was a joint agreement between FAA, USDA, and private sector partners acting through CAAFI that ran from 2010 to 2018. The initiative coordinated several different agencies’ activities related to supply chain development including developing standards for evaluating biomass supply chain risks, modeling potential feedstock transportation and emissions patterns, helping supply chain members nearing commercialization apply for loans, and conducting outreach to growers to increase their willingness to engage in new supply chains.30

Despite the success of this initiative, its authorization lapsed in 2018. Congress should reauthorize the program for another five years so that it can resume its work.

Clean Aircraft Technologies

While most of the emissions reduction from aviation will come from increased use of SAF, we can also achieve emissions reduction by deploying new technologies that enable aircraft to burn less fuel and emit less pollutants from the fuel they do burn. Full or partial electrification of aircraft provides additional opportunities to reduce the use of jet fuel, particularly for shorter-distance air travel.  

CLEEN: In addition to providing RD&D funding for SAF, CLEEN also supports the development of emissions-reduction technologies for aircraft. During the first two funding phases of CLEEN, several new pollution-cutting technologies were commercialized and are now being used in aircraft. For example, General Electric developed a new combustor that reduced NOx emissions, and Boeing demonstrated aircraft wing improvements that provide a two percent reduction in fuel burn.31

An increase in funding for CLEEN would allow more of these low-emission aircraft projects to get funded, in addition to the expansion of SAF projects described in the earlier section. With direction from Congress, it would also enable an increase in the federal cost-share to help projects move forward during the current economic recession. Any additional funding beyond current levels for aircraft-related projects should be focused on technologies that have already gone through early-stage R&D and are closer to demonstration, as these are the technologies that can be scaled-up and deployed the quickest.

ASCENT: Like CLEEN, ASCENT helps advance technologies in both the SAF space and the efficient aircraft space, doing much of the analytical work supporting federal RD&D efforts around clean aircraft technologies. Some of ASCENT’s projects have included modeling and assessing the technologies developed under CLEEN and designing new aircraft engines to increase efficiency. In increasing funding for ASCENT, Congress should ensure it continues to focus on the analytical and modeling work needed to assess the viability of new aircraft technologies, focusing on those that could be scaled up rapidly to reduce emissions as quickly as possible.

NASA: While much of the work being done through CLEEN will lead to near-term emissions gains, the National Aeronautics and Space Administration (NASA) is where more breakthrough advances will come through. This includes electrifying all or some of an aircraft’s power systems, which reduces how much fuel the aircraft needs to burn, as well as other technological advances such as new wing designs, new engines, and lightweight materials.32

One of the most impactful steps Congress could take to move the U.S. forward in this field would be to direct NASA’s aeronautics program to get a full demonstration plane (“x” plane) incorporating a range of technologies—including electrification and the others described above—into demonstration by the mid-2020s. . This would allow researchers to determine how to incorporate a range of different electrified systems into aircraft and help U.S. manufacturers get these new technologies and materials to market ahead of foreign competitors, who are already investing heavily.33,34  This work can also help ensure that next-generation forms of aviation, such as urban air mobility and electric vertical takeoff and landing (eVTOL) aircraft, can take advantage of the newest, cleanest technologies.35

However, NASA’s aeronautics budget does not currently allow for progress on this timetable. In order to get a full x-plane into demonstration within the next five years, Congress should provide a significant and sustained funding increase for NASA’s aeronautics budget.

Department of Energy: Other federal programs that help develop and scale-up electric battery systems for other modes of transportation can also be expanded to include electric aviation. For example, DOE’s Vehicle Technologies Office should expand federal grant funding eligibility to include developing electric systems for use in aircraft. The Advanced Technology Vehicles Manufacturing loan program, which currently helps automakers retool their facilities to produce electric vehicles and related components, should be expanded to offer loans to manufacturers in the aviation sector that could manufacture electrified aircraft systems. If these federal programs are expanded to include electric aviation, Congress should also increase funds proportionally so that the critical RD&D work needed to electrify vehicles already eligible for those programs, like passenger vehicles and trucks, won’t suffer.


Decarbonizing aviation will require a massive, federally led RD&D effort to develop and scale up new sustainable aviation fuels and clean technologies. This funding will need to be accompanied by policies that incentivize broader adoption of clean fuels and technologies that are already commercially ready, which will be the subject of a forthcoming policy memo. There are a variety of federal programs that already support these RD&D efforts; increased funding for these programs will ensure we can continue to make progress towards decarbonization even as the industry deals with the fallout of COVID-19 and the economic downturn. Whether in the context of an economic stimulus package, climate legislation, or annual appropriations bills, Congress should lean on these programs to jumpstart clean aviation RD&D and get us on the path to decarbonizing the industry.

  • Transportation16


  1. “Control of Air Pollution from Airplanes and Airplane Engines: GHG Emission Standards and Test Procedures - Final Rulemaking.” Environmental Protection Agency, 23 Dec. 2020, www.epa.gov/regulations-emissions-vehicles-and-engines/control-air-pollution-airplanes-and-airplane-engines-ghg

  2. “Fast Facts on Transportation Greenhouse Gas Emissions.” Environmental Protection Agency, June 2020, www.epa.gov/greenvehicles/fast-facts-transportation-greenhouse-gas-emissions

  3. “The Growth in Greenhouse Gas Emissions from Commercial Aviation.” Environmental and Energy Study Institute, 17 Oct. 2019, www.eesi.org/papers/view/fact-sheet-the-growth-in-greenhouse-gas-emissions-from-commercial-aviation

  4. O’Hare, Maureen, “Global Air Travel Won’t Recover Till 2024, Says Airline Body.” CNN, 29 July 2020, www.cnn.com/travel/article/air-travel-recovery-2024/index.html

  5. Plumer, Brad and Hiroko Tabuchi, “Coronavirus Could Slow Efforts to Cut Airlines’ Greenhouse Gas Emissions.” New York Times, 6 Mar. 2020, www.nytimes.com/2020/03/06/climate/covid-19-climate-change.html

  6. Robertson, Susan, “Europe Shifting to Clean Aviation Fuel.” Innovators Magazine, 19 Aug. 2020, www.innovatorsmag.com/europe-shifting-to-clean-aviation-fuel/

  7. “Envisioning a Zero Climate Impact International Aviation Pathway Towards 2050: How Governments and the Aviation Industry Can Step-Up Amidst the Climate Emergency For A Sustainable Aviation Future.” International Coalition for Sustainable Aviation, 9 Oct. 2019, Page A-3, www.icao.int/Meetings/A40/Documents/WP/wp_561_en.pdf

  8. “United Airlines Launches First-Ever Jet Biofuels Initiative at LAX with Flight to San Francisco.” Los Angeles World Airports, 11 Mar. 2016, www.lawa.org/en/News%20Releases/2016/News%20Release%20122

  9. “Neste to Supply Sustainable Aviation Fuel to Three Major US Airlines.” Neste, 13 Aug. 2020, www.prnewswire.com/news-releases/neste-to-supply-sustainable-aviation-fuel-to-three-major-us-airlines-301111362.html

  10. Laska, Alexander, “House Bill Clears Clean Aviation for Take-Off.” Third Way, 29 June 2020, www.thirdway.org/blog/house-bill-clears-clean-aviation-for-take-off

  11. “Fuel Qualification.” Commercial Aviation Alternative Fuels Initiative, Accessed Jan. 6, 2021, www.caafi.org/focus_areas/fuel_qualification.html

  12. “Fact Sheet – Continuous Lower Energy, Emissions, and Noise (CLEEN) Program.” Federal Aviation Administration, 4 Mar. 2020, www.faa.gov/news/fact_sheets/news_story.cfm?newsId=22534

  13. Laska, Alexander, “House Bill Clears Clean Aviation for Take-Off.” Third Way, 29 June 2020, www.thirdway.org/blog/house-bill-clears-clean-aviation-for-take-off

  14. “Projects: Alternative Fuels.” Aviation Sustainability Center (ASCENT), Accessed Jan. 6, 2021, ascent.aero/topic/alternative-fuels/

  15. “Clearing the Way for Alternative Fuels.” University of Dayton News, 29 June 2020, udayton.edu/news/articles/2020/06/udri_aviation_fuels_research.php

  16. “A 2019 Look-Ahead: Building on 2018’s Many Successes and Addressing Remaining Industry Challenges.” Commercial Aviation Alternative Fuels Initiative, Accessed Jan. 6 2021, caafi.org/files/CAAFI_2019_Look_Ahead.pdf

  17. “2019 Project Peer Review.” U.S. Department of Energy Bioenergy Technologies Office, April 2020, www.energy.gov/sites/prod/files/2020/04/f74/beto-00-2019-peer-review-report_0.pdf

  18. “Bioenergy Technologies Office FY 2018 Successes.” U.S. Department of Energy Bioenergy Technologies Office, April 2019, www.energy.gov/sites/prod/files/2019/04/f62/FY18%20BETO%20Successes%20Fact%20Sheet_Final.pdf

  19. For a full discussion of DAC and the benefits of funding DAC RD&D, see: Bright, Matt, “Getting Serious About Direct Air Capture.” Third Way, 9 Sep. 2020, www.thirdway.org/memo/getting-serious-about-direct-air-capture

  20. Stukas, Anna and Ellen Stechel, “Direct Air Capture of CO2 and Recycling CO2 into Sustainable Aviation Fuels.” Presented to the Commercial Aviation Alternative Fuels Initiative, June 2019, www.caafi.org/resources/pdf/Direct_Air_Capture_of_CO2_and_Recycling_CO2_into_Sustainable_Aviation_Fuels_June_2019.pdf

  21. Magill, Bobby, “Military Researching Ways to Suck Carbon from Air to Make Fuel.” Bloomberg Law, 15 Jan. 2020, news.bloomberglaw.com/environment-and-energy/military-researching-ways-to-suck-carbon-from-air-to-make-fuel

  22. The SEA FUEL Act, sponsored by Sen. Sheldon Whitehouse (D-RI) and incorporated into the NDAA, authorized $8 million for the program in its first year and $10 million for each of the following three years.

  23. “AFRI Regional Bioenergy System Coordinated Agricultural Projects.” U.S. National Institute of Food and Agriculture, Accessed Jan. 6 2021, nifa.usda.gov/afri-regional-bioenergy-system-coordinated-agricultural-projects

  24. “USDA, Partners Celebrate First Woodto-Jet-Fuel Commercial Flight.” U.S. Department of Agriculture, 14 Nov. 2016, www.usda.gov/media/press-releases/2016/11/14/usda-partners-celebrate-first-wood-jet-fuel-commercial-flight

  25. “USDA Relocations Led to Deep Staff Cuts, CRS Says.” FEDWeek, 11 May 2020, www.fedweek.com/federal-managers-daily-report/relocations-of-two-usda-agencies-cut-staff-deeply-crs-says/

  26. “Biomass Crop Assistance Program.” National Sustainable Agriculture Coalition, December 2019, sustainableagriculture.net/publications/grassrootsguide/renewable-energy/biomass-crop-assistance-program/

  27. Miao, Ruiqing and Madhu Khanna, “Limited Impact of Biomass Crop Assistance Program (BCAP) Under Current Funding Levels.” Farmdoc daily (7):155, Department of Agricultural and Consumer Economics, University of Illinois, 24 Aug. 2017, farmdocdaily.illinois.edu/2017/08/limited-impact-biomass-crop-assistance-program.html

  28. “Alternative Jet Fuel Supply Chain Analysis.” Aviation Sustainability Center (ASCENT), Accessed Jan. 6 2021, ascent.aero/project/alternative-jet-fuel-supply-chain-analysis/

  29. “Federal Alternative Jet Fuels Research and Development Strategy.” National Science and Technology Council, June 2016, Page 7, www.caafi.org/files/Federal_Alternative_Jet_Fuels_Research_and_Development_Strategy.pdf

  30. “Farm-to-Fly 2.0 (F2F2) Fact Sheet: Summary of Activities and Outcomes from the 6-Year Period of Activity.” Commercial Aviation Alternative Fuels Initiative, September 2019, Pages 7-9, caafi.org/focus_areas/docs/F2F2_Final_Report_September2019.pdf

  31. “CLEEN Phase I and II Projects.” Federal Aviation Administration, Feb. 27 2020, www.faa.gov/about/office_org/headquarters_offices/apl/research/aircraft_technology/cleen/media/CLEENI_CLEENII_Projects.pdf

  32. “NextGen Aircraft Design is Key to Aviation Sustainability.” National Aeronautics and Space Administration, Apr. 16 2020, www.nasa.gov/aero/nextgen-aircraft-design-is-key-to-aviation-sustainability

  33. Morgan, Sam, “Electric Plane Certified by EU Regulator in World-First.” Euractive, 12 Jun. 2020, www.euractiv.com/section/aviation/news/electric-plane-certified-by-eu-regulator-in-world-first/

  34. Tovey, Alan, “Rolls-Royce Turns to Germany to Develop Hybrid-Electric Aircraft.” The Telegraph, 5 Nov. 2019, www.telegraph.co.uk/business/2019/11/06/rolls-royce-turns-germany-develop-hybrid-electric-aircraft

  35. Goodrich, Kenneth H, “Dozens of Urban Air Mobility Projects Underway.” Aerospace America, December 2018, aerospaceamerica.aiaa.org/year-in-review/dozens-of-urban-air-mobility-projects-underway/