Continuous Lower Energy, Emissions and Noise (CLEEN) Program Summary and Status Report

Executive Summary

FAA CLEEN Program Overview

In partnership with industry the FAA’s CLEEN Program is developing certifiable aircraft and engine technologies that reduce noise and emissions while increasing fuel efficiency. Technologies developed by the CLEEN Program will result in a fleet of aircraft that have lower noise, use less fuel, and produce fewer emissions, thus supporting the overarching environmental performance goal for NextGen to achieve environmental protection that allows sustained aviation growth.

Historically, advances in aircraft technology have been the main factor in reducing aviation’s environmental impacts. Because of advancements in technology, there has been a 95 percent reduction in the number of people exposed to significant noise and more than a 70 percent improvement in fuel efficiency over the last 50 years. The vast majority of gains in fuel efficiency and noise reductions over the last four decades have come from enhancements in engine and airframe design. However, because of factors such as the growth in the number of operations and the implementation of new flight procedures, community concerns about noise remain a considerable challenge. Furthermore, there is a continued need for emissions reductions and improved fuel efficiency to ensure the sustainable growth of aviation.

Through the public-private partnership of the CLEEN Program (http://faa.gov/go/cleen), the FAA and industry are working together to develop technologies that will enable manufacturers to create aircraft and engines with lower noise and emissions as well as improved fuel efficiency. The technologies being accelerated by the CLEEN Program have relatively large technological risk but also enhanced benefits. Government resources help mitigate this risk and incentivize aviation manufacturers to invest and develop these technologies. By cost-sharing the development with the FAA, industry is willing to accept the greater risk and can better support the business case for this technological development. Once entered into service, the CLEEN technologies will realize their noise, fuel burn, and emissions benefits throughout the fleet for years to come. In addition to the benefits provided by technologies developed under CLEEN, the program leads to improved analysis and design tools that are improving every aircraft or engine product being made by these companies, well beyond individual technology applications.

The CLEEN Program is implemented in five year phases and has goals for noise, fuel burn, and emissions. The first phase of CLEEN was executed from 2010 to 2015. Based on the success of this program, the second phase of CLEEN was initiated in 2015 for a five-year term. The third phase of the CLEEN Program is included in the FY2020 enacted budget and has been kicked off in 2021. To receive funding from CLEEN, industry partners need to contribute at least 100% cost share to the program.

CLEEN Program Goals

The goals of the CLEEN Program are tied to the environmental standards that aircraft and engines are required to meet as a part of airworthiness certification. These goals are summarized in Table 1. As industry meets the goals, the FAA has made the goals increasingly more stringent. Further, additional goals have also been added over time. The third phase of CLEEN plans to target reductions in aviation noise, emissions, and fuel burn. This third phase of the CLEEN Program will also target community noise exposure as well as aircraft engine particulate matter emissions.

Table 1 Timeframe, Goals, and Entry into Service for the Three Phases of the CLEEN Program (Listed goals are for subsonic aircraft)

	Phase I	Phase II	Phase III
Time Frame	2010-2015	2015-2020	2020-2025
Noise Reduction Goal	25 dB cumulative noise reduction cumulative to Stage 5		25 dB cumulative noise reduction relative to Stage 5 and/or reduces community noise exposure
Fuel Burn Goal	33% reduction (relative to year 2000 best-in-class in-service aircraft	40% reduction (relative to year 2000 best-in-class in-service aircraft	-20% re: CAEP[1]/10 Std
NOX Emissions Reduction Goal	60% landing/take-off NOX emissions       (re: CAEP/6)	70% landing/take-off NOX emissions (re: CAEP/8)
Particulate Matter Emissions Reduction Goal	--	--	Reduction relative to CAEP/11 Std
Entry into Service	2018	2026	2031

While the CLEEN Phase I and II fuel burn reduction targets were expressed relative to year 2000 best-in-class in-service aircraft, for CLEEN Phase III, the goal is articulated as a 20% reduction relative to the International Civil Aviation Organization (ICAO) Committee for Aviation Environmental Protection’s (CAEP) CO₂ emissions standard adopted in 2017.^[2] The figure below represents the CLEEN Phase III goal (the red line) relative to the CAEP CO₂ emissions standard (the blue line). The chart also shows the levels of improvement which an ICAO CAEP Independent Experts panel (IEP) set in 2019 as goals for 2027 and 2037 across a range of aircraft size classes.^[3] These targets were based upon a review of current and planned aircraft fuel efficiency technologies and designs.

Figure 1 CLEEN Phase III Goal (red line) Relative to the CAEP CO₂ Emissions Standard (blue line)

With the third phase of CLEEN, FAA will be conducting aircraft technology development from 2021 through 2025, and targeted for entry into service of 2031 and beyond, its fuel burn reduction goal is appropriately targeted between the 2027 and 2037 IEP goals for CO₂ and fuel efficiency.

The figure below shows the CLEEN noise reduction goal in the context of previous and current noise standards.

Figure 2 CLEEN Noise Reduction Goal in Context of Previous and Current Noise Standards

The figure illustrates the FAA’s increasingly stringent Stage 2, 3, 4, and 5 noise standards, represented by the stepped red line. The blue data points in the figure represent the noise levels of previously certified aircraft in their year of certification.

The CLEEN noise reduction goal is to reduce aircraft noise 25 decibels, cumulatively at all three noise certification measurement points, below the Stage 5 standard. In addition, the figure compares the FAA noise standards and CLEEN goals to projections made by the ICAO CAEP IEP in 2019 for 2027 and 2037 across a range of aircraft size classes. In context, the CLEEN goal is aggressive, but important to continue to drive progress towards quieter aircraft in our National Airspace System.

Whereas the first two five-year phases of CLEEN focused on subsonic civil transportation, CLEEN Phase III is open to technologies for both subsonic and supersonic aircraft. The third phase of CLEEN considers to technologies that could reduce noise during the landing and takeoff phases of flight and/or reduce NO_X emissions throughout the flight of supersonic aircraft.

The first two five-year phases of CLEEN, as well as the ASCENT Center of Excellence^[4] and the Commercial Aviation Alternative Fuels Initiative (CAAFI),^[5] have been instrumental to the certification of alternative jet fuels for safe use by civil transportation via ASTM International. At present, the FAA is supporting the certification of new fuels through the ASCENT Center of Excellence D4054 Clearing House at the University of Dayton Research Institute.^[6] The Clearing House supports coordinated testing, evaluation, and review of alternative fuels for approval by ASTM International. These combined efforts have led to the approval of seven fuel types for use by civil aviation. These fuels are all drop-in compatible with today’s aircraft and fueling systems and can be made from a variety of biomass, waste, and fossil resources. Given their similar composition to today’s jet fuel, these alternative fuels do not result in a substantial change in fuel volume used; however, their use can result in a substantial (i.e., more than 50% reduction) non-volatile particulate matter.

CLEEN Phase III will continue supporting testing and qualification of new alternative jet fuels, with a focus on supporting greater than 50% blend levels of these fuels. These efforts reflect the desire of the aviation industry to use 100% alternative jet fuel without the need to blend with conventional jet fuel.

Benefits of the CLEEN Program

The CLEEN Program has matured technologies that have entered the fleet, and industry anticipates that additional technologies will enter into service in the coming years as opportunities arise for their insertion into new aircraft and engine designs. Additionally, the knowledge gained from the development of these technologies is leading to improved design codes and fabrication methods that are  being applied throughout these companies’ product lines, leading to improved environmental performance across the industry.

Some of the technologies matured by the CLEEN Program are entering the fleet in large numbers. For example, GE’s unique lean-burn combustion system, Twin Annular Premixing Swirler (TAPS), pre-mixes air and fuel prior to combustion for leaner burn and fewer emissions than conventional combustion systems. In 2016, the TAPS combustor was introduced on the GEnx engine for the Boeing 787 Dreamliner and the 747-8 aircraft. TAPS was enhanced under the first phase of the CLEEN program, and the resulting TAPS II combustor is now in operation in CFM International's LEAP engine for narrow-body aircraft, which currently has 8,000 orders. Under CLEEN Phase II, GE further developed the next generation TAPS (III) and achieved engine testing emissions demonstration of TAPS III combustion system. The TAPS III combustion system will be implemented in the GE9X-powered Boeing 777X, replacing the GE90-powered Boeing 777, and is expected to enter into service in 2021, enabling NOx emissions 30% below international CAEP standards.

Under CLEEN phase II, GE Aviation developed the Flight Management System (FMS) which automates a wide variety of in-flight tasks, enabling the pilots to safely and efficiently fly an aircraft from its origin to its destination, accounting for traffic, weather, aircraft performance, and required arrival time, while optimizing performance. The FMS uses a variety of sensors to determine its current position and sends guidance commands to the aircraft control systems to guide the aircraft along the approved flight plan. GE’s CLEEN Phase II FMS technologies result in a potential 1% fleetwide average improvement in fuel burn. GE is currently working to incorporate these technologies in future products.

Under CLEEN I, Boeing developed the Adaptive Trailing Edge (ATE) technology, which has been adopted for use in commercial and defense products.  Similar to Boeing, other companies have used technologies developed under the CLEEN program for other commercial and defense programs as well. Due to the sensitivity of these programs, the applications are not publicly releasable.

The CLEEN Program also aids industry in developing the analytical tools to create aircraft and engine designs that have lower noise, emissions, and fuel use. For example, the data and learnings from the CLEEN Phase I fan development work completed by Pratt & Whitney has been incorporated into the analytical tools that they use to design new engines. As such, this investment will lead to noise and fuel burn reductions from every new Geared Turbofan engine that is developed by Pratt & Whitney.

Low emissions combustion development work, conducted under CLEEN Phases I and II across GE, Honeywell, and Rolls-Royce projects, results not only in emissions reductions for the specific combustors developed under the program, but enhances each company’s ability to design and optimize all future engine combustion systems for low emissions.

Collins Aerospace, in conjunction with NASA, developed low drag acoustic fan duct technology and also created new analytical tools that will be utilized for future nacelle designs supporting ultra-high bypass ratio engines. Additionally, these efforts have generated new databases for low drag acoustic surfaces and material properties for novel acoustic structures. In particular, Collins is already introducing the use of the newly generated databases to assist the definition of acoustic specifications on near-‑term applications.

A side benefit of the CLEEN program is that, while maintaining continuous investment on advancing technologies for entering fleet service, the program has enabled development of supporting technologies with lower readiness level that may otherwise have been unsupported, such as developing viable manufacturing alternatives and advancing state of the art materials that can be used in various applications.

According to analysis done by Georgia Tech in 2020,^[7] the technologies matured in the first five-‑year phase of CLEEN will reduce U.S. fleet-wide fuel burn by 2 percent by 2030 and 3.7 percent by 2050,[8] providing a cumulative savings of 13.2 billion gallons of jet fuel. The CO₂ savings are the equivalent of taking 1.11 million cars off of the road from 2020 to 2050. In addition to saving airlines 26.4 billion dollars on fuel, technologies from the first phase of CLEEN will contribute to a 14% decrease in the land area exposed to significant noise, as defined by a day-night noise level (DNL) of 65 dB.

CLEEN Phase II technologies are expected to enter operational service by 2026, providing further benefits to fuel burn, emissions, and noise. An ongoing assessment of CLEEN Phase II’s projected fleet-wide benefits has estimated that the program will reduce fuel consumption 2.5 percent by 2030 and 7.9 percent by 2050, bringing the contribution of CLEEN Phase I and II to 11.6 percent fuel burn reduction in the fleet by 2050.⁸

Cumulatively, CLEEN Phase I and II are estimated to have saved 36.4 billion gallons of fuel by 2050, saving airlines 72.8 billion dollars, and reducing CO2 emissions by 424 million metric tons. These CO2 reductions are equivalent to removing 3.05 million cars from the road from 2020 to 2050.

Figure 3 Cumulative Fuel Savings from CLEEN Phases I and II

Assessment of CLEEN Phase II’s other benefit areas are ongoing. Quantification of the program’s fleet-level noise benefits is expected to be completed in 2022.

Additional information on the CLEEN Program can also be found on the FAA website at www.faa.gov/go/cleen.

CLEEN Phase I and II Aircraft Technology Projects

The following sections summarize the aircraft technologies and sustainable alternative fuels projects undertaken in the first two phases of the CLEEN Program.

Wing Technologies

The FAA CLEEN Program has been working with NASA and Boeing to develop wing technologies that will reduce fuel burn and noise.

Structurally Efficient Wing (SEW)

Description: Under CLEEN Phase II, Boeing developed and demonstrated advanced aircraft wing technologies. The SEW provides large weight reductions through new manufacturing techniques and advanced composite material technology, resulting in a reduction of aircraft fuel burn.

Benefit: Fuel: 3.5% reduction

Figure 4 Structurally Efficient Wing

For CLEEN Phase II, Boeing developed and demonstrated advanced aircraft wing technologies that reduce aircraft fuel burn by up to 3.5 percent. The Structurally Efficient Wing (SEW) provides large weight reductions through new manufacturing techniques and advanced composite material technology resulting in a reduction of aircraft fuel burn. Over twenty years, this technology could result in an estimated 200 million tons of jet fuel savings and avoidance of approximately 660 million tons of CO₂ emissions. In November 2019, Boeing completed full scale ground testing of the Wing Component Test Article at the National Institute for Aviation Research in Wichita, Kansas. The SEW has cleared testing that supports transition of its advanced composite technologies into a broad set of current and future commercial and military applications.

Adaptive Trailing Edge

Description: Under CLEEN Phase I, Boeing developed and demonstrated a prototype Adaptive Trailing Edge (ATE) system capable of tailoring wing performance to reduce noise and fuel burn at different flight regimes.

Benefit: Fuel: Up to 2% reduction Noise: 1.7 decibels (dB) reduction

Figure 5 Adaptive Trailing Edge

The technologies matured by Boeing during CLEEN Phase I included an Adaptive Trailing Edge (ATE) for the aircraft wing. In 2012, Boeing demonstrated that the ATE system provides up to a 2 percent reduction in aircraft fuel burn and reduces aircraft noise levels by 1.7 decibels (dBs). When used fleet-wide in the United States, a 2 percent reduction relative to 2009 fuel usage could save 340 million gallons of fuel with operating cost savings of $1.2 billion. As mentioned previously, technologies from the ATE project are adopted for use in commercial and defense products.

Fuselage Technologies

The FAA CLEEN Program has supported unconventional airframe technologies that have the potential to enable step changes in fuel burn and noise.  Aurora Flight Sciences is advancing composite airframe technologies that would enable an unconventional aircraft configuration. This aircraft concept was originally conceived under a 2008 NASA-funded N+3 study by MIT and Aurora.

Double Bubble D8 Fuselage

Description: Under CLEEN Phase II, Aurora developed and demonstrated composite airframe technologies that will enable an unconventional aircraft configuration

Benefit: Fuel: 29% reduction Noise: 16 EPNdB (Effective Perceived Noise level in dBs)

Figure 6 Double Bubble D8 Fuselage

Aurora, under CLEEN Phase II, developed and demonstrated composite airframe technologies that will enable an unconventional aircraft configuration with the potential to reduce fuel burn, emissions, and noise. The company designed an all-composite fuselage for what is known as the double bubble advanced aircraft concept, a more aerodynamically efficient shape, using materials and manufacturing processes that demonstrate configuration feasibility and weight benefits. The technologies developed under CLEEN Phase II will help enable economical fabrication of the fuselage while ensuring durability of the double bubble aircraft concept. Aurora built and tested a key subsection of the double bubble fuselage configuration to validate its structural performance. The double bubble advanced aircraft concept is estimated to provide a 29 percent reduction in fuel burn as well as potential noise reduction benefits toward the CLEEN Phase II goals. These benefits represent the improvements this configuration change provides, on which additional technology benefits could be applied. This configuration with advancements in engine integration that could be realized beyond the CLEEN Phase II timeframe holds the potential for up to 56 percent fuel burn reduction and 42 EPNdB cumulative noise reduction; however, additional measurements are needed to confirm the noise reduction.

Engine Fan, Nacelle, and Nozzle Technologies

The CLEEN Program has been working with a number of industry partners to develop technologies to reduce fuel burn and noise through improvements in the engine fan, nacelle, and nozzle. Some of the efforts have also considered advanced engine geometries to achieve a step change in fuel efficiency.

Compact Nacelle

Description: Under CLEEN Phase II, Boeing conducted design and test work in collaboration with Rolls-Royce on a compact nacelle inlet and thrust reverser design for ultra-high bypass engines. The technology reduces weight and allows for improved acoustic treatment.

Benefit: Fuel: 1% reduction Noise: Enables improved acoustic treatment solutions

Figure 7 Compact Nacelle

For CLEEN Phase II, Boeing conducted design and test work on a compact nacelle inlet and thrust reverser design for ultra-high bypass engines. The technology reduces weight and allows for improved acoustic treatment. In 2018, Boeing completed a ground test campaign of compact nacelle technology at the Stennis Space Center Rolls-Royce engine test facility. Crosswind testing was conducted for a baseline and short inlet. The findings have been used to validate propulsion-aero design tools and will inform future development programs. Further flight testing will be conducted in 2021 on the Rolls-Royce 747 Flying Test Bed (FTB), which will include angle of attack effects.

Integrated Propulsion System Nacelle Technologies

Description: Under CLEEN Phase II, Collins Aerospace developed an integrated propulsion system nacelle technology to reduce noise, fuel burn, and emissions. The company advanced innovative acoustic treatment technologies and clean fan duct thrust reverser designs.

Benefits: Fuel: 0.46% reduction aligned to 2025 Middle of Market Noise: 2 EPNdB reduction aligned to 2025 Middle of Market

Figure 8 Integrated Propulsion System Nacelle Technologies

Under CLEEN Phase II, Collins Aerospace developed integrated propulsion system nacelle technology to reduce noise and fuel burn. The company advanced innovative acoustic treatment technologies and clean fan duct thrust reverser designs. The combined technology package is expected to provide a 0.46 percent reduction in fuel burn and 2 EPNdB reduction in noise, while also enabling the use of next generation quiet and efficient ultra-high bypass engines. In 2019, Collins completed design and analysis and achieved Detailed Design Review (DDR) approval for a ground test demonstrator that will incorporate acoustic technologies into a modified production thrust reverser. In 2020, Collins Aerospace concluded their work under CLEEN Phase II, following impacts from COVID-19 on the work plan for the project. They plan to complete TRL 6 engine demonstration of the technologies in future years outside of the CLEEN Program. However, selected technologies from the program such as low drag surfaces and zoned liner configurations have successfully reached production ready status and have been incorporated into current production nacelle applications.

Low Pressure Ratio Fan Advanced Acoustic Technologies

Description: Under CLEEN Phase II, GE developed novel acoustic liner and fan noise source strength reduction technologies to combat the reduced noise treatment area available in low fan pressure ratio engines.

Benefits: Noise: 3 EPNdB cumulative noise reduction with no adverse impact to fuel burn

Figure 9 Low Pressure Ration Fan Advanced Acoustic Technologies

Under CLEEN Phase II, GE developed novel acoustic liners and fan noise source strength reduction technologies to combat the reduced noise treatment area available in low fan pressure ratio engines. This work targeted a 3 EPNdB cumulative noise reduction with no adverse impact to fuel burn. In 2019, GE tested novel liner designs in the NASA Langley Grazing Flow Impedance Tube and conducted detailed aeroacoustic design of the fan source strength reduction concept. Follow-on work to test and demonstrate the acoustic technology is planned to take place under CLEEN Phase III.

Ultra-High Bypass Propulsion System Technologies

Description: Under CLEEN Phase I, Pratt & Whitney and NASA completed an ultra-high bypass engine test campaign, demonstrating aerodynamic performance, mechanical, and acoustic characteristics of advanced fan system technologies that would contribute to the overall benefits provided in a new geared turbofan engine.

Benefits: Fuel: 20% fuel burn reduction compared to the Boeing 737-800/CFM56-7B Noise: 20 dB aircraft noise reduction compared to the Boeing 737-800/CFM56-7B Emissions:  60% NOx reduction compared to the Boeing 737-800/CFM56-7B

Figure 10 Ultra-High Bypass Propulsion System Technologies

Under CLEEN Phase I, NASA and Pratt & Whitney developed and demonstrated an ultra-high bypass ratio Geared Turbofan^TM (GTF) engine and associated advanced technologies. Pratt & Whitney also supported qualification of alternative jet fuels via combustor rig and auxiliary power unit testing. In 2017, Pratt & Whitney completed an ultra-high bypass engine test campaign, demonstrating aerodynamic performance, mechanical, and acoustic characteristics of advanced fan system technologies. Geared turbofan engine technologies contribute to a 20 dB aircraft noise reduction and a 20 percent fuel burn reduction because of increased engine efficiency.

Ceramic Matrix Composite Acoustic Nozzle

Description: Under CLEEN Phase I, Boeing designed, fabricated, and demonstrated an acoustic ceramic matrix composite (CMC) primary exhaust system. This advanced material system enables lighter, quieter, more efficient engines. The CMC nozzle was flight tested on the Boeing 787 ecoDemonstrator.

Benefit: Fuel: Up to 1% fuel burn reduction compared to 787 aircraft Noise: Up to 2.3 dB noise reduction compared to 787 aircraft

Figure 11 Ceramic Matrix Composite Acoustic Nozzle

The technologies matured by Boeing during CLEEN Phase I included a Ceramic Matrix Composite (CMC) acoustic nozzle for the engine exhaust. In 2014, Boeing tested the CMC nozzle on a 787 aircraft. This technology can withstand higher temperatures, is made of lighter weight material, and lowers fuel consumption. The CMC nozzle technology can also accommodate acoustic treatments that reduce community noise. The CMC nozzle reduces fuel burn by up to 1 percent and provides up to a 2.3 dB noise reduction. Rolls-Royce provided engines and technical support for this effort.

Open Rotor Engine

Description: Under CLEEN Phase I, GE and NASA developed open rotor designs for efficiency and low community noise. GE leveraged computational fluid dynamics, computational aero-acoustics, and rig scale testing to generate designs that achieved significant noise reductions well beyond what was attained in the 1980’s while retaining cruise performance.

Benefits: Fuel: 26% reduction relative to CFM56-7B powered narrow body aircraft Noise: 15-17 EPNdB reduction relative to Stage 4

Figure 12 Open Rotor Engine

Under CLEEN Phase I, GE also completed scaled open rotor wind tunnel tests with NASA, which resulted in a 15 dB reduction relative to the Stage 4 noise standards and a 26 percent fuel burn reduction relative to specific current jet engines. The results proved instrumental in helping the U.S. negotiate the international agreement on a noise standard at the 9^th meeting of the ICAO CAEP, which was been promulgated domestically as the Stage 5 noise standard.

Advanced Acoustic Fan and Liners

Description: Under CLEEN Phase II, Honeywell is developing a lightweight fan that reduces rotor noise, along with novel acoustic liners that target reductions in both broadband and tonal noise from the fan.

Benefits: Fuel: 1.5% reduction on top of other CLEEN Phase II Honeywell technologies Noise: 12.5 EPNdB reduction

Figure 13 Advanced Acoustic Fan and Liners

Under an optional expansion of their work scope for CLEEN Phase II, Honeywell is developing a weight-reduced fan rotor that simultaneously reduces noise. Honeywell is combining this technology with novel noise reduction liners in the fan case and bypass stream of the engine. Design work will be completed in 2021, with component and engine tests planned to complete by end of 2022, reaching TRL 6 for these technologies.

Engine Core Turbomachinery Technologies

The FAA CLEEN Program has been working with a number of industry partners to develop technologies to reduce fuel burn through improvements in turbomachinery and the use of novel materials used in the engine core.

High Pressure Compressor and Turbine Aero-Efficiency Technologies

Description: Under CLEEN Phase II, Pratt & Whitney developed and demonstrated technologies for the engine compressor and turbine to improve engine thermal efficiency and reduce fuel burn for Pratt & Whitney geared turbofan engines. The development work focused on advanced aerodynamics, cooling, and durability optimization.

Benefits: Fuel: 1.4% reduction relative to a state-of-the-art engine

Figure 14 High Pressure Compressor and Turbine Aero-Efficiency Technologies

Under CLEEN Phase II, Pratt & Whitney developed and demonstrated technologies for both the engine compressor and turbine to improve engine thermal efficiency and reduce fuel burn of the Pratt & Whitney Geared Turbofan engines. The development work has focused on advanced aerodynamics, cooling, and durability optimization.
In 2016, Pratt & Whitney completed a compressor rig test that demonstrated improved high pressure compressor efficiency and validated performance predictions. This compressor design has been incorporated into an engine for both ground and flight test demonstrations. In 2019 and 2020, Pratt & Whitney completed testing of the turbine technologies at the Pennsylvania State University Steady Thermal Aero Research Turbine (START) facility. Combined, the Pratt & Whitney CLEEN Phase II compressor and turbine technologies are estimated to result in 1.4 percent fuel burn reduction relative to a state-of-the-art engine.

Engine Core Technologies

Description: Under CLEEN Phase I, Honeywell developed and demonstrated engine core technologies that increase engine efficiency and reduce engine weight. The technologies include a high temperature impeller, low leakage air seals, and advanced materials.

Benefits: Fuel: 15.7% overall reduction in fuel burn when combined with complementary engine upgrades.

Figure 15 Engine Core Technologies

Under CLEEN Phase I, Honeywell developed and demonstrated engine core technologies that increase engine efficiency and reduce engine weight. Honeywell also tested alternative jet fuels. In 2015, Honeywell completed final engine core and engine endurance testing of all of its CLEEN Phase I technologies, validating fuel burn reduction and maturity for those technologies. Together with complementary engine upgrades, the technologies offer a 15.7 percent overall reduction in fuel burn relative to current engine designs.

Dual Wall Turbine Airfoils

Description: Under CLEEN Phase I, Rolls-Royce developed and demonstrated Dual-Wall Turbine Airfoil technology aimed at increasing thermal efficiency in the turbine section of the engine. Dual-wall turbine airfoils are projected to provide 20% or more reduction in cooling and increased operating temperature capability.

Benefits: Fuel: Greater than 0.3% reduction

Figure 16 Dual Wall Turbine Airfoils

Under CLEEN Phase I, Rolls-Royce developed and demonstrated Dual-Wall Turbine Airfoil technologies aimed at increasing thermal efficiency in the turbine section of the engine, thereby reducing fuel burn. In 2015, Rolls-Royce completed testing of the Dual Wall Turbine Airfoil. The testing showed that the Dual-Wall Turbine Airfoil and CMC Blade Track technologies will realize a fuel burn reduction of up to one percent overall.

CMC Turbine Blade Tracks

Description: Under CLEEN Phase I, Rolls-Royce developed and demonstrated CMC Black Track technology aimed at increasing thermal efficiency in the turbine section of the engine. The blade tracks offer more than a 50% reduction in engine cooling and weight savings compared to a metallic design.

Benefits: Fuel: Greater than 0.4% reduction

Figure 17 CMC Turbine Blade Tracks

Under CLEEN Phase I, Rolls-Royce developed and demonstrated CMC Blade Track technologies aimed at increasing thermal efficiency in the turbine section of the engine, thereby reducing fuel burn. In 2013 and 2014, engine testing of the CMC Blade Track validated the technology’s performance and benefits.

Advanced Turbine Blade Outer Air Seal (BOAS)

Description: Under CLEEN Phase II, Honeywell’s advanced turbine BOAS increases high pressure turbine efficiency, resulting in reduced fuel burn. The technology leverages advanced light-weight, high-temperature materials and coatings and is designed to minimize leakage between shroud and advanced turbine blade tip.

Benefits: Fuel: 22% reduction when combined with other technologies

Figure 18 Advanced Turbine Blade Outer Air Seal

Under CLEEN Phase II, Honeywell is developing and demonstrating an advanced turbine Blade Outer Air Seal (BOAS). Honeywell’s advanced turbine blade outer air seal increases high pressure turbine efficiency, resulting in reduced fuel burn. The BOAS and advanced combustor technology (described below) contribute to an engine level improvement of greater than 22 percent fuel burn reduction relative to a baseline engine. In 2020, Honeywell initiated ground engine test of the turbine shroud for the BOAS system. Further engine testing of a high temperature capable blade alloy is planned for 2022.

Advanced High Pressure Compressor

Description: Under CLEEN Phase II, Honeywell is developing advanced axi-centrifugal high pressure compressor technologies. These  include high efficiency axial stages and an expanded temperature capability impeller (building upon CLEEN Phase I successes).

Benefits: Fuel: Additional 1% reduction on top of other CLEEN Phase II Honeywell technologies

Figure 19 Advanced High Pressure Compressor

Under an optional expansion to their CLEEN Phase II work, Honeywell is developing and demonstrating advanced high pressure compressor technologies for both the axial and centrifugal stages of their compressor. These technologies aim to improve the efficiency of the axial stages and the temperature capability of the centrifugal stage (impeller). The impeller work builds upon successful development of improved components under CLEEN Phase I. These technologies are planned for rig test in late 2021 and TRL 6 engine test in 2022.

Low Emissions Combustors

The FAA CLEEN Program has been working with a number of industry partners to develop improved combustor technologies to reduce engine emissions while avoiding negative impacts on fuel efficiency.

Twin Annular Premixing Swirler (TAPS) II Combustor

Description: GE’s TAPS combustor concept is a low emissions, lean burn system. For TAPS II, GE scaled TAPS wide-body technology to narrow-body applications, made additional design improvements to meet the CLEEN NOx emissions goal, and demonstrated the design in full annular and core engine testing.

Benefits: NOX emissions: More than 60% below CAEP/6 std

Figure 20 Twin Annular Premixing Swirler II Combustor

GE’s unique lean-burn combustion system, TAPS, is designed to pre-mix air and fuel prior to combustion for leaner burn and fewer emissions than conventional combustion systems. The TAPS combustion was introduced on the GEnx engine for the Boeing 787 Dreamliner and the 747-8 aircraft. GE adapted and enhanced this combustor to create TAPS II for CFM International’s LEAP engine in narrow-body aircraft. In 2012, GE engine emissions tests of the TAPS II combustor demonstrated that the technology reduced NO_X emissions more than 60 percent below the 2004 ICAO CAEP NO_X standards, thus meeting and exceeding the CLEEN Phase I goal.

Twin Annular Premixing Swirler (TAPS) III Combustor

Description: Under CLEEN Phase II, GE conducted extensive rig test validation and development of risk mitigation technologies for the TAPS III low emissions combustor, thus enabling the technology to meet the CLEEN II NOx target.

Benefits: NOX emissions: 35 percent reduction relative to CAEP/8 (55 OPR)

Figure 21 Twin Annular Premixing Swirler III Combustor

Under CLEEN Phase II, GE further developed the next generation TAPS (III) and achieved Technology Readiness Level (TRL) 6 emissions demonstration of TAPS III combustion system. The TAPS III combustion system is being implemented in the GE9X-powered Boeing 777X, replacing the GE90-powered Boeing 777, expected to enter into service in 2022, and enables NOx emissions to be 30% below international CAEP standards. The GE9X with the TAPS III combustor was successfully certified in 2020.

Advanced Low NOx Combustion System

Description: Under CLEEN Phase II, Rolls-Royce’s advanced Rich-Quench-Lean (RQL) combustion system employs advanced fuel injection and mixing technologies that will provide significant emissions reduction while simultaneously enabling the increase in Turbine Entry Temperature required by advanced engine cycles.

Benefits: NOX emissions: 40 to 65% below CAEP/8 limits

Figure 22 Advanced Low NOx Combustion System

Under CLEEN Phase II, Rolls-Royce has developed and demonstrated low emissions combustor technology, while also testing alternative fuels. Rolls-Royce’s advanced Rich-Quench-Lean (RQL) combustion system employs advanced fuel injection and mixing technologies that will provide significant emissions reduction while simultaneously enabling the increase in Turbine Entry Temperature required by advanced engine cycles. The project demonstrated a near-term configuration targeting NO_X emission levels 40 percent below CAEP/8 limits and a final configuration with NO_X levels 65 percent below CAEP/8, achieving significant progress toward the CLEEN Phase II NO_X goal. Rolls-Royce  also conducted alternative fuels testing to support ASTM approval of novel fuels. In 2020 and early 2021, final testing of the third generation combustor design was completed.

Compact Combustor System

Description: Under CLEEN Phase II, Honeywell is developing a compact low emissions combustor that uses advanced aerodynamics and fuel injection technologies to reduce engine NOx emissions while reducing weight, and thereby reducing fuel burn.

Benefits: NOX emissions: improved NOX emissions of 2X relative to current state-of-the-art Fuel: 22% reduction when combined with other fuel burn technologies

Figure 23 Compact Combustor System

Under CLEEN Phase II, Honeywell is developing and demonstrating a compact, low emissions combustor. The compact low emissions combustor uses advanced aerodynamics and fuel injection technologies to reduce engine NO_X and particulate matter emissions while reducing weight, thereby reducing fuel burn. The BOAS and advanced combustor technology (described above) contribute to an engine level improvement of greater than 22 percent fuel burn reduction relative to a baseline engine with a factor of two improvement in NO_X margin relative to Honeywell’s state-of-the-art combustor. In early 2021, Honeywell initiated engine testing of the advanced combustor design, and plans to conduct additional rig testing with NASA later in the year.

The low emissions combustor development work, conducted under CLEEN Phases I and II across GE, Honeywell, and Rolls-Royce projects, results not only in emissions reductions for the specific combustors developed under the program, but enhances each company’s ability to design and optimize all future engine combustion systems for low emissions.

Protective Engine Coatings

The FAA CLEEN Program has been working with an airline and coatings manufacturer to develop new coatings to keep engine components at higher efficiencies for longer periods of time, thus reducing fuel burn from the current fleet.

Leading Edge Protective Blade Coatings

Description: Under CLEEN Phase II, The team is developed and demonstrated a protective leading edge coating for gas turbine engine fan blades. This coating protects against fan blade erosion, a source of lost aerodynamic efficiency in engines in service.

Benefits: Fuel: up to 0.4% fuel savings during aircraft cruise and over 1% savings at maximum power

Figure 24 Leading Edge Protective Blade Coatings

Under CLEEN Phase II, a team of MDS Coating Technologies, America’s Phenix, and Delta TechOps developed and flight tested a protective leading edge fan blade coating to reduce erosion and retain fuel efficiency. The team installed coated blades on-wing and conducted a flight service evaluation of the technology, accumulating thousands of operational hours that confirmed the coating’s significant benefits.

Aircraft Engine System Integration

The FAA CLEEN Program has been working with GE to develop technologies to reduce fuel burn through enhanced engine control, flight management and electric systems, enabling the aircraft to perform more efficiently through the entire mission.

More Electric Systems and Technologies for Aircraft in the Next Generation (MESTANG)

Description: MESTANG is an integrated aircraft power system designed to support future “more-electric” aircraft architectures that optimize new power extraction, generation, distribution, and conversion systems.

Benefits: Fuel: up to 6% reduction for single-aisle aircraft on short flights

Figure 25 More Electric Systems and Technologies for Aircraft in the Next Generation

Under CLEEN Phase II, GE also developed More Electric Systems and Technologies for Aircraft in the Next Generation (MESTANG), an integrated aircraft power system designed to support future “more-electric” aircraft architectures. The MESTANG technologies under CLEEN Phase II may reduce fuel burn by up to 6 percent for single-aisle aircraft. In 2020, GE completed a full scale ground test of the system, reaching TRL 6 and completing technology maturation for this project.

Flight Management System Technology

Flight Management System (FMS) Technology

Description: Under CLEEN Phase I, GE completed ground engine and flight tests of FMS-Engine integration technologies that use knowledge of aircraft state and engine health to optimize performance. Under CLEEN Phase II, GE’s FMS software algorithms will optimize aircraft performance during the cruise and descent phases of flight.

Benefit: Fuel: CLEEN Phase I - up to 2.5% reduction CLEEN Phase II - Up to 3.5% reduction relative to legacy systems

Figure 26 Flight Management System Technology

Under the first phase of CLEEN, GE developed a suite of controls and engine health management technologies that leverage the information from various engine and aircraft states (e.g., engine condition, flight stages, and external conditions) to improve aircraft fuel efficiency. These technologies have a potential to reduce fuel consumption by more than 1% in current and future aircraft engines. Some of these technologies are implemented into the CFM LEAP-powered Airbus A320neo and the Boeing 737 MAX and the Passport 20-powered Bombardier Global 7500. One of these technologies will be implemented on the GE9X-powered Boeing 777X, replacing the GE90-powered Boeing 777, expected to enter into service in 2020. These technologies are also implemented in various military applications.

Under CLEEN Phase II, GE Aviation  developed new control policies for the Flight Management System (FMS) to optimize aircraft trajectories. These enhancements incorporate detailed weather forecast data to develop fuel-optimal fight profiles and provide guidance to pilots and the FMS on how to execute them, resulting in a potential 1% fleetwide average improvement in fuel burn. GE is currently working to incorporate these technologies in future products.

CLEEN Phase III Aircraft Technology Projects

The following sections describe the planned aircraft technology development projects under CLEEN Phase III. These efforts have just been kicked off in 2021, so benefits described are estimated and will be refined and validated throughout the program’s maturation of these technologies.

Wing Technologies

Quiet High-Lift System

Description: Boeing is developing flap edge fairings and vortex generators for wing high-lift devices in order to minimize noise.

Benefit: Noise: Up to 0.5 EPNdB noise reduction

Figure 27 Quiet High-Lift System

Boeing is developing a quiet high-lift system, utilizing flap edge fairings and vortex generators to reduce aircraft noise. The technologies will be prototyped and demonstrated in flight.

Aircraft Systems Technologies

Quiet Landing Gear

Description: Boeing is developing landing gear door noise treatment and aerodynamic shields to reduce aircraft noise.

Benefit: Noise: Up to 0.5 EPNdB noise reduction

Figure 28 Quiet Landing Gear

Boeing is developing acoustically treated landing gear doors, and aerodynamic shields for landing gear structures to reduce aircraft noise. The technologies will be prototyped and demonstrated in flight.

Intelligent Operations

Description: Boeing is developing noise-optimized flight path algorithms with integration into the Air Traffic Management System.

Benefit: Noise: 3-5 dBA peak noise reduction Fuel: 2% fuel savings during take-off; 5% during approach phase

Figure 29 Intelligent Operations

Boeing is developing new “intelligent operations” with real-time, noise-optimized flight path algorithms derived from on-board, and ground-based inputs through the FAA Data Comm system. New algorithms for departure and arrival flight paths will be implemented and demonstrated in flight.

Engine Fan, Nacelle, and Nozzle Technologies

Advanced Nacelle: Next Generation Inlet

Description: Boeing is developing a new inlet architecture that will reduce weight, Drag and noise.

Benefit: Noise: Up to 1.5 EPNdB Fuel: 2% fuel burn reduction

Figure 30 Advanced Nacelle: Next Generation Inlet

Boeing is developing an engine inlet with novel structural and systems architecture, which will reduce weight and drag, and improve acoustics relative to existing “short inlet” designs. The new inlet will be prototyped and demonstrated in flight.

Large Cell Exhaust Acoustic Technology

Description: Collins Aerospace is developing a novel exhaust noise attenuation feature involving a “large cell” cavity treatment in the exhaust structure.

Benefit: Noise: Lower noise by 0.9 to 1.5 EPNdB

Figure 31 Large Cell Exhaust Acoustic Technology

Collins Aerospace is developing large cell noise attenuation technology for application to the engine exhaust nacelle structure. This noise attenuation technology is primarily aimed at reducing combustion noise with application to current and future commercial aircraft. Collins Aerospace expect to reach TRL 6 by 2025 via ground engine test in collaboration with Pratt & Whitney. 

Highly Efficient Fan Module

Description: Honeywell is developing over-the-rotor acoustic treatment, a high efficiency booster, and optimizing the fan exit guide vanes and booster stators for combined noise and efficiency benefits.

Benefit: Noise: 1.5 EPNdB Fuel: 1.5% fuel burn reduction

Figure 32 Highly Efficient Fan Module

Honeywell is developing a suite of technologies which they are collectively calling the highly efficient fan module. They will be leveraging prior fan rig testing from a collaborative effort with NASA to inform development of noise reducing over-the-rotor acoustic treatment, a high efficiency booster with optimized stators, and improved fan exit guide vanes. The technologies will be fan rig tested and engine tested in the later years of the project to prove out their noise and fuel burn benefits.

Ultra-Quiet Reduced-Loss Fan Stage

Description: Pratt & Whitney is developing a quieter, more efficient fan module including additively manufactured low-loss acoustic liners and reduced solidity, reduced-loss, lower noise fan exit guide vanes. Areas of interest for the advanced acoustic treatment are highlighted in blue.

Benefit: Noise: Target 3 EPNdB noise reduction combined with P&W combustor technology Fuel: 0.8% fuel burn as part of package with P&W combustor technology

Figure 33 Ultra-Quiet Reduced-Loss Fan Stage

Pratt & Whitney, a unit of Raytheon Technologies Corporation, is developing a reduced noise, higher efficiency fan system under CLEEN Phase III. These technologies focus on reducing aerodynamic losses while reducing noise through new acoustic liners and optimized fan exit guide vanes. These technologies are planned to be matured through rig and engine test throughout the years of the program.

Open Fan

Description: GE Aviation is developing an unducted single fan architecture optimized for noise and fuel burn reduction.

Benefit: Noise: 13 EPNdB cum margin relative to Stage 5 Fuel: 10+% reduction relative to current LEAP engine

Figure 34 Open Fan

GE Aviation is developing innovative open fan technology for that delivers propulsive efficiency while overcoming historical issues with complexity, weight, and flight speed limitations of this propulsion configuration. Under CLEEN Phase III, GE is focused on the aerodynamics and acoustics aspects of the propulsive fan. GE will bring fan blade and vane designs into a rig test program, culminating in high fidelity wind tunnel test validation that will allow for noise and performance measurements as well as projection to a full scale next generation narrow-body application.

Advanced Acoustics

Description: GE Aviation is developing advanced fan duct acoustic liners and fan/outlet guide vane (OGV) technologies to reduce noise.

Benefit: Noise: 2 EPNdB cum. reduction from novel liners, 1 EPNdB cum. reduction from fan/OGV designs

Figure 35 Advanced Acoustics

GE Aviation is developing novel acoustic liners and fan source strength reduction designs to reduce noise for future engine architectures. This work builds on prior work conducted under CLEEN Phase II to prototype these acoustic elements. Under CLEEN Phase III, GE will conduct subscale wind tunnel testing of the fan and outlet guide vane design to evaluate source noise reduction relative to a baseline configuration.

Engine Core Turbomachinery Technologies

Efficient Green High Pressure Core

Description: Honeywell is developing advanced high pressure compressor, low emission combustor, and efficient high pressure turbine technologies for next generation business jet aircraft

Benefit: Noise: 3 EPNdB reduction Fuel: 8.3% fuel burn reduction Emissions: 70% margin to CAEP/8 NOx; 70% reduction in nvPM

Figure 36 Efficient Green High Pressure Core

Honeywell is maturing a suite of engine core technologies under the efficient green high pressure core project. In the high pressure compressor this includes co-optimization of aero-efficiency and noise characteristics. The combustor is being designed to reduce both NOx and nvPM emissions, addressing both CLEEN Phase III emissions goal areas. Finally, advances in the high pressure turbine will improve efficiency and reduce weight using aerodynamic design techniques and improved materials. These technologies will be matured through ground engine test in the later years of the program.

TALON X+ Combustor

Description: Pratt & Whitney is developing an advanced combustion system based on the TALON X that will simultaneously reduce noise and emissions, while improving temperature pattern factor and enabling improved high pressure turbine design and efficiency.

Benefit: Noise: Target 3 EPNdB noise reduction combined with P&W fan technology Fuel Burn: 0.8% fuel burn as part of package with P&W fan technology Emissions: deliver 50% margin to CAEP/8 NOx

Figure 37 TALON X+ Combustor

Pratt & Whitney, a unit of Raytheon Technologies Corporation, is optimizing their TALON X+ combustion system using new swirler designs and cooling design alternatives. The technology will reduce combustor noise, reduce emissions and provide improved pattern factor to the high-pressure turbine, yielding fuel burn benefits. These technologies are planned to be matured through rig and engine test throughout the years of the program.

Compact Core – Low Emissions Combustor

Description: GE Aviation is developing combustor technology that will result in reduced NOx emissions.

Benefit: NOx: Targeting NOx reduction for a future high overall pressure ratio engine cycle, equivalent to 70% margin to CAEP/8 standard at 30 OPR.

Figure 38 Compact Core – Low Emissions Combustor

GE Aviation is developing combustor technology to achieve a reduction in NOx emissions while also reducing non-volatile particulate matter emissions. These environmental goals will need to be balanced with next generation narrow-body objectives for operability, cruise efficiency, and component durability. GE will manufacture and conduct component rig tests to evaluate operability, performance, and emissions of different combustor technology designs.

High Work High Lift Low Pressure Turbine (LPT)

Description: Honeywell is developing technologies for a reduced weight, more efficient and quieter low pressure turbine for future business jet class aircraft.

Benefit: Noise: 0.5 EPNdB Fuel: 2.5% fuel burn reduction

Figure 39 High Work High Lift LPT

Honeywell is maturing technologies to support an engine LPT with improved efficiency, reduced weight, and reduced noise. Application of new advanced aerodynamic and acoustic designs will support these environmental objectives. Honeywell’s approach will build from rig testing to full ground engine test, aligning with the other Honeywell CLEEN Phase III technologies.

Protective Engine Coatings

Particulate & Fluid Erosion-Resistant Fan Blade Coating for Expanded Applications

Description: The team of Delta TechOps, GKN Aerospace, MDS Coating and America’s Phenix, is developing erosion resistant fan blade coatings for various engine applications. The coatings protect the fan blade’s leading edge against particulate and fluid erosion; thus, retaining engine performance, reducing fuel consumption and lowering emissions over an engine’s operational tour.

Benefits: Fuel: 1% or greater fuel burn reduction   and corresponding reduction in greenhouse gas emissions via retaining engine performance over an engine’s operational tour.

Figure 40 Erosion-Resistant Fan Blade Coatings

The team of Delta TechOps, GKN Aerospace, MDS Coating and America’s Phenix is building upon the successes of the CLEEN Phase II test and evaluation and flight demonstration program to expand the coating application to new, used and repaired fan blades across airline fleets. The coating applications will include high and low aspect ratio fan blades, both solid and hollow fan blades, and blades with titanium leading edge strips. The team will demonstrate the coating at TRL-7 by applying the coating on a production-type basis. Flight demonstration will monitor up to three engines for each of the various turbofan blade configurations previously described. The coating’s ability to protect the turbofan blades’ leading edge helps retain engine performance throughout an engine’s operational tour, resulting in reduced fuel consumption of 1% or greater and corresponding reductions in greenhouse gas emissions.

Aircraft Engine System Integration

MESTANG III

Description: GE Aviation is developing more electric aircraft systems that will reduce fuel burn by requiring reduced engine bleed air.

Benefit: Fuel: 3-6% reduction for mid-size aircraft

Figure 41 MESTANG III

GE Aviation is developing a third phase of the More Electric Systems and Technologies for Aircraft in the Next Generation (MESTANG) technology under CLEEN Phase III. The objective of this effort is to replace pneumatic systems with electric powered systems that reduce fuel burn by reducing bleed air requirements on the engine. This project will focus on maturing a 90kW starter/generator, building off of prior work conducted under CLEEN Phase II. GE will conduct requirements definition, design, and manufacturing of a first prototype starter/generator targeting regional, turboprop, and business jet applications.

Hybrid Electric Integrated Generation

Description: GE Aviation is developing an integrated electric power generation system within the engine to enable flexibility in electric power generation and optimize engine performance.

Benefit: Fuel: 3-4% reduction

Figure 42 Hybrid Electric Integrated Generation

GE Aviation is developing integrated electric power generation systems within the engine to enable flexibility in electric power generation and optimize engine performance. This will enable significant operating benefits to the engine including improved operability, improved engine performance, and increased flexibility in off-design operating modes. Under CLEEN Phase III, GE will conduct trade studies to determine the optimal location for a motor/generator and will build a prototype.

Advance Thermal Management

Description: GE Aviation is developing advanced thermal management and waste heat recovery systems to facilitate compressor and turbine temperature increases, thereby improving cycle efficiency and reducing fuel burn.

Benefit: Fuel: Up to 3% reduction relative to traditional architectures

Figure 43 Advanced Thermal Management

GE Aviation is developing Advanced Thermal Management systems such as cooled cooling air and heat recuperation. Under CLEEN Phase III, GE will complete conceptual mechanical and thermal design work for the system elements including a fuel de-oxygenation system, heat exchangers, a thermal transport bus, modern controls, and an associated fuel system. This design work will be conducted in support of future demonstration in a static engine test.

Alternative Jet Fuels

The first two five-year phases of CLEEN, along with the ASCENT Center of Excellence and CAAFI, have been instrumental to the certification and qualification of alternative jet fuels (AJF) for safe use by civil transportation. This testing and evaluation process is conducted in coordination with ASTM International, which manages specifications for jet fuels used by the industry. These combined efforts have led to the approval of seven fuel types for use by civil aviation as of April 2021.

Alternative Jet Fuels

Description: Boeing, Honeywell, GE, Pratt & Whitney, and Rolls-Royce have evaluated a range of Alternative Jet fuels under the first and second phases of the CLEEN program, demonstrating viability and acceptability of these fuels as alternatives to petroleum derived jet fuel.

Benefits: Promotes the development, approval, and deployment of viable renewable alternative fuels for aerospace gas turbine engine applications. Offers potential to reduce aerospace environmental impact and increase energy security.

Figure 44 Alternative Jet Fuels

Fuel characterization and qualification testing for AJF under CLEEN and ASCENT consists of fuel property evaluations and component, rig, and engine tests covering material compatibility, combustion performance, and full engine operability. Under CLEEN Phase I, Boeing, Honeywell, Pratt & Whitney, and Rolls-Royce supported AJF evaluation, fuel characterization and qualification testing for alternative jet fuels.

In 2011, Honeywell completed testing of Hydro-Processed Esters and Fatty Acids (HEFA) fuel, which expedited its approval through ASTM International. HEFA was added to the ASTM International fuel specification in July 2011 for use up to a 50% blend level with conventional jet fuel.

In 2013, Boeing completed evaluations on the effects of HEFA and HEFA blends with conventional jet fuel on non-metallic materials used in commercial aircraft fuel systems. This testing focused on the effects of HEFA on seals within fuel systems, furthering the understanding of key fuel properties that ensure new fuels are drop-in ready with existing and future aircraft fuel systems.

In 2015, Pratt & Whitney completed a series of tests in support of ASTM International approval on a range of AJFs including: Amyris Direct Sugar to Hydrocarbon (DSHC) Farnesane, Kior Hydrotreated Depolymerized Cellulosic Jet (HDCJ), Applied Research Associates (ARA) Catalytic Hydrothermolysis Jet (CHJ), and Swedish BioFuel alcohol-to-jet synthetic kerosene with aromatics (ATJ-SKA). This effort tested fuels produced from a wide range of feedstocks, supporting the desire to have a diverse supply of AJFs. The data from these tests contributed to the approval of two new AJF pathways to the ASTM International specification.

In 2015, Rolls-Royce completed rig testing to understand the performance of fuel system seals and materials in the presence of different jet fuels. The program characterized the impact that various alternative fuels, under realistic engine conditions, had on the sealing force performance of elastomers. The data generated supports the ASTM International approval of a fully synthetic jet fuel. Rolls-Royce’s work on seal performance has continued under CLEEN Phase II.

Under CLEEN Phase II, GE and Rolls-Royce have advanced the understanding of alternative jet fuels, with a focus on advancing additional fuel approvals, including exploration of fully-synthetic options.

GE has conducted combustor rig testing of alternative jet fuels under CLEEN Phase II. This included testing of alcohol to jet synthetic paraffinic kerosene (ATJ-SPK) produced by Gevo Inc., a blend of high freeze point hydroprocessed esters and fatty acids (HFP-HEFA) with petroleum jet fuel produced by Neste Oil, and Virent’s Hydro-deoxygenated Synthetic Aromatic Kerosene (HDO-SAK). Data from GE’s CLEEN Phase II testing supported the 2018 approval of higher blend levels for use of ATJ-SPK fuels. The results of GE’s SAF testing have also supported streamlining of the fuel approval process by validating that a non-proprietary combustor rig, developed under the National Jet Fuels Combustion Program (NJFCP) and supported by ASCENT, can capture key characteristics of different combustion systems from various engine manufacturers.

Rolls-Royce’s robust evaluation program characterized the performance of a fully-synthetic Alcohol-to-Jet fuel under representative gas turbine engine conditions as a viable alternative to conventional jet fuel. This was accomplished through a series of laboratory and burner rig tests, ensuring the candidate fuel will have no negative impact on engine safety, durability, or performance. Rolls-Royce’s CLEEN Phase II sustainable aviation fuels has helped advance the understanding of fully-synthetic SAF.

CLEEN Phase III Alternative Jet Fuel Activities

CLEEN Phase III will continue supporting testing and qualification of new alternative jet fuels, with a focus on supporting greater than 50% blend levels of these fuels. These efforts reflect the desire of the aviation industry to use 100% alternative jet fuel without the need to blend with conventional jet fuel. The following sections describe planned alternative jet fuel activities under CLEEN Phase III. As these efforts are beginning, expected outcomes are provided and will be updated as the fuel testing efforts proceed.

Under CLEEN Phase III, Boeing is supporting qualification of Sustainable Aviation Fuels through lab material compatibility evaluations and flight demonstration. This program will characterize selected new alternative fuel blends and provide test data in support of future ASTM International specifications. Through this effort, Boeing will support continued expansion of certified alternative fuel pathways to promote uptake and sustainability.

GE Aviation will also support alternative jet fuel testing under CLEEN Phase III through several valuable program thrusts. GE will support efforts to qualify new alternative jet fuels with unique chemical compositions, including highly cycloparaffinic fuels. Cycloparaffinic fuels have the potential to provide seal swell benefits that other paraffinic alternative jet fuels lack due to the absence of aromatic compounds. If cycloparaffinic fuels can be shown to provide the necessary seal swell performance as seen in conventional jet fuels, cycloparaffinic fuels would address one of the current barriers that limit paraffinic alternative jet fuels to a 50% blend volume limit. GE testing will characterize combustor operability and emissions impact of the fuels. The planned testing at GE for CLEEN Phase III aims to address additional barriers to support efforts in the alternative fuels community to increase the alternative jet fuel blending limits beyond 50%. Testing results and conclusions will be disseminated to the ASTM International D4054 fuel qualification panel as it provides critical data required as part of the fuel approval process. Additionally, GE Aviation will be exploring best ways to enable standardization of higher blend ratios including 100%.

Conclusion

The CLEEN Program represents a critical piece of the FAA’s environmental strategy and addresses aircraft technology development and maturation of alternative fuels. The first two phases of CLEEN have shown great success, maturing technologies that have entered the fleet, with industry anticipating that additional technologies will enter into service in the coming years as opportunities arise for their insertion into new aircraft and engine designs. More importantly, the knowledge gained from the development of these technologies is leading to improved design codes that are improving every product being made by these companies and the technologies are being used across industry.

Cumulatively, CLEEN Phase I and II are estimated to save the aviation industry 36.4 billion gallons of fuel by 2050, reducing airline costs by 72.8 billion dollars and lowering CO2 emissions by 424 million metric tons. These CO₂ reductions are equivalent to removing 3.05 million cars from the road from 2020 to 2050. The technologies from the first phase of CLEEN are estimated to decrease land area exposed to noise by 14%. These technologies, as well as the use of sustainable aviation fuels, will also dramatically reduce nitrogen oxide and soot emissions from aircraft operations.

The CLEEN program demonstrates the continued commitment of the FAA toward reducing the noise, emissions, and fuel burn from the fleet. The FAA looks forward to the additional fuel burn, noise, and emissions reductions which the third phase of the program will yield as it partners with the industry to develop new technologies through 2025.