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The United States Aviation Climate Action Plan released in 2021 sets a goal of net zero greenhouse gas emissions from the aviation sector by 2050. In order to meet this goal, the United States is unde... The United States Aviation Climate Action Plan released in 2021 sets a goal of net zero greenhouse gas emissions from the aviation sector by 2050. In order to meet this goal, the United States is undertaking initiatives to accelerate development and deployment of measures that reduce greenhouse gas emissions. These include sustainable aviation fuels (SAF) via the SAF Grand Challenge, aircraft technologies via the Sustainable Flight National Partnership (SFNP), and operational measures to improve efficiency via the Next Generation Air Transportation System. The FAA is contributing to the SFNP via aircraft technology developed and demonstrated under the CLEEN Program to make progress towards the emissions goals while simultaneously targeting large reductions in aircraft noise. With the support of the CLEEN Program, the aviation industry is able to expedite the integration of innovative technologies into current and future aircraft. CLEEN helps accelerate technologies through a crucial phase in their maturation, culminating in full scale ground and flight test demonstrations and showing technology readiness for product implementation. At the conclusion of the development effort for a CLEEN technology, each company, having cost shared the development with the FAA, is invested in the technology’s success and confident in its maturity to enter product development for entry into service. Once the products enter into service, the CLEEN technologies will realize their noise, fuel burn, and emissions benefits throughout the fleet for years to come. The FAA initiated CLEEN, which requires a 1:1 minimum cost share match from industry, in 2010 for a period of five years from 2010 to 2015. Upon successful execution of the initial CLEEN effort, the FAA initiated the second phase of the CLEEN Program to run from 2015 to 2020, and subsequently a third phase began in 2021. Since its inception in 2010, the CLEEN Program has been successful in maturating technologies to enter into service sooner than what the industry had anticipated. Several CLEEN technologies have demonstrated significant progress toward the fuel burn, emissions and noise reduction goals, and have entered service in new aircraft and engines.[1] The fourth phase of CLEEN is anticipated to run from 2024 through 2029. Industry must show a viable path to certification and entry into service within five years of demonstration for technologies developed under the fourth phase of the CLEEN Program. The focus of the fourth phase of the CLEEN Program is to mature previously conceived fuel burn, noise, and emissions reduction technologies from Technology Readiness Levels[2] (TRLs) of 3-5 to TRLs of 6-7 to enable industry to enter product development and expedite introduction of these technologies into current and future civil aircraft and engines. The fourth phase of the CLEEN Program is planning a four-tier approach set of goals: Tier 1: Certifiable aircraft technology that improves aircraft energy efficiency by 35% relative to the International Civil Aviation Organization (ICAO) new type fuel efficiency standard for civil subsonic airplanes adopted in 2016 (Committee on Aviation Environmental Protection, CAEP/10) and/or reduces aviation’s climate impacts. Tier 2: Certifiable aircraft technology that reduces noise levels by 25 dB cumulative, relative to the Stage 5 standard for civil subsonic airplanes and/or reduces community noise exposure. Tier 3: Certifiable engine technology that reduces landing and takeoff cycle (LTO) nitrogen oxide emissions for civil subsonic airplanes by 70% relative to the ICAO standard adopted in 2010 (CAEP/8) and/or reduces absolute NOx production over the aircraft’s mission. This must be achieved while limiting or reducing other gaseous or particulate matter emissions. Tier 4: Certifiable engine technologies that reduce LTO particulate matter number and mass emissions for civil subsonic airplanes by 50% over the ICAO new type standard adopted in 2019 (CAEP/11)and/or reduces absolute nvPM production over the aircraft’s mission. This must be achieved while limiting or reducing other gaseous or particulate matter emissions.