Aircraft remain the single largest GHG-emitting transportation source not yet subject to GHG standards in the United States and are responsible for emitting 11 percent of GHG emissions from the U.S. transportation sector and 3 percent of total U.S. GHG emissions. This rulemaking follows on our final endangerment and cause or contribute findings for aircraft GHG emissions which was finalized in August 2016. As a result of these positive findings, the EPA is obligated under section 231 of the Clean Air Act to set emission standards applicable to GHG emissions from certain classes of aircraft engines used in certain aircraft included in the contribution finding. In addition, the EPA anticipates that the 39th ICAO Assembly will approve a final aircraft CO2 emissions standard in October 2016, and that subsequently, ICAO will formally adopt the final CO2 emissions standard in March 2017. Once an international standard is approved and finalized by ICAO, member states, including the United States, that wish to use aircraft in international transportation will then be required under the Chicago Convention to adopt standards that are of at least equivalent stringency to those set by ICAO.