This rulemaking amends the existing United States regulations governing the exhaust emissions from new commercial aircraft gas turbine engines. Under the authority of section 231 of the Clean Air Act (CAA), the Environmental Protection Agency (EPA) is promulgating new emission standards for oxides of nitrogen (NO<INF>X</INF>) and carbon monoxide (CO) for newly manufactured and newly certified commercial aircraft gas turbine engines with rated thrust greater than 26.7 kilonewtons (kN). This action will codify into United States law the current voluntary NO<INF>X</INF> (a two-staged NO<INF>X</INF> standard) and CO emission standards of the United Nations International Civil Aviation Organization (ICAO), and thereby bring the United States emission standards into alignment with the internationally adopted standards. These ICAO CO and NO<INF>X</INF> standards are being added to the current EPA regulations for smoke and hydrocarbon emissions that have been in effect since 1984. EPA is also adopting ICAO's requirement that these standards also apply to applications that otherwise would have been fulfilled by turbojet and turbofan engines (e.g. propfan, unducted fan, and advanced ducted fan). In addition, today's action also amends the test procedures for gaseous exhaust emissions and smoke exhaust emissions to correspond to recent amendments to the ICAO test procedures for these emissions. EPA is also amending its certification test fuel specifications to make them consistent with ICAO's test fuel specifications. All of the affected engines are already meeting the ICAO CO and first-stage NO<INF>X</INF> emission standards that EPA is adopting today. Most engines also meet the ICAO second-stage NO<INF>X</INF> standard; only a few models need minor reductions in emissions to meet this standard. In addition, most manufacturers routinely measure these emissions today even though it is not required by federal regulation. Today's amendments to the emission test procedures are th