Aircraft engine emissions, like noise, are regulated by the FAA and the ICAO. At present only “local emissions” (around airports) are regulated, but this may change in the near future as the effect of emissions at altitude on climate change becomes more significant. Aircraft emissions include many "species" as suggested by the figure below.
Of particular concern around airports is the generation of smoke, unburned hydrocarbons, carbon monoxide, and nitrogen oxides. These are regulated by ICAO and the FAA as described in the following sections for aircraft operation below 3000 ft.
Enormous improvements in aircraft engine emissions have been achieved since the introduction of commercial jets in the late 1950s and 1960s.
Convair 880 (Old Smokey), circa 1960, and Boeing 777, circa 1995.
However, continuous growth and increasing public awareness mean this is not enough, and low-emissions combustor design is a critical aspect of new turbine engine design programs. Furthermore, an international push to reduce emissions above and beyond current regulations and to include emissions impact on cliamte change (e.g. Kyoto Agreement) is gaining support. Commercial aircraft currently generate 13% of CO2 emissions by transportation sources and this fraction is expected to increase as aviation growth exceeds that of ground-based transportation.
Contrails are formed under certain conditions that vary with atmospheric characteristics. Cirrus clouds may form from contrails and cover a substantial area, affecting the balance of energy absorption and radiation through the atmosphere.
In addition to CO2 generation, there is growing concern about water vapor deposition at high altitudes and contrail formation and its effects on radiative forcing (which influences climate change).
The following sections and references (see especially the PDF links below) describe the current regulations, means by which aircraft emissions might be reduced, and methods by which emissions are computed.
Standards limiting the emissions of smoke, unburned hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) from turbojet and turbofan aircraft engines are contained in Annex 16 Volume II (Second Edition, July 1993, plus amendments) [Reference 2] to the Convention on International Civil Aviation. The Annex also contains approved test and measurement procedures.
With respect to subsonic applications, the provisions of the Standards for smoke apply to engines whose date of manufacture is on or after 1 January 1983. For the gaseous emissions, the Standards apply only to engines whose rated output is greater than 26.7 kN. For hydrocarbons and carbon monoxide, they apply to engines whose date of manufacture is on or after 1 January 1986. For oxides of nitrogen, the Standards have three levels of stringency depending on the date of manufacture of the engine. These Standards are summarized later in Section 7.
The regulatory levels are applicable to engines whose date of manufacture is on or after 1 January 1983.
Regulatory smoke number
The characteristic level of the smoke number at any thrust setting, measured in accordance with Annex 16, Volume II, must not exceed 83.6 (Foo)-0.274 or a value of 50, whichever is lower.
The regulatory levels apply to engines whose rated output is greater than 26.7 kN and whose date of manufacture is on or after 1 January 1986 and as further specified for oxides of nitrogen.
The characteristic levels of the gaseous emissions measured over the LTO cycle in accordance with Annex 16, Volume II, must not exceed the following regulatory levels:
Hydrocarbons (HC): Dp/Foo = 19.6
Carbon monoxide (CO): Dp/Foo = 118
Oxides of nitrogen (NOx):
1. ICAO Engine Exhaust Emissions Data Bank, First Edition 1995, ICAO, Doc 9646- AN/943. Updated 1-Oct-2004, http://www.caa.co.uk/default.aspx?categoryid=702&pagetype=90
2. ICAO Annex 16 "International standards and recommended practices, Environmental protection", Volume II "Aircraft engine emissions", 2nd ed. (1993) plus amendments: Amendment 3, 20 March 1997; and, Amendment 4, 4 November 1999.
3. AA241 Emissions Lecture, Nicolas Antoine, 2003.
4. Overview of the NASA AST and UEET Emissions Reduction Projects, John Rohde, NASA Glenn Research Center, March 1, 2002.
5. Development of the technical basis for a New Emissions Parameter covering the whole AIRcraft operation: NEPAIR, Final Technical Report, NEPAIR/WP4/WPR/01, EC Contract Number G4RD-CT-2000-00182, September 2003.
6. FAA, Aviation and Emissions -- A Primer, available also at: http://www.aee.faa.gov/emissions/