In order to perform useful air quality analysis it is necessary to have the most accurate emission factor data available. This report provides updated aircraft engine emission factors and a sample of the calculation methodology used in obtaining these numbers. Modal emission factors have been calculated for a number of gas turbine and piston aircraft engines. Emission factors per aircraft per landing take-off cycle have been calculated for representative aircraft- engine combinations. This group includes commercial jet transports, business jets, turboprops and general aviation piston aircraft.