With the growing awareness of the importance of the environmental protection, a certification for nitrogen oxide emission becomes more and more stringent. The certification of airworthiness is necessary before the aircraft enters into a commercial operation. The requirement of the airworthiness is the basic regulation guarantee for the aircraft safe flight, so it is necessary to study the airworthiness certification method for gas turbine engine emissions. In order to solve the technical difficulty in dealing with the gas turbine engine emission test data, to meet the requirement in the airworthiness certification standard for engine emissions, the "Aircraft Engine Emissions" promulgated by the convention of International Civil Aviation Organization (ICAO), is used to compile the pollutant gas sample data. The calculation of NO_x emission index from aircraft is carried out by using Aerospace Recommended Practice (ARP) of Society of Automotive Engineers (SAE) and the ICAO method based on the ICAO standard LTO cycle. JT3D-7 NO_x emissions are compatible with the existing regulatory standards according to China Civil Aviation Regulations (CCAR-34).