The relative amount to be added to the measured value decreases with the number of engines tested. Table 7-8 shows an example of engine emission data extracted from an ICAO data sheet submitted by the manufacturer with two engines tested. The HC, CO, and SN standards have remained unchanged within the CAEP process. For NO x , the approach was to tighten NO x stringency in accordance with technology gains. Present regulatory procedures based on the LTO cycle were designed to address airport air quality problems, but the CAEP is now pursuing new certification methodologies that take account of the flight mode as well.
Aircraft engines produce many different emissions.
Apart from the major exhaust emissions, a large variety of minor species are produced. There has not been sufficient characterization to date for global inventory purposes, with most of the available data acquired from a selection of military engines using kerosene-type fuel with different specifications. Other emissions are not currently modeled in emissions databases because of very small quantity or the fact that little data exists. This subject is further discussed in Section 7.6 .