The NLR emission model is based on measured emission index values for new engines as published by ICAO for a large number of aero-engines.
The "NLR method" comprises of interpolation in tables derived from the ICAO thrust vs. emission index tables. For the specific engine, the combustor pressure and temperature are determined for each of the 4 ICAO defined thrust settings (this can be done using GSP). This data is entered in the table in the NLR/ICAO tab sheet in the combustor edit window. The emission index is then calculated using logarithmic interpolation with combustor temperature and a simple empirical correction for deviating combustor pressures.
Since the ICAO emission data are specified for standard conditions, application of this emission model is recommended for use only at standard operating conditions (standard ambient temperature and pressure and a standard undeteriorated engine).
For analysing effects of deviating operating conditions, it is best to use the ratio emission models.
NLR emission model
