An investigation of takeoff and climb NOx emissions is reported for a twin-engine, narrow-body, short-to-medium haul commercial aircraft, B737-800. Ten randomly selected flights are considered for two independent domestic routes. The study is based on two approaches: actual flight data records and the International Civil Aviation Organization (ICAO) emission database. First, an empirical model is established in order to identify the relationship between fuel flow and a NOx emission index. Next, the NOx, emissions are analyzed through the vertical air region and the horizontal distance from the runway. Averaged takeoff and climb NOx, emissions for equal vertical regions for each route are found as 27.0 kg and 24.8 kg through the developed models for two routes, A and B, respectively. The results are also compared to those given in the ICAO database which utilizes standard takeoff and climb emission indices. (C) 2013 Elsevier Ltd. All rights reserved.