The aircraft polar must be determined with high degree of accuracy in order to derive the data related to the aircraft performance. At level and constant speed flight, thrust equals drag. Methods for calculating thrust, and therefore drag, are discussed. This work describes the application of two methods for the calculation of in-flight thrust determination and contains a description of the relevant methods and their implementation in a user friendly computer program, together with actual flight data. Inflight thrust calculations for several flight conditions are performed and the results compared.