In-flight deduction of propulsion forces from a turbo-jet engine requires separate assessment of the stream thrust resulting from the passage of fluid through the propelling nozzle and the force due to the passage of (air) mass-flow through the inlet to the engine. Previously@ it was customary to determine values of inlet momentum and thrust forces in-flight using correlations of some form of mass-flow and thrust functions with a semi-dimensional engine operating quantity. Such an approach was found to lead to a variety of difficulties. Distortion of the flow at the face of the inlet was perceived to be a distinct problem. This document considers an alternative procedure@ which relies solely on the calibration of the final propelling nozzle in conjunction with a specific engine in an altitude test facility. The final nozzles of a twin-engined aircraft are calibrated in terms of two effective areas; the one for mass transfer and the other for momentum (force) transfer.