Turbojet engines have been used for many decades in aviation. Although their share in civil aviation is minimal, with the advent of Unmanned Aerial Vehicles (UAV's) there are new applications. Their operation depends on complex aero-thermodynamic laws, and optimum performance is strongly affected by the control system. The authors have previously investigated how the Turbofan Power Ratio (TPR), originally introduced on Rolls-Royce commercial turbofans, can be used in control of single stream turbojet engines. Based on those results, in the present paper the assessment of flight characteristics is introduced, based on mathematical models, which are available of the particular gas turbine type under investigation. Like the corrected rotor speed, the ratio of TPR and actual thrust depends on Mach number, therefore, this function is determined in this paper. The investigations also included a deteriorated model under control of both corrected rotor speed and TPR, which has shown that TPR can partially recover thrust loss thus improving the safety of the power plant. As a conclusion, TPR is worth of utilizing in control systems as it results in more straightforward thrust correlation and reduced performance loss over time. Furthermore, measuring simultaneously with other common engine parameters, the extent of deterioration can be signalized to the crew before it evolves into a catastrophic failure.