Abstract Despite its importance for the aircraft operation and performance monitoring of aircraft and engine the engine thrust is measured indirectly with the current engine monitoring capabilities. Characteristic figures of the engine operation are used as equivalent parameters for an indirect measurement of the engine performance and its control. This is based on correlation between measured parameter and the engine thrust production corrected with operating flight conditions. This method is missing a direct physical connection between the measurement and the engine thrust. Changes in the engine condition influence the correlation and make a safety margin of excess thrust mandatory to guarantee the required thrust production at all points. The missing direct measurement of the engine thrust makes it difficult distinguish between the aircraft and the engine performance assessments because of the deteriorating trends, which can occur on the aircraft (in form of increased drag) and the engine (reduced efficiency due to thermodynamic wear of its components). One requires an exact monitoring of the current engine thrust and condition for maintenance planning and to make economic decisions about its performance restoration. In this thesis, a direct engine thrust measurement system is developed and presented. The system is validated by on-wing engine operation. A calibrated and temperature compensated strain measurement system is used on the thrust transferring parts of the forward engine mount, in order to measure the transferred force between the engine and the air-frame. Validation data is collected from the measurement system installed on the engine of a commercial aircraft during its routine operation for six month. Applicability of the system is proven with the flight test data during take-off and cruise operations, normalised to defined reference conditions, to investigate the reproducibility of the force measured at the engine mount and to prove the direct correlation between the measured force and the produced engine thrust. To be able to detect changes in engine performance due to changes in thrust production, all measurements are normalised to reference conditions by developed methods. The correlation between measured force and thrust is analysed to validate the usability of the system and its achievable accuracy is investigated. It is shown that an accuarate determination of the current engine condition and thrust production is possible with sufficent amount of measurements. The dominant influences and uncertainties in the force measurement are random errors and therefore compensated by statistical analysis. After normalisation to reference conditions, the same amount of force is measured for all data points, allowing the determination of the current engine condition as thrust production at the reference point operation. Monitoring the measured force at reference conditions allows monitoring the engine condition. The direct measured force and the proven physical correlation to the engine thrust enhances the possibilities of the current engine condition monitoring.weiterlesen