The jet engine maintenance process is complex, expensive and time-consuming. It often requires engine disassembly or boroscopic examinations. In order to accelerate the process and reduce the down time of an engine we intend to develop a method to locate and characterize defects and damage at an early state, without having to disassemble the engine. The assumption is that various defects in the hot gas path of an engine have a noticeable impact on the spatial density distribution of the exhaust jet of an engine. The resulting differences in the exhaust jet pattern will be measured with the Background Oriented Schlieren method (BOS). We perform numerical simulations (CFD) in order to analyze the effects of various general defect types on the density pattern of the exhaust jet. The defects under investigation include the malfunction of one burner, the increase the turbine blade tip clearance and burned trailing edges of the blades. The changes in the pattern resulting from the defects are compared to the density distribution of the undamaged initial state. It is shown that different exhaust jet patterns can be linked to the investigated hot gas path defects. Furthermore, a BOS set-up is installed in a test cell of a helicopter engine with a twostage axial turbine to demonstrate the applicability of the BOS method for the measurement of small density gradients resulting from temperature non-uniformities. A cold streak was inserted into the exhaust diffuser to simulate an artificial defect. The completed measurements show that the BOS method is able to detect these small variations. The present paper summarizes the results of different investigations. It presents a combination of BOS measurements of the exhaust jet and CFD simulations of defects within the hot gas path as a promising approach for evaluating the condition of a jet engine.