Apparatus for generating a residuum (r) for detecting erroneous transients, drift or oscillations in the system behaviour of a system of an aircraft, which system receives on the input-side a guide default (w) and a disturbance variable (z) which reproduces external disturbances, and provides on the output side a system output variable (y), comprising: a) a first means comprising a system model for reproducing the system to be monitored which receives on the input-side the guide variable (w), an observer feedback variable (bg) and a disturbance model variable (z), and as a function thereof provides on the output-side an estimated system output variable (ŷ); b) a second means for forming a residuum (r) from the difference between the system output variable (y) and the estimated system output variable (ŷ); c) a third means comprising an observer feedback, which receives on the input-side the residuum (r) and as a function thereof provides on the output-side the observer feedback variable (bg) for dynamic correction of the system model such that the estimated system output variable (ŷ) follows the system output variable (y); and d) a fourth means comprising a disturbance model which receives on the input-side the residuum (r) and as a function thereof provides on the output-side the disturbance model variable (ẑ) for reproducing the effects of the external disturbances on the system model, wherein the disturbance model variable (ẑ) is formed as a dynamic disturbance model variable (ẑ) which is formed by means of a linear combination of a tuning matrix, an integrator 1 s and a disturbance variable matrix, wherein the tuning matrix receives on the input-side the residuum (r) and the disturbance variable matrix provides on the output-side the disturbance model variable (z). a) a first means comprising a system model for reproducing the system to be monitored which receives on the input-side the guide variable (w), an observer feedback variable (bg) and a disturbance model variable (z), and as a function thereof provides on the output-side an estimated system output variable (ŷ); b) a second means for forming a residuum (r) from the difference between the system output variable (y) and the estimated system output variable (ŷ); c) a third means comprising an observer feedback, which receives on the input-side the residuum (r) and as a function thereof provides on the output-side the observer feedback variable (bg) for dynamic correction of the system model such that the estimated system output variable (ŷ) follows the system output variable (y); and d) a fourth means comprising a disturbance model which receives on the input-side the residuum (r) and as a function thereof provides on the output-side the disturbance model variable (ẑ) for reproducing the effects of the external disturbances on the system model, wherein the disturbance model variable (ẑ) is formed as a dynamic disturbance model variable (ẑ) which is formed by means of a linear combination of a tuning matrix, an integrator 1 s and a disturbance variable matrix, wherein the tuning matrix receives on the input-side the residuum (r) and the disturbance variable matrix provides on the output-side the disturbance model variable (z).