A method for monitoring a condition of a VTOL (vertical take-off and landing)-aircraft with a plurality of spatially distributed actuators (2i, 2o), preferably actuators in the form of propulsion units for generating propulsion forces and/or for maintaining a spatial position of the VTOL-aircraft, preferably an electrically propelled, more particularly an autonomous, more particularly a multi-rotor aircraft, wherein: a primary control (4.1) is used for controlling a flight state of the VTOL-aircraft and at least one secondary control (4.2) is used for controlling the actuators (2i, 2o) of the VTOL-aircraft, preferably the propulsion units; during operation of the VTOL-aircraft, the primary control (4.1) generates a primary data set, which is subject to a first uncertainty, which primary data set is entered into an estimation algorithm, and the secondary control (4.2) generates a secondary data set, which is subject to a second uncertainty, which secondary data set is also entered into the estimation algorithm; the estimation algorithm processes the primary data set and the secondary data set and the estimation algorithm generates an estimation result, which estimation result is representative of a condition of the VTOL-aircraft, namely of a health status of at least one actuator (2i, 2o) of the VTOL-aircraft, which estimation result is subject to a third uncertainty, which third uncertainty is equal to or lower than the first uncertainty and/or the second uncertainty; characterized in that the primary data set and the secondary data set represent an identical type of information in a first estimated condition based on the estimation result generated from the primary data set and in a second estimated condition based on the estimation result generated from the secondary data set, respectively, wherein the first estimated condition comprises a first estimated health status of at least one actuator (2i, 2o) of the VTOL-aircraft and the second estimated condition comprises a second estimated health status of the at least one actuator (2i, 2o) of the VTOL-aircraft, respectively; and the primary data set and the secondary data set are matched before they are entered into the estimation algorithm. a primary control (4.1) is used for controlling a flight state of the VTOL-aircraft and at least one secondary control (4.2) is used for controlling the actuators (2i, 2o) of the VTOL-aircraft, preferably the propulsion units; during operation of the VTOL-aircraft, the primary control (4.1) generates a primary data set, which is subject to a first uncertainty, which primary data set is entered into an estimation algorithm, and the secondary control (4.2) generates a secondary data set, which is subject to a second uncertainty, which secondary data set is also entered into the estimation algorithm; the estimation algorithm processes the primary data set and the secondary data set and the estimation algorithm generates an estimation result, which estimation result is representative of a condition of the VTOL-aircraft, namely of a health status of at least one actuator (2i, 2o) of the VTOL-aircraft, which estimation result is subject to a third uncertainty, which third uncertainty is equal to or lower than the first uncertainty and/or the second uncertainty; characterized in that the primary data set and the secondary data set represent an identical type of information in a first estimated condition based on the estimation result generated from the primary data set and in a second estimated condition based on the estimation result generated from the secondary data set, respectively, wherein the first estimated condition comprises a first estimated health status of at least one actuator (2i, 2o) of the VTOL-aircraft and the second estimated condition comprises a second estimated health status of the at least one actuator (2i, 2o) of the VTOL-aircraft, respectively; and the primary data set and the secondary data set are matched before they are entered into the estimation algorithm.