A method for monitoring a condition of a VTOL (vertical take-off and landing)-aircraft with a plurality of spatially distributed actuators , 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 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 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 of the VTOL-aircraft and the second estimated condition comprises a second estimated health status of the at least one actuator 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 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 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 of the VTOL-aircraft and the second estimated condition comprises a second estimated health status of the at least one actuator 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 . <EOS>