An inertial reference system for a helicopter, the inertial reference system comprising: a plurality of inertial reference system sensors (111-113 and 121-123) configured to output sensor data; a mode-selecting processor coupled to the plurality of inertial reference system sensors (111-113 and 121-123) and configured to receive the output sensor data, wherein the mode-selecting processor is configured to perform: a fast Fourier transform algorithm configured to transform the sensor data and output fast-Fourier-transformed-sensor data, the fast-Fourier-transformed-sensor data including a Fast Fourier Transform -peak frequency (f peak) in a zone frequency range of a zone associated with a current-operation mode of the helicopter; a plurality of leveling algorithms configured to execute on the sensor data and output a respective plurality of estimated roll angles and a respective plurality of estimated pitch angles for a respective plurality of operation modes; and wherein the mode-selecting processor is further configured to: input the fast-Fourier-transformed-sensor data; identify a current-operation mode of the helicopter based on the FFT-peak frequency being in the zone frequency range of the zone associated with the current-operation mode; and output one of the plurality of estimated roll angles and one of the plurality of estimated pitch angles associated with the current-operation mode to alignment and navigation filters. a plurality of inertial reference system sensors (111-113 and 121-123) configured to output sensor data; a mode-selecting processor coupled to the plurality of inertial reference system sensors (111-113 and 121-123) and configured to receive the output sensor data, wherein the mode-selecting processor is configured to perform: a fast Fourier transform algorithm configured to transform the sensor data and output fast-Fourier-transformed-sensor data, the fast-Fourier-transformed-sensor data including a Fast Fourier Transform -peak frequency (f peak) in a zone frequency range of a zone associated with a current-operation mode of the helicopter; a plurality of leveling algorithms configured to execute on the sensor data and output a respective plurality of estimated roll angles and a respective plurality of estimated pitch angles for a respective plurality of operation modes; and a fast Fourier transform algorithm configured to transform the sensor data and output fast-Fourier-transformed-sensor data, the fast-Fourier-transformed-sensor data including a Fast Fourier Transform -peak frequency (f peak) in a zone frequency range of a zone associated with a current-operation mode of the helicopter; a plurality of leveling algorithms configured to execute on the sensor data and output a respective plurality of estimated roll angles and a respective plurality of estimated pitch angles for a respective plurality of operation modes; and wherein the mode-selecting processor is further configured to: input the fast-Fourier-transformed-sensor data; identify a current-operation mode of the helicopter based on the FFT-peak frequency being in the zone frequency range of the zone associated with the current-operation mode; and output one of the plurality of estimated roll angles and one of the plurality of estimated pitch angles associated with the current-operation mode to alignment and navigation filters. input the fast-Fourier-transformed-sensor data; identify a current-operation mode of the helicopter based on the FFT-peak frequency being in the zone frequency range of the zone associated with the current-operation mode; and output one of the plurality of estimated roll angles and one of the plurality of estimated pitch angles associated with the current-operation mode to alignment and navigation filters.