1. A method to level an inertial reference system of a helicopter with reference to the local vertical frame, the method comprising: powering up the helicopter; outputting sensor data from at least one gyroscope and at least one accelerometer to a mode-selecting processor; executing a fast Fourier transform algorithm on the sensor data at the mode-selecting processor; and selecting one of a plurality of operation modes of the helicopter as a current-operation mode based on the execution of the fast Fourier transform algorithm.