PURPOSE: To eliminate the influence of turbulence of air flow by momentarily detecting the data regarding flying speed, atmospheric pressure, air temperature, and deflection of wings, and by calculating the inclination angle of an aircraft based on these data.

CONSTITUTION: An aircraft 1 flying at a speed V is provided with a speed sensor 4, a static pressure sensor 5, a temperature sensor 6, and two deflection sensors 3 for measuring the deflection of wings 2, and respective data are sent to a data processor 7. During flight, since the wings 2 deflect due to the lift exerted thereon, the deflection thus caused is measured by the deflection sensors 3, and based on the flying speed V, atmospheric static pressure Ps, and temperature Ts measured by the sensors 4 to 6, the data processor 7 calculates the inclination angle by means of a predetermined algorithm. Thus, without receiving the influence of turbulence of air flow, an accurate inclination angle can be obtained.