System nonlinearities are one of the characteristic features of Avionics systems. Desirable nonlinearities appear currently in automatic flight control , assemblies, regulated power supplies, power control of transmitters , mixers , detectors , ... etc. Undesirable nonlinearities receivers transmitters , ... etc , yield signal distortion, intermodulation,spurious interference signal of annoying effects on board of a/c that is normally crowded with plenty of transceivers using normally crowded spectral band It is, therefore, essential to identify the Avionics system nonlinearities and supply good models for flight control Airborne system designers. Identification of system nonlinearities involves practically two steps; estimation of the nonlinearity order and determination of the coefficients of the assumed describing polynomial. In this work, we give a method for estimating the nonlinearity order for a class of the above-mentioned nonlinearities. Utility of the method is proven through several examlpes .