In this paper a control structure based on feedback (Input-State) linearization has been applied to the elevation subsystem of a laboratory double rotor helicopter. This system is multivariable with 2 inputs and 4 outputs, highly nonlinear and strongly coupled. This article focuses on the elevation subsystem which, in turn, is underactuated with 1 input and 2 outputs. A switching control law between exact and approximate input-state linearization is proposed, which presents good result.