Helicopter performance is key element for the competitiveness of the aircraft and is usually define during flight test. Due to the increasingly complicated design of air intake, estimating the installation effect prior to flight test can be useful to minimize effort and cost. This paper proposed a methodology to predict helicopters engine installation effect that can be used prior to any flight test. For that purpose a two step methodology has been developed using first CFD simulation to determine the aerodynamic and aerothermal disturbances generated by the helicopter air intake and then a thermodynamic engine model to estimate the installation losses. These two steps were validated against in flight measurements and it has been shown that combining all individual effects in the model improve the quality of the installation losses prediction. The global accuracy in terms of installation losses absolute value and shape show that the proposed method provide sufficient accuracy in an industrial context.