In trying to incessantly reduce gearboxes weight, designers can negatively impact the service reliability, particularly in the aeronautic industry, where a weight reduction is often linked with stiffness decrease. Looking for weight savings, gearbox casings are classically made of magnesium or aluminium alloy. In addition, helicopter gearbox design is included the integration of several functions within the same part (like the shafts which gather one or more gears, splines and integrated bearings raceways). These particularities in the design are requiring some specific methodologies for Power Transmission bearing modelling. In this paper, a comparison between tests and simulations results of the aforementioned gearboxes is presented. The first section of the article focuses on the helicopter gearbox description and associated finite elements modelling. Then, the second section provides a comparison between the experimental observations with the results of both FEM simulation and classical analytical simulation. This comparison demonstrates the interest of using FEM simulation to analyze gearboxes behaviour.