A new flight simulation package, named Comsim-Platform and written in the program language C++, has been created to replace the LISP-based PcP (being in use for environmental flight studies at FOI). It re-uses the aircraft model, Platform, of the latter, and it has been provided with a new flight management system, including navigation. All numerical integrations and event notices are handled by the existing C++ based simulation package Comsim. The Comsim-Platform package is intended to act as a basis for various types of flight simulations. The Flight/Navigation application that has been created for testing the functionality of Comsim, with special views on flight/navigation management, has proved its capability. This report documents the usage and functionality of Comsim-Platform and Flight/Navigation. The ease of reproducing the detailed procedures of recorded flights has been demonstrated. This has been used in the validation of the capability of the Comsim-Platform package. Using accurate data for the airframe and its engine(s) of a studied flight, this report proves and documents the soundness of the flight-mechanic basis of the extended Platform model. The final conclusion is that the Comsim-Platform package is ready to be used, with existing version of Flight/Navigation, or with some type of modification/utilization of the latter modules.