1. A method for functionally and physically integrating a constrained aircraft route(s) optimization application into an avionics onboard system, the avionics onboard system comprising a DAL+ digital core computer, having a first criticality level DAL+, integrated into an initial architecture of peripheral computers and of databases having second criticality levels DAL−, lower than or equal to the first criticality level DAL+, and serving as server by hosting a first plurality of generic open services Serv_DAL+(j), and a DAL− peripheral computer for managing the constrained route(s) optimization application, having a second criticality level DAL−, which is lower than or equal to the first criticality level DAL+, and serving as client by dispatching requests for services to the DAL+ digital core computer and/or to the peripheral computers and for databases of the initial architecture through a communications network, wherein the method for functionally and physically integrating the constrained route(s) optimization application comprises the steps consisting in: functionally decomposing the constrained route(s) optimization application OPT_RTE into a second plurality of elementary functions OPT_RTE_FU(i); and determining, on the basis of the second plurality of the elementary functions OPT_RTE_FU(i), a first list of the elementary functions that can be executed in part or entirely by at least one generic open service, and for each elementary function a first sub-list of generic open service(s); and determining an optimal functional and physical distribution of the elementary functions OPT_RTE_FU(i) within the onboard avionics system over the set of possible distributions which minimizes a global cost criterion CG, dependent on several parameters, including at least the additional development cost of the elementary functions integrated within the DAL+ digital core computer; and carrying out the integration of the constrained route(s) optimization application by actually implementing the elementary functions and their scheduling according to the optimal functional and physical distribution determined within the onboard avionics system in the previous step of determining the optimal functional and physical distribution of the elementary functions.