vasFMC

vasFMC is a Open-Source project, which develops an Airbus Flight Managment Computer.
vasFMC is compatible with FSX, brings an Airbus like autopilot overriding the default one of the sim, adds various pages and features compared to the previous version.

So you have an Airbus FMGC or Boeing FMS for aircraft add-ons that don’t carry such advanced systems.