Flight simulation is all about realism (At least for hard core simmers like myself).
One big realism killer is to fly within empty airports, with no other aircraft around or, worst, having fictional airliners going in and out (FSX/P3D default traffic).
FSX and Prepar3D have included in their default package a system that recreates airline and general aviation traffic so your airports don’t look as empty deserts. This traffic interacts with each other and with the ATC control and behaves as an autonomous entity. This traffic is called AI traffic, as it has artificial “Intelligence”.
The default airline AI traffic is made by fictional airlines, with fictional liveries and fictional schedules. Anoying.
This default AI traffic package populates almost all airports in the world with 3 or 4 fictional airlines and general aviation aircraft.
As a rule of thumb this traffic must be deactivated as soon as you install your sim because the airplanes used are not optimized for AI traffic and will hit your sim performance (Low FPS) in medium to large size airports.
How it works.
AI traffic works in a very basic way: Every AI Aircraft has a compiled flight plan which tells the plane to go from airport A to B, using suitable gates, taxiing by predefined taxiways and taking off from the most suitable runway according to the wind (The lousy runway assignment in airports with multiple runways is topic for a separate post).
Planes will travel from A to B almost in a straight line and doing very basic depart and approach maneuvers (Most departures are single turns to intercept cruise heading, same for approaches).
Once landed aircraft will repeat the depart process just backwards.
AI Aircraft models
Any aircraft installed in your simulator can be used as AI aircraft, however this is not recommended, because very detailed models will hit your flight simulator performance specially in large airports.
AI aircraft models can be optimized to be easy with your computer resources and allow have a large airport with lots of airliners and still have a decent sim experience. This optimization is about level of detal (LOD) and polygon and texture optimization.
There is a very organized community that has created almost all existing aircraft models in fsx/p3d versions optimized for AI traffic for free use.
There are also commercial add-ons that provides a full AI aircraft optimized library to be used within your flight simulator.
AI aircraft timetables
AI traffic follow pre-coded flight plans which tells the sim the origin, destination, departure time, speed, altitude and day of the week of the flight. Arrival time is calculated on the fly according to the aircraft speed.
Almost all this input data can be customized by the user, however editing is not an easy task, it has a steep learn curve. I will post later about creating and adding your own airlines.
Every single AI aircraft has a fixed flight plan attached to it. It will fly the same schedule in a loop of 24 / 48 hours or weekly period. Airplanes living in your sim will fly in a loop repeating the same scheduled flights over and over again.
The same way there are freeware and payware optimized models for AI aircraft, there are free and commercial flight plan packages which injects real flight timetables (sort of) to your simulator to be used by AI traffic.
AI aircraft behavior
AI aircraft decisions are way too basic. Do not expect any AI aircraft to do a realistic procedure. ESP AI engine is full of bugs and has a poor decision making process. Some behaviors can be annoying and way unrealistic. However there are certain tweaks to improve its behavior and make it more realistic, I will talk about it later.
AI traffic is a great realism enhancer for your flight simulator, as long as you are able to have a real world replica within it and watch and interact with real airliners while flying. It is up to you and how much time and/or money will you dedicate to customize your AI library.