There are two distict "autopilots" in Il-2 1946, neither of which is an attempt to simulate the presence of mechanical autopilot device in the planes.
The standard autopilot replaces you with an AI pilot. The game default is for this to be automatically engaged whenever you move to a gunner station and be automatically disengaged when you return to the pilot seat, simultaneously returning the gun to AI control. You can also manually engage it at any time and watch your AI buddie do his thing. This can be a good learning tool for newbies because you can essentially sit in the cockpit, watch the gauges and controls, and learn how an "experienced" AI maintains formation, manuvers, and fights. It is also quite useful when you are flying a long range mission and want to use 8x time compression to get where you are going without crashing. This "autopilot" has always been in IL-2
The "level autopilot" option is not the same. It is mainly in the game to assist in level bombing from multi-place bombers with a bombardier station. I think you have to assign it a key in the controls options. Basically, it allows your plane to stay level and straight while you are working with the bombsight. It simluates the plane being flown by a pilot until you are ready to take it over during the final run as the bombardier - otherwise you also have to fly the plane while looking down through the sight and adjusting its speed and altitude settings, which is a real pain. I've tried it, but I generally find the sights difficult to work with as the games's basic printed manual doesn't go into the details of using all the bomsights in the game and they differ from nationality to nationality.
I'm pretty sure "level autopilot" was NOT in the original Il-2, since the original game did not feature any flyable multiplace level bombers with bombardier stations. It was only added with FB.