Remember that even rookie fighter pilots are highly trained killing machines and professionals in their craft.
I disagree completely, totally, 100% with this statement.
Rookies are NOT highly trained killing machines, they are one step above cannon fodder. Witness the decline of both the Luftwaffe pilot quality and the Imperial Japanese pilot quality over the course of the war, as attrition took it's toll and men were given progressively less and less training before being shoved into combat. The results were telling. They were certainly not highly trained, or even very skilled killing machines by any means.
My take on the levels.
Rookies represent that lower quality of training. They can fly, but not that well, and they don't have near as much hand's on time for their aircraft, and zero combat experience. These are the guys that accelerated stall, maneuver too hard and poorly, don't well understand the concepts of energy fighting, much less how to actually do it. Flying straight and level is clearly not going to happen, but they should be much easier to sneak up on from a blind spot. In combat, they waste energy, don't fly "smart", are not be able to keep track of the aircraft health/status and fight well, and make some stupid mistakes.
Average pilots represent just that, the average pilot fresh out of training with a proper amount of time, education, and all around mock combat experience. Haven't been tried in real combat yet, but they understand their rides and would be far less prone to idiot mistakes. They both know to be situationally aware and check six, and should be harder to sneak up on. Real life and death combat will be stressful but their training should be able to help them survive, to a point. They don't know the tricks of the trade that aren't taught in school.
Veterans have seen combat and lived to tell about it. They've flown against the different enemy planes and have seen the differences, and know which tactics to use in each encounter. They know their aircraft very, very well, and can push them to the edge performance-wise. They don't waste ammo on low prob shots, know when it's proper to energy fight vs turning fight. They're damn hard to sneak up on and have high awareness.
Aces are the top dogs. They have multiple kills. They've flown planes home shot to pieces and held together with threads and a prayer. They can make their rides dance in the air, pull off complex, risky moves and succeed. They have eyes on the backs of their heads and seats of their pants. They can force situations that are advantageous to them and their particular aircraft, and can shoot down lesser pilots with better planes. They have almost perfect accuracy from various ranges. Death incarnate, run like hell.
How well IL2 actually models these is up to debate, I think overall it's decent for an AI. My point is and remains, things need to scale. When I want to play around, I'll load up bombers with no weaponry. Toy with stuff? Rookies or average. Normal playing? Veterans. Challenge? Aces. Humiliation? Human opponents. Right now, the game doesn't scale well.
There are SOME small differences between the skill levels when it comes to flight patterns in combat and they can be predictable, but it's not that obvious. To me, the only real deciding factor is the AI's accuracy. Rookies shoot like crap and rarely. Averages shoot like crap but more often. Veterans are a toss up when going head to head, they are accurate enough and you're as likely to get killed as they are. Aces will shoot your butt out of the sky from +1 km away with their deluxe laser accurate guns.