1. Distance from you and direction
2. Position in space
3. Plane capabilities
My biggest trouble isn't keeping eyes on, it's the judgement of E and the timer in my head that is sometimes way off. I had a few sorties yesterday where I knew who shot me and where they came from. I thought I had more time. Stupid eyes and brain.
I'm the same in all of this except I consider Distance and Positioning both as 1. When I stumble into multiple cons I look for who has alt on me, who I have alt on, and who is closest. The highest con could be 5K away, while someone is climbing up below me just 1K away. The closer plane is the immediate threat, but the other con still does have more alt than me making him a fairly big threat, just not an immediate one. Plane capabilities just let's me know who I would like to kill first, but out numbered I'll shoot at whoever I can. It'd be nice to kill the A6M when I'm flying a P-47, but if I can't get shots on him then I'll shoot at the Typhoon off my 12 instead.
The internal timer is a tough thing to get working, mine is broken as well.

For anyone who doesn't understand this, you don't have to keep your eyes on a plane to keep SA on him. In a multi-con engagement it's just physically impossible to look in 2 different directions. So instead you look at the P-51 (as an example) as he misses a pass on you. It appears he is climbing up to alt to set up for another pass. It takes time for him to get turned around, so for the next 3-5 seconds he will not be an immediate threat as he is busy climbing and turning around. Take this time to check up on that Spitfire that was turning in on you. After 3-5 seconds an internal alarm clock should start ringing in your head (ok, you don't literally hear a ringing, but you know what I mean

). It's telling you "Hey! That P-51 should be turned around by now and is probably making a pass on you! You should check on him and see what he's up to!"
I love this thread! It's making me think of things I've never actually thought about before!
