Check the field altitude where you are fighting- I thought that for a long time when I used to go into the DA occasionally, until I noticed after one particularly horrendous engagement (P-40E vs Bf-109G) that the field alt was 10K. So we were starting the fight at 10K, and since we were both fighting close and dirty right above the ground, the alt never occured to us. We never noticed the ground was almost 2 miles up.