I rather like Charon's thoughts above on how the experience of the pilots themselves is not as important as the air wing as a whole. You could work this as readiness value for each airbase as a whole. When plane is shot down your value goes down as they will be replaced with a less experienced pilot, whereas successful missions increase it as your pilots and ground crew become better. At certain readiness thresholds you could have skill tree abilities for both the ground crew and the pilots that you could pick such as quicker refueling or slightly longer range for cannon weapons. Also higher readiness could translate directly into higher accuracy and evasion as well. You could even transfer readiness between bases to bolster a bad base or one that recently took a lot of casualty's. A ten percent transfer could be explained as you just sent them a couple good pilots and a few experienced technicians. Also as an aside it would be cool if there were other aircraft in the combat zone. It would spice it up a bit if there were civilian craft that you could try to bait the et's away from, or military craft from the local government that could try and assist you.