for AI you have to consider its health and approach direction to the player
for simplicity's purpose we'll divide the two AI status of enemy: combat and defensive
Defensive will be defined by the enemy's avoidance of direct view of player aircraft and the attempt to flee player if provoked. player direction can be determined by calculating instantaneous velocity and then assuming the direction it is in as the direction the player is facing.
Combat will focus on the enemy's attempt to trail the player's back and fire missiles. It will only fire missiles if the player is within the firing cone of the enemy aircraft. The AI should also have a secondary tactic to head-on attack the player if the player vector is towards the enemy and the enemy is a certain distance away. provoked defensive enemies will also enter combat if the player ends up within their firing cone.
Different levels of combat can be determined through use of variables such as distance before attempt to head-on combat, health level before entering defensive mode, turning speeds, airplane max speed, missile damage, missile speed, and missile turning speed.