several times ive been collided with, or collided and not hear or see it, but still took damage. its a mystery to me on who goes down as a result and what the parameters are. as far as ive seen, the rammers who have it down to a science can collide into you with the belly of their plane into your canopy or abouts and you take the damage and they fly off. there are a lot of dirty players in AH and the collide model is one tool they use.
YOUR computer is the one who controls collision detection for YOUR plane. If YOUR computer detects YOUR plane has intersected with another object, then YOUR computer assigns the damage to YOUR plane.
Like someone else has said, it is impossible for anyone to know the exact amount of separation between two planes, in real time.
The amount of damage assigned is based on a lot of variables. Graphically, it is grossly represented, at this time. In other words, you might only have a percentage of damage done to a part, but the part is graphically shown as being completely gone.