If you hit, only you take damage. It's only in your flight engine. The other computer, due to minor lag, slight delays, or just different latency to the server, does not see the ram, and thus does not have it.
HOWEVER, if the ping is low and the latency non-important, and BOTH planes ram (it registers a collision on both ends) that means that both computers said "Oh, my plane hit yours" and apply damage to themselves (and only themselves) for the same impact.
So if you rammed something, only you saw it. Chances are (for me 99% of the time) the other guy didn't ram you at all.