Bus, I don't see how you were spouting rubbish. PNR depends on fuel and distance, no matter how many engines you've got. Topbunk, there can be all sorts of reasons for turning back apart from an engine failure. If you are going from A to B, then the point where you no longer have fuel to get back to A is PNR. If you can go all the way from A to B and back again, then there is no PNR.
And whats this "even you may understand"? No need to be grumpy - season of goodwill, what?