You're quite right Shoe, and that's why I used power/thrust. Not everybody is familiar with the what some might call the subtle difference, although it isn't all that subtle. I intentionally didn't intend going too deep for a couple of resons; First is that it's very complex and takes a lot of space and time, and second is that I don't know it all and may well knot myself up. I probably did know it many years ago, but it's not terrribly important in day to day flying of airline category aircraft. Not too much vertical flight is done in 737s/747s etc. I would have used a slightly different equation, and that is thrust > weight + drag, because the drag and weight vector resultant points vertically downwards, but it means the same thing.