Thanks guys...Sorry for delay in replying. I've been at work. (Jabbrrwok) .. max distance is c.100nm so no problem with long routes. Can you give me a formula that will work. I've been to (charley) Avaition formulary but it is not giving accurate results. I might be entering the formula incorrectly (using Excel). If you can give me a worked example it might help. (Chocks Wahay) I've worked that one out before, I'll look it up and give it to you. Ta again.