Wednesday, 4 February 2009

Point of No Return (PNR)

Similar to Point of Equal Time or Critical Point (CP), the Point of No Return is a straight forward equation easily resolved on your Dalton flight computer.
The Point of No return (PNR) depends on the fuel endurance time. After passing the Point of No Return the remaining fuel will not be sufficient for a return to the point of departure. The flight can only be continued to the destination or to an alternate airfield. It is, of course, also possible to calculate the Point of Safe Return based on the available fuel after deducting the required reserve.
The simple formula for PNR is:
T (PNR) = E X GS Home/[GS Out + GS Home] (Time formula)
Where: T (PNR) = Flying Time to PNR & E = Endurance.
The formula can also be transposed as:
T (PNR)/E = GS Home/[GS Out + GS Home]

Example: Endurance 6h30min. GS Out = 240kt. GS Home = 210kt.
On the flight computer:
Align 450 (240 + 210) on the inner scale with 210 (GS Home) on the outer scale. Read 182min outer scale against 390min (6h30min) on the inner scale.
Result: The PNR will be reached after a flight of 182min (3h02min). If the point is to be located geographically, determine the distance to the PNR by means of GS Out. In this example the distance to PNR will be 728nm. (182 mins @ 240kt.)


James said...

Good stuff on both the PNR and PET.
Time to get the Dalton out and do some more revision!
Also, wonderful inspiring pictures again...

Blog Archive said...

Thanks James, Your feedback is very welcome and encouraging. It is the feedback (via comments) that keeps me going! Maybe I should do a piece on the Dalton flight computer?

Fred said...

Would it be possible, do you think, to touch on radio navigation? I'd like to get your take on VOR and NDB navigation and such things as time-to-station, etc., etc.

Anonymous said...

Shouldn't this be based on total range rather than total endurance? (It would give you an answer in miles, then, and you would have to divide by GS-Home to get time). I'm thinking that if you actually did this you would be flying best range AS rather than max endurance AS, because you would care about whether you could get there, not how long you would remain aloft.

Blog Archive said...

I would not argue with that. The idea of these formulae is to keep-it-simple. The use of the Dalton Computer is to utilize the flight plan (and winds) as known. This flight plan is probably based on Long Range Cruise - LRC. Having established your PNR, or indeed your CP, you can now fly for range at best range IAS. But you will be able to reach your departure field at the IAS you had to this point.
Thanks for your comment.