PDA

View Full Version : Off track PNR


Len-ticular
5th Nov 2015, 05:21
Hi there all,


I was hoping that there might be someone out that might be able to explain the steps (required calculations) required to calculate a PNR for an off track alternate.


ie Flying 500nm between point "A" and point "B" and there is an alternate 90 nm off track midway along track.


Any help would be very much appreciated.

c100driver
7th Nov 2015, 20:14
Do you mean an ETP?

A PNR is totally dependant on the fuel on board.

Lucky8888
8th Nov 2015, 00:16
Hi there all,

I was hoping that there might be someone out that might be able to explain the steps (required calculations) required to calculate a PNR for an off track alternate.

ie Flying 500nm between point "A" and point "B" and there is an alternate 90 nm off track midway along track.

Any help would be very much appreciated.

Are you sure you want PNR and not ETP? Considering you indicated an "alternate" perhaps you are referring to an ETP. There is a difference.

The Point of No Return (PNR) is the farthest point to which the aircraft can go and be able to return to the departure point with adequate holding, approach, landing, and alternate fuel.

An Equal Time Point (ETP) is the geographical point in a flight whereby the aircraft has the same flying time to an alternate airport as the flying time to another airport, typically an alternate.

DJ77
8th Nov 2015, 08:11
Rough solution.
1- Extend route A -> B to the point (C) where you would reach fuel exhaustion if you were not landing at B (hopefully, this point is past B in the direction of A->B).
2- Your PNR is simply the ETP between C and your alternate.

Hope this helps - DJ

compressor stall
8th Nov 2015, 10:03
Called Off Tack Diversion or Latest Point of Safe Diversion.

It's used for the latest point / time you can divert to an en route alternate (NOT a destination alternate) and arrive with reserves intact.

It's not a simple calculation, but it's not too difficult once you get your head around what you're calculating. Consider:

Melbourne to Perth. Perth weather is above minima but with 60 holding but you’re really not comfortable. You want to know the latest point in the flight that you can divert to Alice Springs and land with fixed reserve only. You don't have enough fuel to fly all the way to Perth then divert to Alice Springs(yes this is legal in Oz but that's another story....).

Data
Melbourne to Perth = 1461nm
Perth to Alice Springs = 1069nm

Using basic 2,000kg/hr fuel burn
GS 400kts MEL-PER and 475kts PER-ASP

FOB at departure = 10500kgs

Useable fuel = [10500-1000(Fixed Res)]/1.1 = 8636kg

Fuel required from MEL-PER-ASP =
(1461/400*2000)+(1069/475*2000)
7305+4501= 11806kgs

Approx distance to LPSD = [Useable Fuel/Fuel required for entire journey*distance destination to alternate}
8636/11806*1069
=781nm from MEL. Call this point L

Now check fuel required for MEL-L-ASP
MEL to L is 781nm @400 kts = 3905kgs
L to ASP is 760nm @ 450kts (crosswind now) = 3377kgs
Total is 3905+3377=7282kgs

You have 8636kg to play with.

Thus you have 1354kgs more for which you’d go another approx 40 mins (=2000kg/1354kg*60). Divide this by 2 and make L2 20 mins further on = =1/3 of an hour.

400*1/3 = 133nm

L2=781+133=914nm from MEL
L2 to ASP = 764nm

Fuel Req MEL-L2-ASP=
4570+3820=8390kgs

That’s within 5 mins of the Useable Fuel you had on departure. If it was more out than that, you’d refine again.

You can refine the average ground speeds for the diversion and reduced fuel burns further into the flight to get as accurate as you wish.

Capn Bloggs
8th Nov 2015, 10:44
I'm taking that with me tomorrow! :D