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PNR Calculation

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PNR Calculation

Old 1st May 1999, 12:40
  #1 (permalink)  
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PNR Calculation

Off on my nav course in a week but im having a real problem getting my head around PNR calculations.
Anybody got any 'thicko proof' methods of calculating the answer to these type of problems.
Old 1st May 1999, 14:05
  #2 (permalink)  
Join Date: Jun 2001
Location: Glasgow no more!
Posts: 6
constant fuel flow D = (E * O * H)/(o+H)
d=(endurance (hrs) * g/s out ) g/s home)
G/s out + g/s home

flow = kg per gnm

G/s = gnm per kg

[This message has been edited by Bubbles (edited 01 May 1999).]
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Old 2nd May 1999, 05:44
  #3 (permalink)  
Captain Custard
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Socca, I couldn't resist reading your thread, with a name like that!! Trig forever!

My experience is that Bubbles technique doesn't work! Now I'll explain why! Sorry Bubbles! It does work with a consant fuel flow scenario, but with, say, a normal cruise out, and an abnormal cruise back, it gets a bit messy.

A method that is easy (after you've done it a coulpe of times), accurate and will work for any type of PNR (3 eng, 2 eng, Depress) and at any "out" condition is as follows:

1. At a suitable point after TOPC, ie the START POINT, note the distance from the departure airfield (assuming that's the one you'll go back to if you have a problem). Note the fuel on board.

2. From the fuel on board, subtract the Fixed Reserve (30 minutes Depressurised PNR, 10 minutes Engine Failure PNR).

3. From the remaining fuel, divide by 110% for an Engine Failure PNR. To be super technical, we could assume that there would be little variable reserve needed for the OUT segment to the PNR, but to err on the conservative side, we take off 10% for the whole of the remaining fuel.

4. Calculate the SGR OUT by dividing the Fuel Flow OUT by the Groundspeed OUT.

5. Calculate the SGR BACK by dividing the Fuel Flow BACK by the Groundspeed BACK. This will be at the BACK configuration.

6. Calculate the fuel required from the START POINT back to the ORIGIN (at the SGR BACK).

7. Subtract the fuel required from the START POINT back to ORIGIN from the fuel available.

8. Divide the fuel remaining by the sum of the SGR OUT and the SGR BACK. This is the distance from the START POINT to the PNR. Add the distance from the START POINT to ORIGIN and that's the PNR from the ORIGIN in miles.
Old 2nd May 1999, 07:41
  #4 (permalink)  
Flat Side Up
Posts: n/a
Many variations of this are possible. Best to decide which is the most fuel critical scenario. generally it is the Deoressurised return PNR. The engine(s ) out is not so critical fuel wise because a drift down descent will be the norm.

CC's method is the way to go but it can be tidied up a little by avoiding te SGR out and SGR back bit.

Having arrived at the start point note Fuel on board. Calculate the fuel to return to departure in the non-normal condition plus 10%(Vrbl Res) and deduct this amount plus the Fixed Reserve.

The remainder amount represents the fuel from Start point to PNR and back to Start point including 10% (Vrbl Res)= 110% of fuel burn. Say you have 25300 kg ,pullout your trusty prayer wheel aka Jepp CR something. Place 110 on the Inner against 25300 on the Outer and read against 100 on the Inner = 23000kg

Suppose Normal TAS 480 F/Flow 4400kg/hr
And Return TAS 340 F/Flow 4000kg/hr

Then GS Out = 520k @ F/F 4400kg/h
And GS Back = 300K @ F/F 4000kg/h

Calculate(Prayer Wheel Or Calculator)

[GS Out/GS Back * RETURN F/F] + Normal F/F

[520 /300 * 4000] + 4400
6933 + 4400 = 11333kg

This is the fuel needed to fly OUT for ONE Hour normally and RETURN non normal

Then It gets easy..Prayer Wheel.

Place 11333 on the Inner opposite 23000 on the Outer hold (don't shift) it there and READ ON THE INNER opposite the following:-

60 pointer = Time Out = 122min
GS Out(520 = Distance Out = 1055nm
F/F Out(4400) = Burn Out = 8950 kg

Prove it! By setting Gs Back 300k on the Inner opposite Distance out 1055nm on the Outer.
Then again:-

60 pointer = Time Back = 210 min
GS Back 300k Set vs dist 1055 Done
F/F Back (4000)= Burn Back =14050kg
Proof is that Burn Out plus Burn Back (To Starting point) add up to fuel available for this

Fuel Burn Out 8950kg
Plus Burn Back 14050kg
Fuel Avail 23000kg

Can be used for any number of engine out or non normal configurations. Very theoretical!
Old 2nd May 1999, 15:07
  #5 (permalink)  
Join Date: Jun 2001
Location: Glasgow no more!
Posts: 6
fair enuf guys, thats what you use in the real world, but for the exam, thats the formulae they want!
Thats only for a single leg PNR though.
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Old 3rd May 1999, 07:14
  #6 (permalink)  
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For the exam use what he instructor sugests. But in my real world captain custard shows the way. It is also easy to visualise what you are doing. As for the 10% additional fuel taken out of the calc as you reach your PNR. you can re calc closer to your PNR or use some of that fuel as your fudge factor that will be used during the emergency decent for dep cruise if required. I've never experienced one for real, but in the sim everything seams to take a moment or two before bearings are regained . Depending on speed it will also take some minutes to do the 180 turn, while burning critical fuel.
Old 4th May 1999, 03:41
  #7 (permalink)  
Captain Custard
Posts: n/a

If the exam problem is a multi-config PNR (eg depress or engine failure), then you'll have to use mine or FSUs system. I did it in Senior Comm Flight Planning and got 95%. besides, are they after the formula, or only your working and the correct answer?

FSU: I don't like prayer wheels! The little metal bit in the middle makes my glass tip over!

Old 4th May 1999, 03:57
  #8 (permalink)  
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Thanks for your advice gentleman.
Now if I can only remember all this on exam day aswell as the nav, met, radio aids and dit dit dit I should be home free !!

Now where did I put that ch lon formula!!
Old 22nd Jul 2000, 21:23
  #9 (permalink)  
Posts: n/a
Thumbs up

As I understand it, I think you're finding the ratio of normal vs non-normal fuel consumption per hr (per 2 hrs?)of flight.

Once the ratio as a rate of consumption is known then it's applied to the available fuel - much as a simple FF/time/consumption is done.

Very tricky, Flat Side Up!

[This message has been edited by Tinstaafl (edited 28 July 2000).]

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