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PNR query

  • Rich C
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Rich C created the topic: PNR query

Hi,
I came across this question in a BT online exam, got it wrong, and still cant work out what I am doing wrong. Heres the details:

TAS 170 kts
Charter flight
fuel flow 60LPH (applied for all calculations)
20 kt hwind
fixed res 45 mins (@ 60LPH)
taxi 5L
hold req departure 30 mins (@ 60LPH)
hold req destination 60 mins (@60LPH)
150L @ startup
find the distance to PNR

So Iwork out flight fuel and safe endurance:
150 - 45 - 5 = 100L. As its a charter I apply v.r so I now have 87L.
So here is where i start getting confused. I have to take the hold requirements into account. that means 87-90= -3???

So I try starting with 150L, and subtract the hold fuel first: 150-90= 60.
I then minus the f.r and taxi: 60-45-5= 10L
I then apply v.r: =8.7L
8.7 (say 9)@ 60LPH= 9 mins safe endurance.
PNR (time) = safe end x gs home / gsh+gso = 5mins
5 mins @170 (-20kt hwind) = 5 mins @ 150kts = 15nm.

This of course, is wrong.
then given answers are 27, 34, 61 and 85.

Any help is greatly appreciated.
Cheers
Rich
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bobtait replied the topic: Re: PNR query

Firstly collect all of the information you need to do the calculation.
60lph makes it easy because we can use 1 litre per minute.

Ground speed out = 170 - 20 = 150kt
Ground speed home = 170 + 20 = 190kt
Flight fuel available = total fuel at start-up - taxi - fixed reserve - holding / 1.15
Note that the holding we use is the holding that applies to the departue aerodrome. When you calculate a PNR the departure aerodrome is your destination.
Flight fuel available = 150 - 5 - 45 - 30 / 1.15 = 61 litres
Safe endurance = 61 minutes.

Now time to the PNR = 61 x 190 / 340 = 34 minutes.

If you wanted to know the distance to the PNR your would use 34 minutes at 150kt = 85nm.
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  • Rich C
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Rich C replied the topic: Re: PNR query

Thanks (again) Bob for the explanation.
Of course ! I am not going to reach the destination ie point B. I will fly out from point A toward B but will turn around and head back to A where I have to take the hold requirement at A into account.

And I should have subtracted the hold fuel before applying 1.15 VR

Cheers
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