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Rich C created the topic: PNR query
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
fuel flow 60LPH (applied for all calculations)
20 kt hwind
fixed res 45 mins (@ 60LPH)
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.
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.
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