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PNR Question 12, Page 4.52
Cessna182
Topic Author
Cessna182 created the topic: PNR Question 12, Page 4.52
Hello,
I'm working through CPL Performance again to refresh myself and am a little confused with question 12 on page 4.52.
Initially I tried working it out like so:
- 500nm at 170 knots will take 176.5 minutes.
- since we are burning 28GPH it means I need 81USG of flight fuel.
- why isnt 176.5 minutes the answer? I know I'm incorrect but I seem to be getting confused as to what the difference is between flight fuel and safe endurance. Are they the same thing but one is represented as time and the other as fuel? The reason why I think 176.5 is correct is simply due to the fact that it's the time we can fly without including any of the reserves or holding fuel.
I've looked at the working at the back which I do get, however I'm confused as to why I'm getting a different answer doing it this way.
Also, with the beetle diagram questions, you are saying to use the least weight out of the three for takeoff. I've come to a question where the least weight is the LDG limit + fuel burn off in flight. Is that saying that we will have nothing but the flight fuel inside our tanks at take off and no reserves? Seems a bit strange if that's the case
Richard replied the topic: PNR Question 12, Page 4.52
Hi Cessna,
I assume you are referring to Q12 in the "Revision Exercises on ETP and PNR".
When you are calculating a PNR, you don't care about the distance to the destination. You simply want to know:
"How far can I fly outbound and yet still return to my original airport with my reserves intact."
Therefore the 500nm to Bravo is irrelevent. An interesting thing about PNR's is that if you have a lot of fuel on board and a short sector, your PNR will most likely be beyond your destination.
Safe Endurance is an expression of how long it will take you to burn the flight fuel you have available but for a PNR it is the flight fuel available for a return to your origin, not a flight to the destination.
As for the Beatle diagram, no, we are not landing without any reserves. We are simply working out how heavy can we be when we start the flight, assuming all the flight fuel is burnt by the time we land and yet we will not exceed the limitations imposed by the destination runway or the airframe of the aircraft.
The reserves will still be in the aeroplane and that's part of your minimum fuel on board at take off calculation.
