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MissSoph created the topic: Clarify safe endurance
I think I may be getting a bit muddled
With safe endurance when calculating PNR….
So it states that to find safe endurance… find the trip fuel available…. Convert the trip fuel to minutes at cruise rate… cool….
So I have been given…..
Distance A to B -244nm
Hold B -60 mins
Fuel on board at start up -190litres
Fuel flow - 50 liters per hr….
Usually trip fuel= distance x fuel flow \ ground speed……
But isn’t safe endurance about useable fuel on board… so I’m not really looking for the trip fuel….
Now if I hadn’t been given how
Much fuel was in the tanks… then yes… I have to find the fuel I need for the trip…. So trip fuel…. Using the equation above ….
Have I got this correct? Or so I slit my wrist now )
Please don't slit your wrists just yet. Put that box cutter down!
A PNR calculation has nothing to do with the planned trip.
If you decide to return home from the PNR you wont be going to the destination. The only thing that matters is how much fuel is in the tanks when you dip them.
So forget all about the original planned trip. The distance A to B has nothing to do with it. In fact it's quite possible that the PNR could be beyond the destination if you are carrying more than minimum fuel.
In the case of the PNR calculation, the 'trip' is from the departure point to the PNR and back. The term 'trip fuel' could be misleading because it has nothing to do with the original planned trip. However, it is an official CASA definition.
So the trip fuel for a PNR is simply
(TOTAL USEABLE FUEL AT TAKE OFF - TAXI - FIXED RESERVE - HOLDING WHEN YOU GET BACK TO A) ÷ 1.1 (if it's a charter).
Convert that into minutes and you have the safe endurance.
MissSoph replied the topic: Clarify safe endurance
Thanks bob…. Box cutters are out of reach… so should I assume then that if I get a question about PNR I will always be given the total fuel at start up/ total fuel in the tanks? Would
I ever have to calculate how
Much fuel I need from being
Given the distance, fuel flow, ground speed( found from TAS)
Yes, you will always be given the total usable fuel at start-up or take-off. If you were told that you departed with the minimum fuel required, you could simply find the time interval for the flight (ground speed and distance), and that would be your safe endurance since you would have only the reserves left when you arrived at the destination. I've never heard of such a question being asked in the exam.
Hi bob As regards the safe endurance calculation If a question gives distance and fuel on board And the fuel on board minus all the reserves And the distance /trip does not use Anywhere close to the fuel left and if we had added the 10% variable Couldn’t we have added a variable amount onto discretionary fuel or margin fuelAs well as the trip fuel
Once again, a PNR calculation is concerned with only one thing..Of the fuel on board at take-off, how much can you legally burn? The calculation is done before take-off, so you cannon assume that any of the reserve fuel can be available for the trip to the PNR and back. The original planned trip has nothing to do with the PNR calculation.