Good evening,

I have stumbled across question 12 (page 122) of the PNR revision questions in the Performance textbook.

Its asking to find the safe endurance, given the usable fuel on 145USG at start up.

In the explanation for the answer, it has removed the fixed reserve, taxi, and holding fuel from the total fuel, and then divided the remaining fuel by 1.15 to remove the variable reserve.

Now this is where the problem lies. I was under the assumption that Variable fuel applies to flight fuel required only. Now i calculated the flight fuel using echo fuel consumption data, and the flight fuel required is less than the remaining fuel available when the fixed reserve, holding, and taxi were taken away from the total fuel on board. I assumed there was fuel margin on board.

So my question is this.. should variable reserve be removed from all fuel remaining after removing the other reserves, or should it be calculated using the flight fuel required for the trip only?

Kind Regards,

Alex

When you are calculating a PNR the intended destination has nothing to do with it. The only thing that matters is the amount of fuel you have on board when you leave. The PNR simply tells you how far away from base you can fly before you cannot return with all reserves intact. The PNR may actually be beyond the destination. So the original trip fuel has nothing to do with it.

You simply take the total fuel you have in the tanks and calculate how much of that is available as trip fuel (ignoring the destination).
So (Total fuel at start up - taxi - fixed reserve - holding at the departure if required) ÷ 1.1 gives you the total trip fuel available. You then use that to calculate the safe endurance. The 1.15 you mention has now been changed to 1.1. (10% variable reserve, not 15%).

In the current book the answer looks like this

Thanks for your reply Bob. I just don’t understand why the variable reserve is taken off the whole of the remaining fuel, when the variable reserve only applies to trip fuel. So your effectively taking more fuel out of the equation and reducing PNR? Effectively you’ve got margin in the tank which your taking 10% away from as variable reserve.

Regards,

Alex

I think I get what you mean though, destination is irrelevent and there all fuel is trip fuel, once taken reserves away.

That's correct.