G'day Bob,

Got this question (or something very close) on my CFPA exam last week and I have been racking my brains since as to how I was wrong. It was something like this:

Your flight planned track from Alpha to Brave on a charter flight is 218M and the distance is 410nm. Given the following details, find the distance to PNR from Alpha.

Wind 310T/20
Magnetic Variation 12E
TAS 135kt
Fuel Flow 160L/h
Fuel on board at start 710L
Taxi and start up 10L
Fixed reserve at cruise rate.

I worked out safe endurance of 189 minutes. Time to PNR was 98 minutes. 98 mins at 130kt is 212nm. Out of the answers available, none of them matched this (It's possible I have done something wrong somewhere), so I chose the one closest which was wrong and I missed out on the 4 marks allocated to this question.

Where did I go wrong?

Also, with a PNR calculation, do i minus out the variable reserve or can it be burnt? If my calculations are right, then I am thinking that this variable reserve component of the safe endurance is where I could have gone wrong.

Thanks heaps Bob!

A PNR is NOT considered to be an emergency. People fly routes that have a PNR almost every day. It is simply the point from which you cannot return to your departure point without burning into your reserves. When you calculate the flight fuel available for a PNR calculation, you must take all of the reserves out of the start-up fuel.

So flight fuel (assuming a charter fight) = (Total fuel at start-up - taxi - fixed reserve - any holding that applies to the departure aerodrome) ÷ 1.15. Note that when you calculate a PNR, the destination has nothing to do with it. A PNR assumes you are going to return to your departure aerodrome, so the only holding that has to be taken out is holding at the departure aerodrome. Also the distance to the destination (410nm) has nothing to do with the PNR calculation.

If you return from your PNR to the departure aerodrome, ATC will not do you any favors, you will be treated just like any other arrival and you should have planned to still have all reserves intact and any holding requirements accounted for.

Also in the example you have given remember that the wind is always a true direction. So a wind of 310°T and a variation of 12°E means the wind is from 298°M. Your FPT is 218°M so the wind is 80° across the track. You would have to check ERSA CON wind component tables to get the head/tail wind component to calculate GS home and GS on. That gives you a headwind component of 3 knots. So GS out = 135 - 3 = 132kt and GS home = 135 + 3 = 138kt.

Thanks Bob, makes sense!

Hello Bob,

Long time observer and first time poster in here. I appreciate all your books, I am self studying my CPL theory and they got me through the exams so easily (bar 1! unfortunately I failed performance once,and next test is this week tuesday)
I like to ask something related to this particular topic. If we are to carry holding for Bravo, when we calculate our PNR are we supposed to do:
(Fuel at start up - Taxi - Fixed Reserve) ÷ 1.15 considering we could be using our holding fuel for Bravo on the way back to Alpha?

Thanks in advance

Yes, you may be using the holding fuel over Bravo if you return from the PNR. But if you return from the PNR you won't be going to Bravo will you? Just remember that when you calculate a PNR your destination has nothing to do with it. Of course if the destination had required holding you would have included it in your original flight plan, but if you decide to return from your PNR, you will be abandoning your original flight plan.