'Critical Point' and 'Point of No Return'

[compressor stall]

A lot depends on whether you are studying this for practical useage in flight or for an exam.

A PNR is the furthest you can fly out and still make it back to your dept point with reserves intact.

For preflight calculation:

1. Calculate Fuel on Board (FOB) preflight
2. Subtract taxi fuel from ans1 FOB at takeoff.
3. From ans2, subtract Fixed Reserve, wx/traffic Holding at Deptarture aerodrome (which will end up being destination if you return)
4. Ans3 / 1.1 (For Variable reserve of 10%) This Answer is your Safe Useable fuel for your flight. You want to use ALL of this and not a drop more.
5. Ans 4 less climb fuel (from POH)
6. This fuel figure is your safe useable fuel from top climb
7. Calculate the fuel required to return to departure point from this same distance from departure aerodrome
8. Subtract this fuel from Ans6
9. Calculate FuelFlowOut/Gsout plus FuelFlowBack/Gsback This figure is how much fuel you will use for every nm past topc to return to that point
10. Ans6 divide by Ans9 gives you an answer which is your distance to your PNR.

Note the return configuration can be in any case – DP, Engine out, etc.
Inflight – FOB less fixed res, holding then divide by 1.1. Minus fuel req’d to return pres pos to dept aerodrome. Repeat step 9 and 10. Voila!

It does not make allowances for turning the aircraft around 180 degrees at the PNR. You would need to subtract this fuel figure from the Safe Useable Fuel at step 8.

It also makes no calculation for increased range (if any) for engine out drift down...you can add that in at your leisure.

Remember, the further into the flight you calculate, the further the PNR should be - because of the Variable Reserve is only calculated on the fuel in front of you. (provided the GS is less than 110% of the planned).

The second scenario you describe is in my language a LPSD – Last Point Safe Diversion.

You want to fly as far along the track from departure point A to destination B before diverting to C and at C with your reserves intact.

From a point Enroute (say X)

Useable fuel = (FOB-Fixed Reserve-Holding at C) divided by 1.1 (VR)

Calculate: Useable Fuel / fuel req’d X-B-C * Distance to X to B = Approx Distance X to LPSD

Draw this point on the map. Call it L. Calculate fuel req’d to fly X to L then C. Compare it to your Useable fuel. Calculate the difference, then work out how many miles you will travel with that much fuel. Divide this distance by two.

If the fuel required X-L-C is more than Fuel on Board, move L back towards by the distance above. Away if less…

You can also do it for pre dept by following the intial steps for the PNR re the climb.

Hope that helps!

(with thanks to Checkboard).