2 Formulas to find Accurate Specific Ground Range (SGR) and when to use them in ATPL Flight Planning
Specific ground range (SGR) is very important for the ATPL Flight Planning exam, especially relating to PNR questions. SGR is a measurement of the kilograms of fuel burnt per nautical mile flown. So if your SGR was 10.5, you would burn 10.5kg per nautical mile flown, so if you fly 300nm, you will burn 3150kg. Easy!
You may have read about Specific Air Range (SAR), this is a SGR before you account for wind. With the right SAR and head/tailwind component, it is possible to estimate flight fuel/ PNR position very accurately.
It is extremely important to use accurate SGRs when correcting your estimated PNR position and not estimated SGRs.
Below are the two options available to calculate accurate SGRs for your flight plan.
1. Zone Fuel and Distance
This is the formula you will use most often. I personally don't write down fuel flow in my flight plans as I have it saved in the memory function on my calculator (that is a whole other discussion) so the only fuel information on hand is zone fuel.
You can divide your zone fuel by the distance of that zone and that will give you an accurate SGR. You can use this for a few things, the main one being PNR calculations.
2. Fuel Flow and Groundspeed
The second option is used much less often. It involves dividing your fuel flow by your groundspeed. It will give you the same answer as above but using a different method.
I only recommend using this method when the question gives you fuel flow directly. An example would be a 2 mark question where they give you a fuel flow, a TAS, and a wind component and they ask for zone fuel. You simply multiply the distance of the zone by the SGR.