We start with a distance in Nautical Air Miles — NAM — of 3,044 — Page 96, Lesson 92

I want to walk you through a worked example that shows how we use the cruise tables to find fuel required for a leg. This is Step 6 and Step 7 from one of the practice examples. We start with a distance in Nautical Air Miles — NAM — of 3,044. You go into the cruise table looking for that number. The table lists aircraft mass against a corresponding cruise NAM value. You will almost never find the exact NAM figure you're looking for. The rule is: take the lower figure. In this case, 3,044 NAM isn't in the table, but 3,036 NAM is. That 3,036 NAM corresponds to an aircraft mass of 51,100 kg when overhead the point labelled "B". That gives us the mass at "B". We already know the mass at "A" from earlier in the example — 54,400 kg. Step 7: the difference between 54,400 kg and 51,100 kg is 3,300 kg. That 3,300 kg is the fuel required for that leg, before any correction. Now, there is a correction to apply. You should decrease the fuel required by 1.2%. One point two percent of 3,300 kg is 40 kg. So subtract that 40 kg, and you get a total fuel required of 3,260 kg. Let me also walk you through the calculation box at the bottom of that example, because it shows how the NAM-to-NGM conversion works. We have a True Airspeed — TAS — of 418 knots. The Ground Speed — GS — is 368 knots. The Nautical Ground Miles — NGM — is given as 500. The formula to convert NGM into NAM is: NAM equals TAS multiplied by NGM, divided by GS. So 418 times 500, divided by 368, gives 568 NAM. Now, the next example on the page is a full worked problem. Let me teach that one too. The question: Aircraft mass at "A" is 51,200 kg. Aircraft mass at "B" is 48,500 kg. Cruise at Mach 0.78 at Flight Level 350, with an ISA deviation of plus 20 degrees Celsius, and a 50-knot tailwind. We need to find the True Airspeed, the ground distance, and the Specific Fuel Consumption. Step 1: Find the correct page in the cruise table — page 59. Step 2: Find the TAS. The table gives a base TAS of 449 knots for those conditions. Since we are ISA plus 20, we add 20 knots, giving a TAS of 469 knots. Step 3: Aircraft at "A" is 51,200 kg. Enter the table with that mass and extract the cruise NAM — 3,279 nautical air miles. Step 4: Aircraft at "B" is 48,500 kg. Enter the table and extract the cruise NAM — 2,788 nautical air miles. Step 5: Subtract to find the NAM flown between "A" and "B". 3,279 minus 2,788 equals 491 NAM. Step 6: Convert those 491 NAM into NGM — Nautical Ground Miles. The formula this time is the reverse: NGM equals NAM multiplied by GS, divided by TAS. We have a 50-knot tailwind, so GS is TAS plus 50 — 469 plus 50 is 519 knots. So NGM equals 491 multiplied by 519, divided by 469, which gives 543 NGM. Step 7: Calculate the Specific Fuel Consumption — SFC. The book notes that strictly speaking, this is not the performance definition, but for this calculation you need to remember that SFC is the fuel required divided by the ground distance flown. Specific Air Range, by contrast, is the fuel required divided by the air distance flown. The fuel required is the difference in mass between "A" and "B" — 51,200 minus 48,500 equals 2,700 kg. The ground distance is 543 NGM. So SFC equals 2,700 divided by 543, which gives 4.97 kilograms per NGM, to two decimal places. Now the third example on the page. Aircraft mass at "A" is 55,500 kg. Outside Air Temperature is minus 59 degrees Celsius. Wind is light and variable. Cruising using Long Range Cruise — LRC — at Flight Level 310. The aircraft is to fly for 35 minutes. We need the fuel consumed from "A". Step 1: Find the correct page — page 29. Step 2: Correct the OAT of minus 59 into an ISA deviation. ISA at that altitude is minus 47, so minus 59 is ISA minus 12 degrees. Step 3: TAS from the table is 437 knots, then correct for the ISA deviation — minus 12 gives 425 knots. Because the wind is light and variable, we assume TAS and GS are both 425 knots, and NAM and NGM will also be equal. Step 4: How far can you fly in 35 minutes at 425 knots? 35 minutes is 35 sixtieths of an hour. 425 multiplied by 35 over 60 gives 248 nautical miles. Since NAM equals NGM here, that is 248 NAM. Step 5: Aircraft at "A" is 55,500 kg. Enter the table with that mass and extract the cruise NAM — 4,047 nautical air miles. Step 6: Subtract the 248 NAM flown from the starting NAM. 4,047 minus 248 gives a cruise NAM at point "x" of 3,799. Step 7: Enter the table looking for a distance of 3,799 NAM. Again, you won't find it exactly, but 3,800 is close enough. That corresponds to an aircraft mass of 54,100 kg, 35 minutes after "A". Step 8: The difference between 55,500 kg and 54,100 kg is 1,400 kg — that is the fuel required. Strictly speaking, you should then decrease the fuel required by 0.72%. That is 10 kg, giving a total of 1,390 kg. So the fuel required from "A" for that 35-minute LRC segment is 1,390 kg.