192°(T) + 7E Var — Page 565, Lesson 566

I want to walk you through a set of worked navigation problems — these are the kind of calculations you'll be doing on the real thing, so let's go through each one properly. First, question 169, answer c. We have a True track of 192°(T), and a variation of 7° East. Remember the rule: Variation East, Magnetic Least. So we subtract the 7° East variation from the True track. 192 minus 7 gives us 185°(M)ag Track. That's the magnetic track, not the heading yet. Now question 170, answer b. We're still on that 185°(M)ag Track. We have 5° of left drift — that means the wind is pushing us to the left of where we're pointing. To compensate, we need to aim off 5° to the right. So we add 5° to the magnetic track: 185 plus 5 gives us a Magnetic heading of 190°(M). That's the heading we actually steer. Question 171, answer d. This one's about convergence. The datum meridian is Greenwich, and the aircraft is 115° West of that datum, in the Northern hemisphere. So the convergence is 115° East. Now the rule: Convergence East — True Least. That means when convergence is East, the True track is the smaller number — it's less than the Grid track. So we subtract: 344°(G)rid minus 115°(C)onvergence gives us 229°(T)rue. Question 172, answer a. This is a rhumb line distance problem between two points at the same latitude — that's what we call Departure. The formula is: Departure equals change of longitude in minutes, times cosine of the latitude. Here the change of longitude is 10°, so we multiply 10 by 60 to get minutes — that's 600 — then multiply by cosine of 60°, which is 0.5. 600 times 0.5 gives us 300 NM. Question 173, answer b. Don't forget: ISA at FL410 is still -56.5°C, even though you're higher than 36,090 feet pressure altitude. So if the SAT is -46.5°C, you use the CRP-5 computer to work it out. Question 174, answer b. We convert 22 US gallons per hour to imperial gallons on the CRP-5 — that comes out to about 18.3 imperial gallons per hour. Then use either the CRP-5 or your calculator to get the total endurance. Question 175, answer b. Here we have two separate problems. A and B are at the same latitude but different longitudes — so it's a departure problem again. Departure equals change of longitude in minutes times cosine latitude. That's 1 degree times 60, times cosine 60° — 60 times 0.5 gives us 30 NM. Then A and C are at different latitudes but the same longitude — that's a simple change of latitude problem. One degree of latitude is 60 NM. Question 176, answer b — use the CRP-5. And question 177, answer d: 135 times 1.75, divided by 60, equals 3.94. Now, let me make sure you've got the two big ideas locked in. First, the variation rule — Variation East, Magnetic Least — that's how you go from True to Magnetic. Second, the departure formula — change of longitude in minutes times cosine latitude — that's how you find the distance along a parallel of latitude. These two rules will keep coming back throughout your navigation work.