You're at 7,500 feet and you need to be down at 1,000 feet — Page 486, Lesson 487

Let's start with the descent calculation, because that's the concrete worked example that opens this part of the lesson. You're at 7,500 feet and you need to be down at 1,000 feet. That's a loss of 6,500 feet. Your rate of descent is 800 feet per minute. So 6,500 divided by 800 gives you 8.125 minutes in the descent. That's the time you'll be descending. Now, at 156 knots, in 8.125 minutes you'll cover 21.1 nautical miles. But you don't want to arrive at the DME at 1,000 feet — you need to be down at 1,000 feet 6 nautical miles before the DME. So you add that 6 NM to the 21.1 NM, giving you a range to start the descent of 27.1 NM from the DME. That's the answer. Now let's move into the chart work — Lambert and Polar Stereographic charts, and the effects of convergence. For all practical purposes, a straight line drawn on a Lambert Conical Orthomorphic chart — that's a conformal chart — or on a Polar Stereographic chart at high latitudes, can be considered to be a great circle. But the type of plotting you're concerned with deals with aircraft that fly rhumb line tracks. A rhumb line is a track that crosses all meridians at the same angle — it's a constant true track. So there's a distinction here: the straight line on the chart is a great circle, but you're flying rhumb lines. Now, measuring tracks. If an aircraft is to fly from A to B, you join the two positions with a straight line — that's the great circle — and you measure its direction using the mid-meridian. That gives you the rhumb line track on the 1:1,000,000 scale chart. So the key point: measure at the mid-meridian to get the rhumb line track. Plotting headings and tracks. If you have a known heading or track to plot, you do it using the nearest meridian to the position from which it has to be drawn. Any subsequent error is ignored — it's small enough to disregard. Measuring distances. On a chart where scale cannot be considered constant, you must measure distances using the latitude scale locally. That's because on these projections the scale varies with latitude. Plotting positions takes a little longer than on a Mercator chart, because the Lambert graticule is not rectangular. Here's the procedure. First, mark the longitude of the position to be plotted in the longitude scale to the north, and also on the one to the south of the position. Then lay a straight edge between those two points. Then, using a pair of dividers, plot the latitude along the length of the straight edge, upwards or downwards from the nearest parallel of latitude. Now, plotting ADF bearings. If the change of longitude — that's ch.long — is 2° or more, you draw a line parallel to the aircraft's meridian through the NDB, and plot as shown in Figure 28.4. If the change of longitude is less than 2°, you plot from the meridian of the NDB directly. That's explained in more detail in the next paragraph. So the key distinction there: the 2° change of longitude threshold determines whether you need to draw that parallel line through the NDB or whether you can plot straight from the NDB's meridian.