Wednesday, 27 August 2008

PPL Ex 18 (iv): NAVIGATION: (Measuring the Track Page 2)

Remember: Click to enlarge!

Having measured the track angle it is now necessary to measure the track distance. The 1:500,000 ICAO Aeronautical Chart is a Lambert Conformal Conic Mercator Chart. The lines of Latitude are at ½ ˚ (30nm) intervals. Each 1˚ is equal to 60nm. Do not measure your track distance along the lines of Latitude. The lines of Longitude are also at ½˚ intervals. The distance must be measured along a line of Longitude (i.e. vertically). E.g. even if your track line is (say) 090˚(T), the distance of the track line must be measured using a North/South graticule line. If you observe these lines closely it will be apparent that each 1 degree of latitude is 60nm with small marks to one side (the west side) of the line at each 1nm, with slightly larger marks at each 5nm and a cross line at each 10nm. Of course, provided that you use a ruler of the correct scale you can also measure the distance in this manner. In the illustration you may notice that I have attached a small dayglo arrow on the ruler pointing to the 1:500,000 scale (as this is the scale of chart that I always use), and a dayglo arrow at the North (360˚) mark on my Douglas Protractor. These are time-saving devices! Having measured the Tracks (˚T) and Distances (nm) we now write them down on the Navigation Log for each ‘leg’ of the route. On inspecting a typical Nav Log you will notice that there can be 14 or more columns! Don’t worry – the whole process of figuring out your navigation plan is quite logical. Before proceeding any further, however, I believe that it would be a good idea to explain TRUE AIR SPEED or TAS, because TAS features early in these columns. Ignore the blue writing if you already know all this stuff!

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