Plotting — Page 478, Lesson 474

We're starting a brand-new topic now: Plotting. This is where we take all the navigation theory and actually put pencil to chart. Let me set the scene for you first, because it explains why we're doing this at all. So far you've seen two extremes of navigation. On one end, solo-pilot VFR flying using the 1 in 60 rule — that's the mental-math, rule-of-thumb world. On the other end, you have the fully automatic systems: INS, IRS, FMS, GPS, and RNAV — the glass cockpit jet airliner world. But there's a whole middle ground. There's a large group of aircraft that are more sophisticated than a light training aircraft, yet less sophisticated than a full jet airliner glass cockpit. These intermediate aircraft are routinely fitted with VOR, DME, ADF, and a Slaved Gyro Compass. And for those aircraft, you need to plot. Now, what do you plot on? You plot on a Radio Navigation chart, or a specialist plotting chart. Usually that's a Mercator's chart or a Lambert's Conical Orthomorphic chart. But here's the key point for your EASA ATPL: the requirement is specifically for the Lambert's Conical Orthomorphic, and occasionally the Polar Stereographic. So the techniques I'm going to teach you are for those two charts — not for Mercator's. The notes deliberately leave Mercator's out. And the actual charts you'll use for most of the exam questions are the Jeppesen E(LO)1 and E(LO)2, plus the E(LO)1A. One very practical point: if you're sitting the EASA exams, you must bring your own copy of each chart. The CAA will not provide them at the exam venue. So that's a checklist item for you. Let me now run through the equipment you need for this section of the course. You'll need a navigational computer — that's your flight computer, the circular slide rule type. You'll need a navigational straight-edge, for drawing clean lines. A plotting protractor — and note it's the square type specifically. Compass and dividers, for measuring distances and transferring positions. And for the actual marking: 2H and HB pencils — 2H for plotting, HB for writing — plus a soft eraser. So that's your kit: navigational computer, straight-edge, square plotting protractor, compass/dividers, 2H and HB pencils, and a soft eraser. Now we move into the definitions, abbreviations, and symbols. And the very first question is fundamental: True or Magnetic? Plotting is normally carried out using true north as the datum. Now, it is possible to plot some types of radio bearing directly in magnetic, without converting to true. But the usual practice is that true tracks and bearings are used. So keep that in mind — true is the default datum for plotting. Next, the definition of Heading. Heading is the direction in which the nose of the aircraft is pointing, measured in degrees from 000 to 360, clockwise. And it's measured from one of three norths: true, magnetic, or compass north. The abbreviations are Hdg(T), Hdg(M), and Hdg(C) — so Hdg(T) is heading relative to true north, Hdg(M) relative to magnetic north, and Hdg(C) relative to compass north. Now here's the conversion logic, and this is important. If you're given Hdg(M) — a magnetic heading — you must convert it to true by taking into account the variation on the chart. Variation is the angular difference between true north and magnetic north at your position. If you're given Hdg(C) — a compass heading — you must convert it to true by taking into account both variation and deviation. Deviation is the error introduced by the aircraft's own magnetic fields, the compass installation error. So remember the chain: compass heading needs deviation and variation to get to true; magnetic heading only needs variation. Then we have Fix. A fix is the ground position of the aircraft, obtained from two or three visual or radio position lines, or by radar. So it's your confirmed position on the ground, established by intersecting position lines. And finally, Position Line. A position line is a line somewhere along which the aircraft is known to be at a particular time. So you don't know exactly where on that line you are — you just know you're on it at that moment. When you cross two of them, you get a fix. Let me show you what this looks like in practice. That figure is an example of the plotting you'll be doing. So the core idea to hold onto: you're working in true north as your datum, you're converting headings through variation and deviation as needed, and you're building up fixes from position lines. That's the foundation we'll build on as we go deeper into the plotting techniques.