Airways — Page 158, Lesson 158

I want to walk you through the information on airways charts and the various legends you’ll need to interpret. Let’s start with how we plot locations on these charts. The position of airfields, navaids — that’s navigation aids — and reporting points, along with other features, can be readily plotted by referencing the latitude and longitude grid printed on the chart. That grid is your fundamental coordinate system for fixing any point. Now, isogonals — these are lines of equal magnetic variation. On some charts they’re indicated only at the edge, but on others they’re extended fully across the chart as a continuous dashed line. You’ll need to note those for magnetic heading corrections. Shorelines and large inland lakes are shown as well. You should also pay attention to the border and miscellaneous information printed on the chart — things like airspace boundaries, special use airspace, and other operational notes. Let’s talk about Grid Minimum Off-route Altitude, abbreviated as Grid MORA. This is a critical safety value. A Grid MORA clears all terrain and man-made structures by 1000 feet in areas where the highest elevation is 5000 feet AMSL — that’s Above Mean Sea Level — or lower. In areas where the highest elevation is 5001 feet AMSL or higher, the Grid MORA clears all terrain and man-made structures by 2000 feet. So the clearance margin increases once the terrain itself is higher. If a Grid MORA is shown as “Unsurveyed,” that means the information is incomplete or insufficient — you cannot rely on that grid square for terrain clearance. If a Grid MORA value is followed by a plus/minus sign, that denotes doubtful accuracy, but it is believed to still provide sufficient reference point clearance. You treat it with caution but it’s not unusable. Now we move to the High Altitude Chart Legend. The symbols in this legend are additional to the ones from the preceding legends — they apply specifically to High Altitude Charts, but all the earlier legend symbols also still apply. Next we have Area Charts. This legend is applicable to Area Charts and is additional to the legends that came before it. You must familiarize yourself with the Area Chart symbols because questions will be posed on them in the examination. Pay particular attention to the symbols for: departure and arrival routes, the Speed Limit Point, man-made structures, and the depiction of communication frequencies. Now, Area Charts may also include Generalized Terrain Contours. Terrain information may be depicted on an area chart when the terrain within the chart’s coverage rises more than 4000 feet above the main airport — the example given is the Madrid Area Chart. But here’s a crucial point: the absence of terrain contours does not imply the absence of terrain or structures. Just because you don’t see a contour line doesn’t mean the ground is flat or clear. Furthermore, the contour information does not assure clearance above or around terrain or man-made structures. There may be higher uncharted terrain or man-made structures within the vicinity. Terrain information is useful for visual orientation and general visualization of the terrain — it helps you picture what’s down there — but it does not replace the minimum altitudes dictated by the airway and air route structure. You must always follow the published minimum altitudes, not just the terrain contours. Finally, we have the JM — SID and STAR Legend. SID stands for Standard Instrument Departure, and STAR stands for Standard Terminal Arrival Route. Since the examination will include questions on selected SID and STAR charts, you must have a thorough knowledge of this legend. These charts provide a graphical illustration — note, not to scale — of the governing authority’s prescribed procedures. So the drawing is schematic, not a precise map, but it shows you the exact routing, altitudes, and restrictions you are expected to fly.