Topographical Chart — Page 132, Lesson 136

I want to walk you through the topographical chart and how we establish minimum flight altitudes. Let's start with the miscellaneous information printed on the chart itself. First, you'll find Flight Information and Meteorological Services — these are the frequencies for ATIS, which is the Automatic Terminal Information Service; Flight Information Service; and Weather Information. These are provided within various Flight Information Regions, or FIRs, at nominated centres. You'll find this data in the inset chart at the bottom left of the main chart. Next, General Aviation Forecast Areas are shown in an adjacent chart, and the numbers you see there refer to telephone numbers for obtaining those forecasts. Note the Airspace Classification Germany diagram and table. Only classes C, D, E, F, and G are used in Germany — so no class A or B airspace exists there. Also note the Phonetic Alphabet and Morse Code table, and the Semi-circular Cruising Levels on VFR Flights — including a separate set for France. For VFR Routes Within France, bearings and tracks are given as magnetic, and distances are in nautical miles. Finally, note the table of Airspace Designators and Control Frequencies. Now, the excerpt gives several example questions — Examples 3 through 8 — asking you to decode airfield information, symbols, navaid data, and airspace dimensions. The answers are on page 141, but we won't go through those now; the key point is that the chart contains all this information in its margins and insets. Let's move to the core of this section: Establishment of Minimum Flight Altitudes, referenced to EU-OPS 1.250. When you select a flight altitude or flight level that gives adequate clearance for a given sector, you must consider eight factors. First, the accuracy with which an aircraft can determine its position — if your navigation is less precise, you need more buffer. Second, the inherent inaccuracies of altimeters and their indications, plus the corrections required to account for temperature and pressure variations relative to ISA, the International Standard Atmosphere. Third, the characteristics of the terrain — is it flat, mountainous, or rugged? Fourth, rotor turbulence and standing waves — these are wind effects near mountains that can affect your actual height above the ground. Fifth, the accuracy of the navigational chart itself — no chart is perfect. Sixth, the vertical extensions of Danger, Restricted, and Prohibited areas — you must avoid them if they cannot be overflown, so you need to know their top and bottom altitudes. Seventh, the vertical extensions of the types of airspace — different classes have different floor and ceiling limits. Eighth, the highest ground or obstacle within the promulgated distances either side of the planned track. And ninth, the ICAO Standard Semi-circular Cruising Levels, which you can see in Figure 9.1. Now, the Minimum Grid Area Altitudes, or Grid MORA, are printed on the chart. You've already been introduced to these earlier. They can be used in three ways: as a rapid means of assessing the appropriate flight level or altitude; as a means of cross-checking terrain clearance values that you've obtained using the stated methods; and as a rapid means of re-assessing safe clearance of terrain — for example, when a pilot becomes unsure of their exact position in relation to their intended track. As a student, you will be required to find the highest obstacle within a given distance either side of track. Normally that distance is 5 nautical miles either side of track. So you take your planned route, look 5 NM to the left and 5 NM to the right, find the highest obstacle in that corridor, and that becomes your reference for minimum altitude. Let me show you the semi-circular cruising levels and how they relate to terrain clearance. This figure shows the standard semi-circular cruising levels and the bottom of the chart layout. Now, when the QNH is higher than the standard 1013.25 hPa, you get less height gained above the terrain — we'll see that in the next figure. And when the QNH is lower than 1013.25 hPa, you get more height gained. The temperature difference from ISA at the cruising pressure level also affects your true altitude. So in summary: the topographical chart gives you all the reference data — frequencies, airspace classes, VFR routes, and the Grid MORA values. When establishing a minimum flight altitude, you consider position accuracy, altimeter errors, terrain characteristics, turbulence, chart accuracy, airspace and danger area vertical limits, the highest obstacle within 5 NM either side of track, and the semi-circular cruising levels. The Grid MORA gives you a quick check and a cross-reference for your own calculations.