Airways - Miscellaneous Charts — Page 196, Lesson 205

I want to walk you through the Atlantic Orientation Charts, specifically the AT(H/L) 1 and 2 charts. These are specialised charts used for route planning and en route navigation between major transatlantic aerodromes — so think of them as your primary navigation tool when you're crossing the North Atlantic. Both charts are drawn on a Lambert Conformal Conic Projection. That's a type of map projection that preserves shape over small areas and is commonly used for aeronautical charts because straight lines drawn on it approximate great circle routes well. The scale of the AT(H/L)1 chart is 1 inch equals 132 nautical miles, and the AT(H/L)2 chart is 1 inch equals 136 nautical miles. Notice the slight difference — they're very close, but not identical. Now, how do you measure distance on these charts? You have three methods available. First, you can extract the values that are printed parallel to the published tracks and/or sectors of those tracks — so the chart itself gives you distances along the established routes. Second, you can use the nautical mile scale printed at the left and right-hand margins of the chart. Third, you can use the adjacent meridian scale — that's the scale along the nearest line of longitude, which on a Lambert Conformal projection is a good reference for distance. Let's look at the information these charts carry. The front panel shows several things. It shows the coverage of the chart pictorially — so you can see at a glance which area the chart covers. It also includes a NAVAID LEGEND, which explains the symbols for navigation aids. There's a selection of VOLMET broadcasts in radio-telephony, or RTF — that's the voice weather broadcasts for aviation. Navaid information, listed alphabetically, is provided at Panel 1 on the AT(H/L)1 chart and at Panel 8 on the AT(H/L)2 chart. The charts also carry critical operational information. You'll find NORTH ATLANTIC & CANADA MINIMUM NAVIGATION PERFORMANCE SPECIFICATION — that's MNPS — which sets the navigation accuracy requirements for operating in that airspace. Then there's the NAT ORGANIZED TRACK SYSTEM, abbreviated NAT OTS — these are the daily published tracks across the North Atlantic. NORTH ATLANTIC COMMUNICATION information is there, and COMMON PROCEDURES FOR RADIO COMMUNICATION FAILURES. All of that information is located at panels 7 and 8 on the AT(H/L)1 chart. Additionally, NORTH ATLANTIC CROSSING CLEARANCE PROCEDURES & FREQUENCIES are published at panel 9 on the AT(H/L)1 chart. And finally, you'll find POSITION REPORTING PROCEDURES, STANDARD AIR-GROUND MESSAGE TYPES & FORMATS, INCREASED WEATHER REPORTING, and SPECIAL PROCEDURES FOR IN-FLIGHT CONTINGENCIES IN MNPS/RSVM AIRSPACE — that's Reduced Vertical Separation Minimum airspace. All of these are critical for operating safely and in compliance with procedures when crossing the North Atlantic.