Great Circle Routes: Why Your Sim Flight Plan Curves on the Map
Open a transatlantic route in NG ROUTE — say New York to London — and the magenta line bends north over Greenland. New pilots sometimes assume the planner made a mistake. It did not. The curve is a great circle, the shortest path on a sphere, shown on a flat web map that cannot represent the globe without distortion.
Flat maps lie gently
NG ROUTE uses a standard web Mercator projection, the same family as most online maps. Mercator preserves angles locally but stretches area toward the poles. A rhumb line — constant heading — appears straight. A great circle appears curved except along the equator.
Airliners prefer great circles (with small airspace tweaks) because less distance means less fuel and time. Your simulator FMC also flies great-circle legs between fixes even when the map overlay looks indirect.
Northern hemisphere example
From KJFK to EGLL, the true shortest track arcs north toward higher latitudes where the circumference of the Earth is smaller. Flying a visually “straight” eastbound line on the map would require constantly changing true heading and would cover more ground.
Track the heading readout in the sim during cruise: it drifts as you follow the great circle. That drift is normal.
Southern hemisphere mirror
Sydney to Johannesburg curves the other way — south toward the Indian Ocean — for the same mathematical reason. The NG ROUTE distance figure along airways already accounts for the network path; the map curve is a visualization artifact, not extra mileage invented by the tool.
When airways override the pure great circle
Published airways do not always follow a perfect great circle. Organized tracks over the Atlantic (NAT) shift daily in the real world; static navdata may route you via fixed fixes that add a few nautical miles for separation or radar coverage. That is still more realistic than a single DCT line drawn by hand.
Practical briefing tip
Trust the DIST field and nav log leg distances over your eyeball measurement on the map. For session planning, compare ETA before and after changing cruise level — wind matters more than map curvature for block time.
Try it yourself
Generate KJFK–EGLL and YSSY–FAOR in NG ROUTE. Zoom the map slowly and watch how the path hugs latitudes. Then load the plan in the sim and observe heading changes in the MCP. Connecting map theory to cockpit behavior is one of the fastest ways to build IFR intuition.