Why flight paths aren't straight lines

HostWhenever I'm on a long flight, I end up staring at that little map on the screen in the back of the seat. It always strikes me as weird that the plane takes this big, looping path over the ocean instead of just cutting across in a direct line. It feels like we're taking the long way around for no reason. What's actually going on when a pilot or a computer picks the path for the day?

GuestIt's one of those things that looks wrong only because of the way we look at maps. Most of the maps we see in books or on our phones are flat. But the world is a ball. If you take a piece of string and stretch it between two points on a globe, you'll see the shortest path isn't a flat line. It's a curve that bows toward the north or south pole. We call these great circles. To our eyes, looking at a flat map, that curve looks like a detour, but in the three-dimensional world, it's the most direct shot possible. If a pilot flew what looked like a straight line on a flat map, they would actually be burning way more fuel and taking a much longer route.

HostThat makes sense for the shape of the earth, but I have noticed that even on the same route, the path changes. I flew to London once and we went way up over Greenland, but on the way back, we stayed much further south. If the shortest path is that string on a globe, why would it ever change?

GuestWell, the earth might stay the same shape, but the air is always moving. Think of it like a boat in a river. If you're rowing with the current, you go much faster with less work. Up where planes fly, there are these massive rivers of fast-moving air called jet streams. They can blow at two hundred miles an hour. If a flight planner sees a strong jet stream heading east, they'll tell the pilot to hop right into the middle of it. Even if that path is hundreds of miles longer in terms of distance, the tailwind pushes the plane so fast that you arrive earlier and use less gas. Coming back the other way, you do the opposite. You might fly a huge loop just to stay out of that wind so you aren't fighting a wall of air the whole way home.

HostSo it's about the wind, but the sky is huge. Is it really just a free-for-all where pilots hunt for the best breeze, or are there actual rules about where they have to stay?

GuestThere are very strict rules. One big one is about safety, especially when you only have two engines, which most modern planes do. There's a set of rules with a clunky name, but it basically means the plane has to stay within a certain distance of an airport where it could land if an engine failed. You can't just fly over the loneliest part of the ocean if there's nowhere to go in an emergency. The path has to stay close enough to land so that if one engine quits, the plane can coast to a runway on the other one.

HostI honestly thought we were past that. I mean, engines are so reliable now. Do we really still have to hug the coastlines just in case?

GuestWe do, though the rules have loosened up as engines have gotten better. It used to be that you had to stay within sixty minutes of a place to land. Now, some planes are allowed to be three or four hours away from the nearest airport. But that still shapes the path. You also have to think about the land below. There are some parts of the world, like the Himalayas, where planes try not to fly. The mountains are so high that if the cabin lost air pressure and the pilot had to drop to a lower height where people could breathe, they might hit a peak. Plus, you have to deal with which countries let you use their air. Some places charge a lot of money to fly over them, and others might be off-limits because of a war.

HostThat sounds like a lot of traffic jam potential. If everyone is trying to hit the same jet stream or avoid the same mountain, how do you keep them from bumping into each other?

GuestIt's much more organized than people think. The sky actually has invisible highways. Over the Atlantic, where there's no radar to see every plane, they use a system of tracks. Every morning and every evening, air traffic controllers look at the weather and the wind and they lay out a series of lanes. They tell the airlines, okay, here are the five paths for today. All the planes heading to Europe have to stay in these narrow lanes, spaced out by time and height. It's like a giant conveyor belt in the sky. If you want to go a different way to save fuel, you might be out of luck if that lane is already full.

HostIt's funny to think of the sky being crowded when it looks so empty from the ground.

GuestIt really is. The big goal for the future is to move away from those fixed lanes. As satellite tracking gets better, we might be able to let every plane pick its own perfect path based on the wind of the second, rather than sticking to a highway that was drawn on a map that morning.

HostThose glowing lines on the seat-back map aren't just a mistake in the drawing, they're the result of a massive game of connect-the-dots played with wind, math, and safety rules.