Why do planes fly in arc-shaped patterns, instead of just a straight line to the destination?

Question

If as you were looking at a 2-d map (Bird's eye)

Answer 1

The Earth is a sphere. It's not as far to go around a sphere near the top and bottom as it is in the middle. So if you were to fly around one, you might want to take the shortest route and fly in an arc to help you do that.

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How is iit that Thomas Bs answer, 9 spots below mine, says pretty much the same thing I do yet I have 0 thumbs up and 4 down, while he has 4 up and 1 down? I'm not trying to knock his answer, I just don't know what people are reading when they are "thunbing."

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Also everyone who says that the result is from taking a 3D globe and flattening it to a 2d map is the reason for the arc, please read my answer, Thomas Bs answer and John Bs answer (3-4 below Thomas B). The arc isn't the result of flattening a map. This is actually the way flights are routed. It is not our opinion, it is the reason they do this.

Answer 2

I think what you're talking about here is great circle navigation.

A great circle is a circle on the surface of a sphere that has the same circumference as the sphere, dividing the sphere into two equal hemispheres. Equivalently, a great circle on a sphere is a circle on the sphere's surface whose center is the same as the center of the sphere. A great circle is the intersection of a sphere with a plane going through its center. A great circle is the largest circle that can be drawn on a given sphere.

Great circles serve as the analog of "straight lines" in spherical geometry. See also spherical trigonometry and geodesic.

The great circle on the spherical surface is the path with the smallest curvature, and hence an arc (an orthodrome) is the shortest path between two points on the surface. The distance between any two points on a sphere is known as the great-circle distance. While this is the shortest distance between two points, if one was to travel along one it would be hard to manually steer as the heading is constantly changing (except in the case of due north or south or along the equator). Thus Great Circle routes are often broken into a series of shorter Rhumb lines which allow the use of constant headings between waypoints along the Great Circle.

When long distance aviation or nautical routes are drawn on a flat map (for instance, the Mercator projection), they often look curved. This is because they lie on great circles. A route that would look like a straight line on the map would actually be longer.

(And no, I can't take credit for the answer, Wiki which I copied it from explains it here better than I can).

Answer 3

The maps you see with the arc shaped lines showing the flights, I think I have seen those too, are to show the flight over a round earth. The maps are not flat maps but a flat map of a globe. The flight paths are straight but the arc you see is the aircraft following the curve of the earth.

Answer 4

Planes appear to fly in an arc-shaped pattern on a flat map, because a flat map does not really look like the round earth. Planes fly in what is called a great circle route. If you projected this great circle route on a globe, it would be a straight line which curved around the earth.

Answer 5

The planes do, in general, fly in a straight line. When the flight path is projected onto a flat map (2D) the path looks to be curved. However, the actual flight path is straight to the destination. (within the bounds of air traffic limitations).

Answer 6

It's called a "great circle route". You'll need a globe to really see it to make sense. For instance, San Francisco and Paris, France are at about the same latitude so you'd think that all you have to do is fly west to east and that would be the shortest distance. But if you grab a string and stretch it from San Francisco to Paris, you will find that instead of flying over New York which is also east of SF, you will be heading up over Idaho into Canada and over Greenland and finally into Ireland and Europe. You'll be headed north east right after departure from SF and you'll be headed south east as you begin to see the Eiffel Tower. Try it and you'll see what I mean.

Answer 7

it comes from the distortion of the map makers. If you look at a polar coordinat map, planes fly in straight lines on them.

Another way to think of this is that the earth is a circle. We put a cylinder around it and then project outward to the cylinder. This causes a loss of curvature and adds distortion to the top and bottom of the map. This is why it looks like planes fly in arcs rather than straight lines.

Look up mercador projection and polar projection on wikipedia for a good explanation.

Answer 8

Agreeing with Cherokeeflyer, Jack C has made it perfect. I'll just add that all these confusion arises because of the map projection. The popular Mercator projection is notorious for that. For the person who said it is not about 3D to 2D mapping, check out the size of Greenland in the offending map, you can see it is almost the same size of South America, while in reality Greenland is 15 times smaller.

None of these confusions would arise if a gnomonic projection is used.

Answer 9

It seems like an arc because the WORLD-MAP is flat where as the GLOBE is a sphere, if you mark the paths on a globe, they'll look straight,

+ And Sometimes MAP-Printers give a cure to show that it is an Airplane path, i.e. the curve shows that the path is in the AIR...

Answer 10

A straight-in approach to a R/W is preferred to save fuel and to expedite traffic In a controlled area.
An A/C approaching from the opposite direction to the landing R/W will take up a pattern (Downwind, crosswind and final) in order to land, that way he/she will have the landing area in site at all times.