Why do Planes Fly in an Arc?

When looking at the flight path of an aircraft between two points on earth, one will quickly realize that the path that the aircraft takes to its destination is never straight, rather being an arc across the globe. While one may think that this is an inefficient choice that will lead to higher costs and lengthier flight times, there is actually a very good reason as to why this is done. To put it simply: aircraft fly across the globe in an arc due to the curvature of our round planet.

When looking at a basic 2D map, if one were to fly from Madrid, Spain to New York City, United States, the path between them would be almost completely straight using the 40 degree latitude line that nearly connects the two locations. Nevertheless, the actual flight path would follow a slight arc where the plane “moves upward” on the 2D map before starting to “drop downward” on the map toward New York City. With a spherical Earth, calculating the shortest distance between two areas can be more complex, as what seemed like a straight shot across a 2D map would actually result in a longer flight time than executing an arc.

Modern flight paths are often conducted with the use of great circles, those of which are the largest possible circles that one can draw around the planet. For Earth, the largest possible circle is around the equator where the planet is the widest. This same principle can be used to draw distances around any area, and finding the shortest distance between Madrid and New York city results in an angled arc that deviates from the standard longitudinal and latitudinal lines we are used to. By using this standard practice, operators can create efficient flight paths that save fuel, time, and money during each operation.

While this can now seem cut and dry, it is also important to know that the shortest distance between two points is not always the best choice. No matter the starting point and end destination, flight planners will have to account for all air traffic, weather, airport proxities, and other factors that may exist between both points so that safety can be upheld. This means that a longer path may be taken in certain instances to keep passengers safe. For example, there may be a very short path between two areas, but a massive body of water sits between them. To account for the possibility of needing to conduct an emergency landing, the flight plan may be adjusted so that the aircraft remains closer to emergency landing spots.

Another major reason as to why the shortest distance is not always possible is because of the aforementioned air traffic. To abide by FAA rulings, only 5,400 aircraft are allowed to be in the sky at peak operational times, and air space is divided into corridors for safety. With the help of air traffic controllers, these corridors are monitored with clearance being given to pilots so that they know when to enter air space to avoid other aircraft.

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