# Naked Science Forum

## General Science => General Science => Topic started by: Mariana on 12/02/2019 09:14:53

Title: What determines the directions of flights?
Post by: Mariana on 12/02/2019 09:14:53
Palesa wants to know:

What determines the directions of flights?

If the earth is round, why is it that flights don't always travel the shortest route? For instance, why does a flight from New York City travel eastwards and not westwards to China for faster travel time?

Do you know the answer?
Title: Re: What determines the directions of flights?
Post by: evan_au on 12/02/2019 09:58:06
Quote from: Palesa
What determines the directions of flights?
The quickest, cheapest and least-fuel flight will normally take the shortest path.

The shortest path on a sphere is called the "great circle" path. It doesn't match the straight-line path on a flat map - for example, the shortest path between two points in the Northern hemisphere is often over the North Pole, a path that looks "impossible" on a flat map. This is the case for New York (JFK) to Beijing (PEK) - the shortest path is almost exactly over the North Pole.

JFK-PEK_Great_Circle.PNG (82.36 kB . 577x547 - viewed 604 times)
See: http://www.gcmap.com/
For example: http://www.gcmap.com/mapui?P=PEK-JFK

There are reasons that an airline might deviate from this path, for example:
- To take advantage of favorable winds
- So they are never too far from an airport, in case of an emergency
- To avoid nations that don't want you flying into their airspace (at all) or just in particular areas (eg military bases)
- To avoid warzones

But they would prefer a path close to the great-circle path.
Title: Re: What determines the directions of flights?
Post by: alancalverd on 12/02/2019 15:03:38
All true, plus, if you bend the flight path slightly eastwards you can get a 200 knot tailwind from JFK to PEK.

Additionally, magnetic north is about 10° west of the US east coast, so to fly true north from New York you actually fly about 10° east on your compass. All flight directions are given as "magnetic" so having reached cruising altitude you will be initially instructed to "fly QDR 010" to stay in your airway.