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 288 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.