Why do we fly across the globe latitudinally (horizontally) instead of longitudinally?

Question

For example, if I were in Tangier, Morocco, and wanted to fly to Whangarei, New Zealand (the antipode on the globe) – wouldn’t it be about the same time to go up instead of across?

ETA: Thanks so much for the detailed explanations!

For those who are wondering why I picked Tangier/Whangarei, it was just a hypothetical! The-Minmus-Derp explained it perfectly: *Whangarei and Tangier airports are antipodes to the point that the runways OVERLAP in that way – if you stand on the right part if the Tangier runway, you are exactly opposite a part of the Whangarei runway, making it the farthest possible flight.*

In: 2270

Answer 1

You are correct that *in theory*, if you had exactly antipodal points, and you could fly in any direction from one to the other, then because of Earth being an oblate spheroid (longer circumference along the equator than across the poles) it would be shorter to fly straight north or south.

In practice, there are regulations for where aircraft can fly, for safety reasons. In addition to the political reality that if a flight overflies X country, the airline must pay that country an overflight fee (covers the use of air traffic control, plus the possibility that the country may be used for an emergency landing). Some countries charge excessive overflight fees in an attempt to discourage overflight.

The main regulations are known as “ETOPS”, which technically apply only to 2-engine aircraft, but the principles apply to aircraft in general. During flight you never want to be so far from available airports, so that in case of emergency you can go to those airports and land. And in polar areas, there usually aren’t many available airports for emergency landings.

But probably the biggest reason is the jet stream. The jet stream, which essentially is wind at an altitude that aircraft fly at, make it practically shorter to fly if you can follow it (since it gives the aircraft a tail wind). Jet streams go from west to east. (This is why you see eastbound flights try to follow the jet stream, while westbound flights try to avoid it.)