If you were to pull up the route from Air New Zealand’s latest flight from Auckland to New York, you may notice something peculiar; NZ2 did not fly in a straight line.

Those familiar with aviation or geography may already know this is because planes typically fly the “great circle” route, which is the shortest distance between two points on the surface of a sphere.

Since Earth is three-dimensional, a direct route looks curved when plotted on a two-dimensional map.

However, look closer at NZ2′s flight route and you’ll see it’s not just curved but curved twice into an “S” shape. Compare it to several days of flights and the degree of “curve” in the “S” varies considerably.

So, what’s going on? Is it an issue with the auto-pilot? Did the pilots accidentally veer off course or do powerful winds nudge the plane from its plotted route?

According to Air New Zealand’s chief pilot Captain David Morgan, it’s none of the above, although it is related to the wind.

Firstly, the variation across different days is a result of pilots making the most of advantageous winds. Since jet streams and winds change and move across the Earth, the route can also change in order to avoid turbulence or align with a jetstream.

As for the “S” shape, Morgan uses an analogy to explain.

“If I’m in a boat rowing upstream at 5 knots but the river is going downstream at 5 knots, I’m not going anywhere. To go upstream at 5 knots, I’ve got to row at 10 knots.”

It’s a similar concept when it comes to a plane but, instead of water flowing downstream, it’s air flowing along the earth’s latitudinal lines.

“There is flow of air from west to east, in these latitudes,” Morgan said. “What you don’t want to do is fly long distances into the wind.”

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When a plane flies into the wind, it requires more power (and thus, expensive fuel), to maintain a certain speed.

By flying in an “S” shape, the plane can avoid a certain amount of resistance from the air while still making an efficient route.

“If we want to fly from New Zealand to the US, knowing there’s this airflow coming across latitudes, what we tend to do is go across, up and then over,” he said, describing the “S” shape one can see.

The flow of air along the earth’s latitudinal lines is also why flying to a destination can take more or less time than flying back.

“If I’m flying from New Zealand to Australia, I’m heading into a headwind and it’s going to take longer,” Morgan explained, as the plane will fly from east to west, opposite to the airflow.

“That’s why it takes three hours to get to Australia and only two hours and 40 minutes to get back.”

It’s also why NZ1 from New York to Auckland can take up to an hour longer than NZ2 flying in the opposite direction.