I enjoy messing around with the flight radar app, and I saw something on there that I though was odd. I live in the SF east Bay Area, and the flight paths over my house include those flights from LAX to China, South Korea, Japan, etc. Last night I noticed a flight from LAX to Singapore. The app shows the plane’s current track, as well as its future track. The track on this particular flight (which was heading north - northwest over my house) showed a large arc, over the North Pacific, almost to the Aleutian Islands, turning south, hugging the east coast of Japan, finally ending in Singapore. My question, why the huge arc, and why head so far north, when Singapore is so far south? I know there will be some “curve” of the flight track, just due to the earth being a sphere, but this track seemed so strange.

If you take the factor of the earth being a ball, and then understand the track in that perspective, you see it is the shortest/safest route (taking into account alternate/emergency airfields). And to bear in mind, "unfriendly skies".

Never forget KAL 007.

Works the same way when laying out a track for a ship...it’s called a “great arc” or “great circle” When we laid a plot to go from Norfolk to Gibraltar...it looks like a straight line because we use flat paper. In reality the earth is round and when the track is viewed on a globe it is an arc.

https://en.wikipedia.org/wiki/Great-circle_distance

To illustrate, if you happen to have a globe handy, take a piece of string and pull it tight from one location to the other. Then take possible emergency landings and unfriendly airspace into account as monkey said.

-Rob

As posted, the ‘great circle route’ is a primary factor. A few other biggies are current winds & ‘over-flight’ costs. Most countries have costs associated with flying over.

https://simpleflying.com/overflight-fees/

Overflights fees can vary widely, they go into the mix when dispatch plans an efficient flight. Weather or turbulence may contribute, besides just winds.

Even a shorter flight, Wash Dulles to Seattle will vary most days. Besides the route, the ‘cost index’(planned speed factors) will fluctuate day to day. Since weight & winds/temp isn’t the same, altitude & ‘step climbs’ are planned for various points.

It's a great circle routing, but the routing also takes into account winds aloft. A flight west bound goes against winds, so routing will be adjusted to avoid the strongest winds and areas of turbulence. The flight is also adjusted to allow for diversion in the event of an emergency.

Overwater flights use "equal time points," or ETP's, that are points along the route of flight that allow for a diversion in the event of a medical emergency, depressurization, or engine failure (or other emergnecy). On a long trip from SFO or LAX, there are often two sets of diversion points, and these may dictate where the aircraft flies, to allow for a reduced diversion time, given the fuel available for the flight.

ETP diversion fuel legal requirements are slim: one can arrive at the diversion field and have only enough fuel to hold for fifteen minutes. That's not much at all. Given that ETP diversion points require the flight to turn around and go back to one airfield, or press on to another, it's a balance against what's required for the trip as well as what may be required to depressurize and descent to a lower altitude, and still make it to an emergency alternate airfield, with the necessary legal reserves.

Additionally, overwater flights require an extra 10% fuel reserve above those normally carried, for the overwater portions of the flight. To carry this extra fuel requires a higher fuel burn: typically about 4% of the weight of the extra fuel is burned to carry the extra fuel, which amounts to a potential significant extra cost, loss of revenue weight for passengers and/or cargo," and a performance penalty, as well. The heavier weight also limits climb altitude, which affects the ability to be efficient in cruise.

Each of those factor is applied to the routing, and trips to Asia involve an arc that takes the flight up toward alaska, then toward Japan, arcing back down past korea, toward Hong Kong or Singapore, etc. Following this path offers multiple alternates at oceanic points, Alaska, Japan, etc.