Transpacific Route Question

[DavidK99]

I’m a frequent, curious traveler with no affiliation to your industry. I hope it’s ok to ask this here.

I often fly between LAX and Southeast Asia, and enjoy watching the plane's route on my monitor. Yesterday as my wife and I were flying Eva BR15 LAX to TPE, we were studying the display closely. Today after landing we looked at a globe, and were puzzled that our route seem to fly several hundred miles north of what appears to be the shortest path. Doing web searches made things more puzzling.

The Flight Aware map of BR15’s route shows it passing just south of Kodiak Island and the Kamchatka Peninsula.

But when I stretch a piece of string between those cities on a globe, it looks like the shortest route is actually a few hundred miles south of there — well south of the Aleutian Islands, not north of them. When I entered LAX and TPE on the “Great Circle Mapper” site it showed a route similar to my string check.

Can someone explain why the actual flight path are so different from what seems to be the shortest route for this long flight? (Maybe the flight tracks are designed that way to maintain radar contact, or to allow emergency access to other airports en route? Maybe it has to do with the jet stream?)

In terms of flight time and fuel usage is the apparently longer route a benefit, a compromise, or does it not make much difference?

Thanks in advance.

[pax britanica]

As a fellow interested passenger, more Atlantic orientated, the answer is that route choice is very complicated. First, there is the great circle -shortest path, but that might not be fastest depending on winds. After that you have issues like ETOPS regulations, avoidance of airspace over some countries, military exercises, ATC charges which can vary by route. Most of these do not vary from day to day but the weather does vary and can make routes very different from one day to the next if the jet stream is in the 'wrong' place or there is known significant bad weather-ie turbulence/thunderstorms on route.

Thats a layman's view based on asking a few pilots back in the day when you could have those conversations in the air

[DaveReidUK]

Pacific Organised Track System (PACOTS)

[hamil]

Unless there is a specific route restriction -- weather, closed airspace, volcano activity, etc., the shortest distances in aviation operation are based on NAM (Nautical Air Miles). In other words, it's the total ground distance adjusted to wind component.

In winter in the northern hemisphere, flying westbound from USA to SE Asia requires a good flight planning in order to obtain the most efficient route, essentially avoiding strong headwinds, so this is possibly one of the reasons you flew a little bit out of track compared to your previous flights. Frequently the flight time difference between 2 airports -- for example LAX-NRT-LAX -- is over 3 hours (the jetstream in the Japanese airspace can reach speeds above 200 kt).

Many reasons can also be considered, but without some important operational details it's impossible to know what exactly happened that day.

[Dave Therhino]

The web site windy.com has a great graphical presentation of current and predicted winds at the surface and at various altitudes. The slider to designate the altitude you want displayed is on the right side of the page. Slide it up to flight level 350 and you will see what hamil is talking about above.

[Jn14:6]

May also be due to the volcanic eruption in the Aleutians?

[DavidK99]

Thanks for the replies...lots of good info. I'm going to check the links that were posted and do some other amateur digging, then report back.

I don't think this was a one-off because I've taken that flight about 20 times and the route always seems to be about the same. And I think the return flights follow roughly the same route, which makes me question whether winds are the factor. But I don't want to assume anything and an curious to learn more, so will look into this and post again later.

All of your responses are much appreciated.

[swh]

Originally Posted by DavidK99

Can someone explain why the actual flight path are so different from what seems to be the shortest route for this long flight? (Maybe the flight tracks are designed that way to maintain radar contact, or to allow emergency access to other airports en route? Maybe it has to do with the jet stream?)

It would be due to forecast winds.

I ran a 777-300ER flight plan with an almost direct route from LAX to TPE along airways very close to the great circle (about 1.7% greater), with yesterday’s winds. The shortest flight plan was 6018 nautical miles including the SID from LAX and STAR into TPE, the average wind on that flight path was 267/69 knots, an average of 62 knots of headwind. The 62 knots of headwind reduces the ground speed so the aircraft flies an equivalent of 6899 air nautical miles. Flight time of 14 hours and 13 minutes.

Using the filed flight plan, it was 3.6% longer than great circle, 6139 nautical miles. The average wind on that path was 269/61 knots, average of 48 knots of headwind. That 48 knots of headwind reduces the ground speed so the aircraft flies an equivalent of 6796 air nautical miles. Flight time of 14 hours and 2 minutes.

The longer ground path due to the wind direction actually increases the average ground speed, flight time is reduced by some 11 minutes, a saving of 1686 kg in fuel.

[DavidK99]

swh, thanks very much for your response. That makes it all quite clear. Looking at recent BR12 TPE-LAX flights on flight aware, I see that they have taken a much more southerly route than the BR15 LAX-TPE tracks -- far south of the great circle route. It makes perfect sense now.

As an outside-the industry frequent traveler, this site is an amazing resource and I've learned a lot from browsing it over the years. Air travel is an experience like no other...reading up on things and paying attention to details can really be eye opening.

Many years ago I picked up a great book called Science From Your Airplane Window, and have read it several times. I'd strongly recommend it to other "spectators" here who are fascinated and curious about air travel.

Thanks again for the helpful replies.