Question about routing
 

I've been looking at the LGW - CUN flights over the last couple of weeks.

I have noticed that often, you might expect for aircraft to follow a similar route as other aircraft doing that route, but they have been vastly different. For example the Thomson flight yesterday took a southerly route, the BA flight took a middle-ish route and then a dog leg south and the Thomas Cook flight did a very northerly route over Greenland. This changes all the time, for example the Thomson flight today is doing a northerly route.

My question is what determines these routes if not weather? Why would they go a southerly route one day and a northerly next if the weather has not changed particularly etc etc? And is there a way to know roughly in advance which route they might take for the purposes of selecting your seat to see the 'best' views of things? Not just this route, but other routes too?

Lady,
It is primarily the winds, including the infamous jet-stream, blowing from west to east over the Atlantic. Flights from Europe would like to avoid headwinds, while flights in the other direction, don't mind a little push. (Recognising that both head and tailwinds can cause turbulence.)

I get that but why would flights leaving within 30 minutes of eachother take 3 different routes? Surely the winds haven't changed that much?

Just a few thoughts:

Attempt to reduce Navigation charges (ATC charge for their services, some charge much more than others).

Aircraft equipment/temporary minor defects might restrict the route of flight (perhaps a minor defect means the aircraft has to stay within a relatively short distance of en-route airfields).

Maybe a decision has been made not to a route the flight through an area of possibly severe turbulence/inclement enroute weather.

Might be something as simple as having plugged in all the variables Company A's computer reckons going north is cheapest/best, whereas company B's computer has come to a different conclusion....

There are also over 2500+ transatlantic flights a day, see here.

So basically it could be anything!

Would I be correct in saying at certain times of the year that certain routes are more likely to be taken? E. g more northerly routes in Autumn for example?

You might want to post your question in the ATC forum. Many SLF will speak from their own experiences while the ATC guys can likely give you a more day-to -day, month-to-month perspective.

I do know that the jet stream tends to be further south in the Winter ...

Perhaps the Captain will make a decision on the day as to whether go on a certain route based on facts like winds, aircraft type, ATC restrictions and other performance related issues. For example a 787 has a better fuel range than an A330.

Thanks for your input everyone, I've asked in ATC as you suggested ExxB.

As with most this in aviation it will likely come down to cost. As mentioned airwy charges and the like.

Depending the loads to have you may be capped for a portion of the flight at a certain flight level. If this flight level is in a strong head wind then it may be more efficient to fly longer distances to avoid.

Depending on the flight planning suite the airlines use the crew will be presented with the route the airline wants them to flying based on whatever is most important to that airline be it cost or speed.

Lady

By all means ask but ATC don't make the strategic decision as to which route "we" take. In the hours running up to a flight the airline will run multiple variations of flight plans through their flight planning computers, probably with varying lateral and vertical profiles, and ultimately pick the plan that works best (and as fa2fi says it's usually just down to cost)- that plan then gets "filed" with ATC. ATC can in effect veto certain parts of routes (e.g. due to forecast/actual route congestion, volcanic ash, etc, etc..) and/or they can insist we follow set routes in places (e.g. Organised Tracks on parts of the north Atlantic, inbound/outbound routes at airports), but they rarely (if ever) intervene and insist that we follow a specific route from departure to destination, certainly not on a long haul flight.


fa2fi has made a good point about "capped" levels and the like. Just as a +1 to those comments don't forget (?) that this is a three dimensional business and jet streams are three dimensional beasts. At any given point there might be a howling westerly head wind at 30,000 feet whereas it might be slack calm directly above the same point at 40,000 feet. So the "high flyer" going from A to B might quite happy and efficiently fly a direct line over a jet stream, whereas those also going from A to B not able to cruise at as a high a level for whatever reason, (e.g. weight and aircraft performance) may find it's more efficient to take a longer route around the jet.

There's really not any great mystery to this, just don't make your seat choice based on the previous year's/week's/day's flight plan...