North Atlantic Tracks

[magus69]

This may be a silly question. I was looking at the North Atlanic tracks on a regular map. When I transpose the tracks onto a globe, they make a curve or arc across the ocean. When flying these routes, is the aircraft in a very, very slight continuous left or right bank for a longish period or are the routes flown in a series of straight lines over the earth's curvature with an alteration of heading now and then.

[D120A]

Not a silly question and I hope that, when the experts answer this post, the two points I am about to make aren't deemed to be silly too.

The first is that, unlike your globe (I suspect), the earth is not a perfect sphere. It is an oblate spheroid, fatter at the Equator than at the poles. Thus any real great circle track will appear not to be a straight line on a spherical globe.

The second point concerns Coriolis acceleration. Any object moving at a constant velocity in a rotating reference frame (and the Earth turning once a day is a rotating reference frame) will experience an acceleration, both the magnitude and the precise direction of that acceleration being given by the vector product (2 times the earth rotation vector times the aircraft velocity).

It's difficult to explain vector products on a flat sheet of paper, but suffice it to say that the Coriolis acceleration experienced by an aircraft travelling straight and level at 500 mph directly over the North Pole (in any direction) is 0.107 ft/sec-squared, towards the tip of the port wing. (Tracking Eastwards along the Equator, the Coriolis acceleration is vertically downwards. Tracking North or South over the Equator it is zero.) Thus any aircraft straight and level in the northern latitudes will experience an acceleration (and hence drift?) to port. But I am not sure if that would affect tracks - rather, I would expect it to affect the heading you would fly in still air in order to achieve a certain track, and that heading would vary with latitude.

For what it's worth! As always on PPRuNe, I await the expert answers with extreme interest!

[rab-k]

No "expert", but as one who designs such things, perhaps I can offer an explanation.

When designing the North Atlantic Tracks there are numerous factors which must be taken into consideration, but we basically start off with Company Preferred Routes. These take the form of Minimum Time Paths between destinations in North America paired with European departure aerodromes. These are drawn up by airlines, using forecast Met data to calculate the route that provides the minimum flying time for each pairing. The MTPs are then forwarded to us in Oceanic ATC where they often appear on our system looking not a whole lot different from great circle routes.

Once we have a sufficient number of MTPs from the operators we can establish where the bulk of the traffic wishes to route and then design a set of tracks to maximise airspace utilisation. Factors we must also take into account include ensuring separation standards are maintained, (obviously), complying with requirements from adjacent ATC centres, avoiding areas of military activity, etc. For numerous reasons the Organised Track Structure typically routes via whole degrees of Latitude at intervals of 10 degrees of Longitude, therefore the final design will conform approximately to the MTPs.

The routes flown are from waypoint to waypoint, rather than along a fixed track. Therefore due to the curvature of the earth if, for example, an aircraft routes from 55N020W to 55N030W, it will cross 25W at approximately 55º05'N. It would not, in this case, follow the 55th parallel along its' entire length between 20W and 30W. To put it another way, the tracks are a series of great circle routes between consecutive waypoints, typically at intervals of ten degrees of Longitude.

Therefore to say that "the routes are flown in a series of straight lines over the earth's curvature with an alteration of heading now and then" pretty well sums it up. Although when you say "alteration of heading now and then", a great circle route involves a constantly changing heading, as opposed to a rhumb line, but before I get way out my depth, perhaps I can point you in the direction of:

http://en.wikipedia.org/wiki/Great_circle
http://en.wikipedia.org/wiki/Great-circle_navigation
http://en.wikipedia.org/wiki/Great-circle_distance
http://en.wikipedia.org/wiki/North_Atlantic_Tracks

Hope that clarifies rather than confuses. :ok:

[galaxy flyer]

Rab-K

Just an historical question-do you know when the track system was first implemented? I know it was 2 degrees laterally and Ocean ships until the early '70s, but what is the history before then.

GF

[twistedenginestarter]

I must admit this is something I've been struggling with recently. If I pop Heathrow and Kennedy in my hand-held GPS, it gives me a direction to fly. Is that a rhumb line or a great circle route it's giving me?

[dash6]

Great circle.

[rab-k]

First occasional use of north-Atlantic tracks in 1961. Daily organised track structure introduced and twice daily NAT Tracks published from 1965. http://www.xmatc.com/atcdata/html/46424.html

[galaxy flyer]

Thanks, Rab-K. Twenty years of crossings in the C-5 and I never knew the history of the tracks. Great stuff.

GF

[magus69]

So, if the heading is constantly changing because a great circle route involves a constant heading change, does the pilot put the aircraft into a very slight bank? It seems that it would be easier to maintain this curved track if you were banked. If you see what I mean. One other reason I asked this question in the first place, was that I had the impression that one wing was always a bit lower than the other when I went across the pond. Interesting comments, thanks.

[dash6]

No bank involvd. Great circle is a straight line over the earth. Rhumb line is a constant heading in relationship to longitude.Since the Earth "tapers" the rhumb line would require constant turns.In practice rhumb line is a piece of fiction on a chart.:)

[skiesfull]

If the next waypoint requires a different initial true track, then at the waypoint, the a/c will bank slightly to take up the new initial true track. Most of the published Organised Track System routes involve a change of heading (bank) to fly to the next waypoint. I cannot remember flying across the Atlantic with the same initial true track after every waypoint. Nothing is perfect!

[magus69]

Thank you for your answers, that's solved my illusion! :)