I recently flew commercial to Scotland and was tracking our progress on my iPad using Foreflight. I made a direct-to line that followed the great circle route, but noticed that we meandered off of it quite a bit.

The only explanation I can think of is that the crew was trying to take advantage of favorable tailwinds by following the jet stream. Is this assumption correct?

And if so, can an autopilot be programmed to follow a route dictated by a winds aloft forecast?


Most flights across the Atlantic don't fly direct great circle routes; they use the North Atlantic Track system, which is an array of "lanes" you might say, usually 60Nm apart, like one of those slot car racing places with 6 or 8 lanes of cars. Just about everybody that can make the trip non-stop will be in the tracks at various altitudes. Off each coast are an array of fixed entry point fixes called Oceanic Entry Points as jumping on or jumping off points.

The north/south position and the precise routing of the tracks is set up each day, with the focus on eastbound traffic in the morning and westbound in the evening, and are moved up and down the take advantage of winds or weather, especially jet streams.

Jet streams flow along air mass boundaries at the top of the troposphere and change constantly as air masses move. The tracks are set up to try to exploit or avoid them depending on whether they are favourable or not (as well as other weather systems).


can an autopilot be programmed to follow a route dictated by a winds aloft forecast?

Yes, but the question is how it is done, and these days it's better to think of it as not just the autopilot, but a Flight Management System (FMS).

In the case of the North Atlantic Track system explained by John K, the waypoints that define the route an airplane is to take are defined by latitude and longitude.

In my day (old fart-retired in 1999) the waypoint lat/long values were read to us over the radio, and we entered them into inertial navigation system system (actually 3 of them) which then controlled the autopilot—they voted so to speak.

These days there are a better ways of getting the assigned route into the FMS, and which hopefully someone familiar with them can explain.

The point is, though, that the FMS is not looking at the winds aloft forecast and deciding the route, it's simply flying a route given to it by some means that has been ordered by a ground controlling facility to take advantage of the winds.

Now, could an FMS be programmed to receive the winds aloft forecast and decide from that the best route. Probably so, but when you add in the complications of separation for all the other aircraft doing the same thing, I don't think we're there yet. In other words, automated ATC is still down the road a ways to say the least.


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