Green dot and others.
The FMC always gives you great circle track between two points no matter the distance between them. So between 55N40W and 55N50W, on a rhumbline track you would stay at a constant 270 degrees, while on a great circle track (as per the FMC) your ITT (inital true track) will start somewhere greater than 270 (say 273) and end somewhere below 270 (perhaps 267). Sometimes the NAT tracks are very close to great circle tracks, but most often they are not. As has been pointed out earlier, however, this is not a concern, as they are planned to make use of (or avoid!) the prevailing winds as much as possible.
Rgds,
DB
PS! Rainboe, I'm sure you meant to say that on a Mercator chart, great circle tracks are curved upwards towards the North pole (in the northern hemisphere), not towards the equator.
