Yes, if you fly a constant heading of 090°T in still air you will follow a line of latitude (a rhumb line), but (i) the air is not still, as the wind changes you will have to change your heading (ii) very few light aircraft compass systems (none?) allow you to fly true heading so you will actually have to work out variation and fly magnetic heading. Variation changes so you will also have to change your mag heading to allow for that and (iii) both the above arguments apply equally to a true track of 000° which would allow you to fly up a meridian (both a great circle and a rhumb line).
If you are flying on a DI these are affected by transport wander, earth rotation and the possibly erroneous effect of the latitude nut. Flying due north or south transport wander would be zero, flying along the latitude the latitude nut was set to compensate for would remove that error.
If you are flying with an IRS/FMS combination coupled to the autopilot everything is *easier*. Flying due north or south you can just follow the great circle track, flying east or west you would have to build an approximation to the rhumb line with a series of short (50NM to 80NM) great circle tracks. This is marginally less *easy* but would still give much greater navigational accuracy than trying to stumble along a line of latitude in a light aircraft with a compass and whizz wheel.
Maybe this is just a discussion question with no correct answer, to see how much you know?
Last edited by Alex Whittingham; 2nd Jan 2013 at 16:21.