Read post #2 boy! Rhumb lines are flown doing airplots where you just fly headings, note position when you can and deduce from that what the wind has been doing to you so you can work out what it is, and hence aply that wind to your next leg. Really only maritime search and rescue and 'find the submarine' flights pre-INS era.
The B747 started flying the Atlantic in 1971 with 3 Carousel or Litton INSs. It went very well, although they used to pick up significant inaccuracies until you could update with radial/DME on the other side. I recall on the 747 in the 70s, INS failures were quite regular. Considering all the flight instruments were fed off them and they provided artificial horizon information (except standby), they were essential equipment. I think we were flying NATs from when I started on the Atlantic in 1971.