All previous posts correct.
Just to add a little more info, some aircraft, indeed the more modern ones have IRSs in lieu of INS. That stands for Inertial reference sytem - lasers and smoking mirrors! INS applies mainly to the older DC-10s, L1011s, DC-8s, 747 classics.
Not all aircraft fly the standard NAT tracks depending on their final destination and the allocated Flight Plan route. These are called "random" tracks and usually cross the standard ones diagonally. For example LGW to TAB or GND. Some but not all operators have 180 minutes ETOPs capability. Should there be an aircraft defect which cannot be deferred because of the strict requirements, then you may find you can only do 138 or 120 minutes from the nearest suitable airfield on your route. This means a more circuitous route and hence more time!
All the stuff about RVSM, HF radio links, position reporting and SELCAL is spot on. The North Atlantic (NAT) is actually divided up into 4 sectors: Shanwick, Gander, Santa Maria and New York. Which you use depends on your route. Since most modern aircraft are 2-pilot operations, operators will require one of the pilots to keep a running plot on a trans-oceanic chart. This way you will find out soon enough if you are on track without any VORs/NDBs to rely on!
Still amazes me that after coasting out over Lands End and getting your oceanic clearance and ETOPs entry point, after 8 hours or more of nothing but ocean, you can still accurately find the correct Caribbean island and runway
!!
Good luck!
767 Driver