Any twin engine can fly with one engine out. The over-water part is that it can fly on one engine for a long distance to get back to an airport - which tends to be further away if you're flying over the ocean. Basically having a bit more margin in the design for extended endurance with an engine out.
That generalizes to 'any plane with more than one engine can fly with one engine out'.
> The over-water part is that it can fly on one engine for a long distance to get back to an airport - which tends to be further away if you're flying over the ocean.
That's where certification comes in: not all twin engine planes are ETOPS certified.
> Basically having a bit more margin in the design for extended endurance with an engine out.
Yes, and that translates into much more work than just 'a bit more margin' under the hood, to get that margin you also will need to take this into account during the design phase of the aircraft, crew training, maintenance schedule etc. Incidents under ETOPS conditions are rated more severely than those in other situations.
While I appreciate that modern aircraft are incredibly safe, I don't think many people really appreciate what it means for a flight and aircraft to be ETOPS 240 or ETOPS 330 certified. That means if something happens in the air, you have 4 - 5.5 hours of flying before you reach an acceptable runway. You really are on your own.
Something to think about next time you're over the pacific.
