Originally Posted by Pascoe

Damn - sorry KARFA - for some reason didn't spot your above post. Apologies.

Predictably it's a bit more complicated than that. There are all sorts of protocols for this but one that you may often see reference to is ETOPS, which stands for Extended range Twin engine Operational Performance Standards. Basically a set of operating procedures to allow twin engined airliners (where the issue or a single engine failure over water / ice is more serious than in a 4 engine bird) to operate safely.



In addition to the requirements about flight time to emergency / alternate fields (which are defined as permitted maxima at single engine cruise speed, based on the exact ETOPS certification the plane has) there are also as I understand it extra protocols around maintainence checks etc. The aircraft even have ETOPS painted on the undersides in strategic spots just so, when they're in hangar getting a look over, there's no confusion as to what needs inspecting and to what level of detail.

So in theory a non ETOPS certified plane shouldn't be flying an ETOPS routing. AA got in trouble with this a while back flying LAX to HNL and had to get another bird out there for the return.

Apparently in aviation slang it can also stand for 'Engines Turn Or Passengers Swim'. Got to love that. But it is not specifically a ruling about flights over water as I understand it - the assessment is on flight time to a suitable alternate. And therefore there are also standards for 4 engine planes (typically ETOPS 180 I think).

As you'd imagine there is lots online on this. The above is merely a summary of the small amount I know about it. If I got any of this wrong, experts please feel free to correct (gently please haha)