White Knight - I don't think it's confusion in so much as predictable thread drift. The perceived wisdom (or lack there of) of continuing on after an engine out on a quad pretty much inevitably morphed into a discussion of the relative merits of a quad vs. a twin.
Back in the early days of ETOPS, there were lots of studies related to engine failures, when they happened, and when did they contribute to significant incidents or accidents. For example, it was discovered that relatively few engine failures occurred during cruise - most occurred during takeoff/initial climb, or top of descent/descent - when ETOPS wasn't a significant concern.
But - in what was probably the most controversial finding - the probability of an accident as the result of a non-common cause engine failure was greater on a quad or a tri than a twin.
The reason was pretty simple: Commercial Airliners are designed to fly just fine after the shutdown of one engine - in fact when we do system safety assessments, we consider basic engine shutdowns no worse than major (even on a twin). But, an engine can do something worse than simply quit - it can catch fire or fail uncontained - and failures of that type have caused a number of crashes over the years. The more engines on an aircraft, the greater the odds that one of those engines will fail in a catastrophic manner (I said it was controversial
).
Now I fully expect to get flamed for posting this by the "quad centric" group, but the studies were vetted by a number of experts and was eventually accepted by the Feds as the foundation of current ETOPS.
Listen, I work twins and quads (at the moment 747, 757, and 767 - in the past 737 classics and 777). They are all safe, and there are relative advantages both ways. But it's not only simplistic to state that adding engines makes an aircraft safer, its not supported by the data.
