Please note that nowhere in my post did I advocate a safe number of engines.
I simply pointed out that a factor to consider is that the more engines you have the more likely you are to experience an IFSD. It is, of course, not the only factor to consider. In addition to the probability of an IFSD, you obviously need to consider the consequences of an IFSD.
The fact is, that all else being equal, a quad (that's flying on 3) is 50% more likely to experience an engine failure than a twin (on 2).
But if a quad can divert on 2 as safely as a twin on 1, then this increased probability of diversion may not be a significant issue.
The probability of an IFSD is factored into the calculations behind ETOPS. The probability of an IFSD should similarly be factored into the calculations behind a quad (that's flying on 3) continuing its flight. The fact that a tri-jet might routinely travel the route is not necessarily relevant since the consequences of an IFSD are different for a tri-jet on 3 than for a quad that's flying on 3.
Last edited by stagger; 26th Apr 2006 at 16:41.