I might be accused of being a bit anal here, but what the hell.....
Nigel
or 2. I have yet to recall one due to an independent double engine failure (by indpendent, I mean where both engines fail for their own reasons.)
There was that recent incident over the Atlantic where the ETOPS engine failed because of leak in fuel line.
Soon after, the second engine failed because?????? Well we don't know yet...... but it wasn't
primarily because of a hole in the fuel line on the other side.
Probably neither failed for their
own reasons, but they both failed nonetheless, and the cause is probably independent.
Some would say this could happen to a quad too, of course.
So human error excluded, I'd say the reason ETOPS twins are safer than quads is because there is far more margin built into each engine of the ETOPS plane than each engine on the quad. An ETOPS plane has to have engines which will allow them to climb like a bat out of hell with one engine. A quad can do only that with three. One might say that each engine on a quad is pretty underpowered.
If you hit windshear or have a GPWS and you have all engines turning, then I'd rather have all my engines to be totally outrageously overpowered. An overpowered twin (with all engines) may clear the ridge the OK powered (with all engines) quad impacts.
As an aside - which is what this thread has become - that B777 which got 180 ETOPS before being put into service flew from Hawaii to the mainland on one engine. If an engine failed on the 777 over the desolate wastelands, I would rather the commander made the judgement call that he fly across Siberia and land in Tokyo. Statistics would be on his side. Engines are far more reliable than dodgy controllers, dodgy weather services, dodgy approaches and dodgy runways. Better food in Tokyo too, but the booze costs would be excessive
[ 19 November 2001: Message edited by: slim_slag ]