I think that AnFi's hypothesis are both valid:
i) that as engines get more reliable, the advantage of two engines over one will decrease. The advantage will only become zero when engines are 100% reliable (almost certainly never) but there may come a point when the chances of engine failure is so small that the advantage of a twin is outweighed by all the other things than can go wrong.
ii) that the chances of a twin engine failure is higher than you might think (both engines will be the same age, subjected to the same stresses and impurities in fuel/oil etc.)
However, I think that AnFi is drawing the wrong conclusion. AnFi's conclusion is that a single is effectively as safe as a twin. I think the correct conclusion is that a twin is effectively as unsafe as a single.
Obviously, I'm not saying ban all helicopters - just that those flying twins should not be just as cautious as those flying singles e.g. be prepared for an auto even in a twin etc.
Matthew
Last edited by mdovey; 27th Mar 2014 at 10:10.