Nick
your mention of three engine helicopters brought to mind a conversation many years ago when the EH101 was first being talked about. One of the pilots made the ('I think') valid point that we make so much fuss about cat 'a' performance, yet the North Sea had never to our knowledge had a performance related accident. However we had just gone through a rash of gearboxes falling apart at their input shafts. It appeared to us that the only effect of adding a third engine was to increase the possibility of an aircraft ditching by 50%.