Indeed the rules are different for two engine aircraft as opposed to 3 or 4. With the loss of an engine in a twin there is an immediacy to landing that is reflected in the rule of landing at the nearest suitable. It is what constitutes suitable at that time that is the matter of some consideration and thinking. One has to be VERY careful in overflying a suitable airport in that situation if one is down to a single motor. Doing so would involve a lot of explaining at the tea and biscuit time that would be sure to follow such a decision.
As far as the discussion of the evolution toward large long-range twins economics do play a significant role. Three and four engine aircraft of course do not need to comply with the absolute rigor of twin operations and thus are afforded some savings for the planning, not to mention, the expense of an actual divert. We only need to look at the cost that Delta surely must have incurred during the Ascension divert. If they had to hire out a 747F to bring in a new engine that alone would have been expensive to state the obvious. Add to that the cost of the discomfited passengers, the aircraft launched to come and get them, and that two airframes are not (at least for a couple of days) in the normal rotation of generating revenue and the one side of the balance sheet takes a bit of a hit.
I always like to remark that long-range twins share a lot in common with quantum physics. Both are predicated on probabilities and translated from the indeterminate through the language of statistics! There is a trade-off that each carrier must make for the planned routes.
I actually enjoy the finer point inherent in that trade-off. When I tour an air museum with guests I always remark, whether it be a Beech Staggerwing or the Concorde, that an aircraft is a compromise. Perhaps a wonderful and ingenious compromise, but a compromise nonetheless. The same applies for the trade-off of the twins to four motors--there is some compromise being made.
As far as being far over the Laptev Sea on a twin in the middle of winter, well...the airplane does not know (so to speak) whether it is over land or water but I sure do!
Speaking of statistics, I am still trying to grasp Boeing's position that more diverts are made for reasons other than engine failures. I can understand that, but having chatted with one of the gents who was on the 3 hour Mid-Pacific Odyssey on one engine, the statistics take on a different meaning at that time. It is often difficult for humans to overcome the counter-intuitive. In other words, sure more diverts are made for sick passengers or hydraulic failure, but the question is what happens on those few, and perhaps not so happy times, when an engine does fail? Do I really wish to be 204 minutes (or more if Boeing and the other manufacturers -prevail in this discussion) from an aerodrome? There does need to be some comfort level after all...
Last edited by Uncle Fred; 16th Jan 2013 at 00:27.