stilton wrote:
Indeed, the levels of redundancy built into modern transports, particularly the 747 allow for continued flight in the event of a loss of significant systems, up to and including powerplants. (That is why they have four!)
They have four because at the time they needed four to get them in the air since more powerfull engines were not available.
Optimum cruise is with four engines. That's a commercial argument and thats the reason of the design. They could easily have build it with less redundancy so it would be even more intersting commercially speaking (lesser weight) but they didn't do that for safety reasons.
That is why the redundancy is build in for safety and not commercial reasons. It's obvious but hard to acept if you do not want to see it that way.
Once one engine is out the remaining redundancy must be used to get the plane on the ground safely and not to continue for commercial arguments only this is the key argument against the continuance of the flight.
That and only that is the main reasoning when one has to answer the question of What is the safest course of action.