Originally Posted by
Good Business Sense
Glofish, your conclusions are not applicable - you're now into another universe with probability stats if you start to look at two engine failures on a four engine aircraft on the same flight.
You probably have a far higher chance of being hit by a meteorite.
As an industry we generally don't do, if, if, if and if and that's what is being done in this thread - there is no evidence that there is a problem here nor that engine out flight has been dealt with other than with successful outcomes.
The statistical probability calculations argue that we've more chance of an engine failure in a 3 or 4 engine aircraft etc - well, that's good, the 747 gets "safer" after it's engine failure.
Don't forget that the engine failure stats/data on which ETOPS was founded did not include many failures such as the engine stopping due to a gearbox malfunction etc - the engine had stopped and it was not possible to start it again but as it wasn't a "core engine" problem it didn't count
PS there are regs to be complied with if you lose another engine including drift down on two ..... at high weights on two engines most aircraft like the 747, A340 etc need a drift down altitude of circa 10-15,000 feet - there is still a safety net.
There are definitely times when mathematicians need to be re-educated in the real world. Just up-thread we had a brief exchange about an L1011 losing all three engines
https://en.wikipedia.org/wiki/Easter...nes_Flight_855 .
You see if there are times when Murphy walks around doing the before flights carefully breaking each engine (
*) the fault tree can get very short and simple. This is the probability of a
human failure which unfortunately can be far higher than people allow for. I won't bore you with anecdotes but there are lots. So yes - if your engines are all perfectly serviced and there are no outside influences the reliability is such that the summed probability of multiple failures is vanishingly small - but common mode failures are not affected by the number of engines slushy fuel, wrong fuel or oil or fuel replenishment etc etc can lead to all of your engines going embarrassingly quiet all for the same reason - often at close to the same time.
That is why when a supposedly redundant (dualed) items
both fail you are in different territory and simplistic probability no longer applies. Such a common failure means that there is the potential for an external input that was common to all engines. That's when people earn their pennies making the go/no-go decisions and the crew of course have their own 'flesh in the game' - these decisions should be soundly based.
Fault tolerance, resilience and redundancy is an interesting field
(
* Muphy's Law - "
If there is a way of doing something wrong - someone somewhere will do it that way."
Not to be confused with
'
Sod's Law:-
"If anything can go wrong - it will." )