In rec.aviation.owning Robert Briggs wrote:
wrote:
Robert Briggs wrote:

The whole point of ETOPS is that the *requirements* for the
engines are rather stricter than those for airliners with
three or more engines, since once you've got a single failure
the other fan had jolly well better keep turning.

While the probability of all engines failing will increase with
the probabilities of individual engine failure, that number will
always be less than any individual probability.

This is a consequence of the laws of probability and nothing else.

There is more than simple "the laws of probability" at work here.

In pre-ETOPS days, three or more engines were required for, say,
transatlantic airliners because the risk of multiple engine
failures was deemed to be too high.

ETOPS certification requires engines which are *demonstrated* to
be more reliable than those on 707s, early 747s, and the like.

It is by no means impossible for four independent engine failures
on an older aeroplane to be more likely than two on an ETOPS kite.

Of course, now that manufacturers are building ETOPS-certified
engines there's not a great deal of point in deliberately making
*less reliable* versions for airliners with three or more engines,
so it is rather likely that *newer* 747s are less prone to losing
all four engines independently than twins are to losing both.

Mathematics doesn't care about certifications or maintenance
programs. Programs are based on the mathematics.

If the probability of an engine failing is 0.1, the probabilities for
all engines failing a

1 0.1
2 0.01
3 0.001
4 0.0001

If the probability of an engine failing is 0.7, i.e. really ratty engines,
the probabilities for all engines failing a

1 0.7
2 0.49
3 0.34
4 0.24

Things like ETOPS exist because the numbers say getting the probabilities
low is a good thing.


--
Jim Pennino

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