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

The probability of an ETOPS plane losing both engines in a single
flight due to unrelated failures is extremely remote. That doesn't
mean it can never happen, but it is less likely than winning the
lottery.

Not quite; the probability of all engines failing decreases with the
number of engines if all engines have the same probability of failing.

That looks fair enough at first sight, but, as you go on to say, it
is "highly dependent on the probability of the individual engine
failing".

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.

With three or more engines, a second failure during diversion is
much less likely to be catastrophic.

Of course, if you take two pairs of ETOPS engines, fit them to a
four-motor aeroplane, and maintain them to ETOPS standards then
the probability of losing all of them from unrelated failures is
exceedingly small - *way* down in the noise of multiple failures
with a *common* cause.

After posting it occured to me that the above was an incorrect statement.

It should be, the probability of all engines failing decreases with the
number of engines.

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

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

In the real world, we attempt to keep those probability numbers low
so that such an occurance becomes highly unlikely.

The probability of getting 3 jackpots in a row on a Vegas slot machine
is a number greater than zero, but does not form a valid basis for a
retirement plan, for example.

--
Jim Pennino

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