Diesel aircraft engines and are the light jets pushing out the twins?

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

Remove -spam-sux to reply.

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.

Dean Wilkinson wrote:

Jim,

Please don't reply in such a way as to make it appear that I said something
that I did not. It was Matt Whiting that made the comment about losing all
engines on a twin being more likely than losing all four. I didn't say
that, and I know that is not true.

How do you know this is not true? If I recall correctly, the
probability of independent events occuring simultaneously is equal to
the product of the probabilities of each event occurring. If we rule
out common cause failures such as fuel exhaustion and look at only
random failures, the the probability of all engines failing
simultaneously is the product of the probability of failure of each
engine separately. Assuming that each engine has the same probability
of failure, means that with two engines the probability of both failing
is P^2 whereas with four engines the probability of all failing is P^4.
Since 0=P=1, P^4 will be less than P^2.

As someone else said, the probability of having AN engine fail on any
given flight is higher with more engines, but I believe the probability
of ALL engines failing on a given flight is less with more engines.


Matt

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

Remove -spam-sux to reply.

Ernest Gann in "Fate is the Hunter" described one event in which he
lost nearly all power on 3 of 4 out of LGA once, and another time that
all 4 quit simultaneously with a load of passengers over the Pacific.
The first event was caused by the mechanics testing a new type of
spark plug, which they "unfortunately" had time to install on 3
engines. The second was a glitch in the fuel system.