From: (Louis K. Scheffer)
Organization: California Institute of Technology, Pasadena
Date:         28 Oct 96 03:06:35 
References:   1 2 3 4
Followups:    1 2
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  or MIME structure (Robert Dorsett) writes:
>>The original question was is an ETOPS flight riskier than a flight on a
>>three or four engine airplane?  The answer is still no,

I suspect the answer to this involves procedures and cockpit automation and
human factors much more than engine reliability.  I know the limiting
factor on most redundant computer hardware is the human element once a failure
has occured.  Suddenly, the person in charge has to do things differently,
using less well rehearsed procedures, and the odds of a mistake rise
dramatically.  There are many classic cases of this; where one engine goes bad,
and the pilot then shuts off the good engine (thinking it's the bad one), or
where a landing gear warning light goes on, and the crew tries to fix this
while the plane flies into the ground, and so on.

I would suspect that a modern ETOPS plane, where the automated cockpit does
the 'right thing' upon an engine failure (where the right thing was thought
through by experts who were not under time pressure), is probably safer
than a 4 engine plane where an engine failure needs to be treated with
a long checklist and modified procedures.

So it is possible that both of these statements are true - that one specfic
failure mode (all engines failing) is more likely on an ETOPS flight, but
that the overall risk is lower.  Since the human factors elements are very
hard to quantify, this is very hard to prove one way or another.

   -Lou Scheffer