From: (DLawler)
Organization: America Online, Inc. (1-800-827-6364)
Date:         21 Oct 96 02:51:37 
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>> to keep them in peak operating condition.  Boeing has argued that the
>> evidence shows the more engines and aircraft has, the more likely there
>> will be a problem with one or more of the engines, and that consequently
>> 3+ engine airplanes are no more safe in that respect than 2 engine
>> airplanes.
> By this argument, double or triple redundancy in instruments or control
> systems should also be removed, since the more electronics or the more
> hydraulic lines you have, the greater the chance there is for one of them
> to fail.

Two engines is double redundancy.  The question to be answered for every
mechanical system is how much margin of safety is needed.  To answer that,
engineers look at historical data, engineering models, operating
environment, etc and try make the best judgement possible.  And EVERY
system is different.  Hydraulic lines are QUITE a bit different from
engines, and comparing the redundancy between the two on purely a numbers
basis is pretty specious.

Certainly the more duplication you have, the greater the margin of safety.
 But you reach a point of diminishing returns.  Maybe three engines are
more safe, but do they add so much more safety that they justify the added
costs, etc., or is it a minor improvement that can be offset by the
benefits of two engine operation?  The FAA has decided that two engines
are enough for ETOPS flight provided numerous steps are taken to prove
their safety.

Does this mean that a two-engine ETOPS plane will never have an accident
resulting from engine problems?  No, there are no guarantees in anything.
But I've never heard of a two-engine failure on a two-engine ETOPS plane
resulting in an accident, and there are far more dangers in everyday life
than flying on one.

- David Lawler