From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
Date:         01 Oct 96 23:56:37 
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>Are ETOPS flights as safe as 3 and 4 engined aircraft, except the obvious
>that they have less engines and hence less chance of a failure!

If the probability of an engine failure is the same, then yes, with
more engines you have a higher probability of having an engine fail
within a given amount of time.  But those assumptions aren't true.
Often, twins have higher thrust versions of the same engine (the
PW4000 used on the 747-400 has 56,000 lbs of thrust, the 767-300's
version is 60,000 lbs, and the 777's is 77,000 lbs and up).  They
don't just run the engine harder, of course, but core temperatures
and stresses are probably higher at full rated thrust.  Lufthansa
used this argument to justify the A340 over the A330, even where
ETOPS wasn't a significant concern.

On the other hand, if everything is working, a twin's engines don't
have to work as hard on takeoff.  Since an airliner must be able to
complete a takeoff and climb on n-1 engines once it has attained V1
speed, a twin can achieve (approximately) the same performance with
both engines running at only 50%.  A four-engined aircraft must run
its engines at 75% of rated thrust to match its engine-out

One might also argue that the added scrutiny and maintenance care
that an ETOPS-rated aircraft receives makes it *safer* than a 3- or
4-engined airliners that just gets the normal maintenance.

Karl Swartz	|Home
Moderator of sci.aeronautics.airliners -- Unix/network work pays the bills