Re: 2 engines vs 4 engines planes

From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works
Date:         22 Oct 93 01:05:31 PDT
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In article <airliners.1993.673@ohare.Chicago.COM> acm@SkyNet.GUN.de (Dietmar Hanke) writes:>What happens if one of the 2 engines of a B-767 fails somewhere over the 
>atlantic? Which distance can this jet still fly by just using one engine.

This question seems to be coming up a fair amount again ...

If one of the two engines fails over the Atlantic (or Pacific, or vast
unpopulated areas of Asia, or anywhere else lacking a handy airport)
it must limp to safety on the remaining engine.  The rules regarding
operations over vast areas without convenient airports are known as
EROPS (Extended-Range OPerationS) and the specific rules for twin-
engined aircraft are often called ETOPS (Extended Twin OPerationS, or
my favorite, Engines Turn Or Passengers Swim. :-) )

ETOPS ratings are generally discussed not in terms of distances but in
flight time, on one engine, from the closest usable airport.  The very
earliest ETOPS ratings were for 90 minutes (60 minutes is just normal
operations) which could make it across the Atlantic but with a rather
long and inefficient route.  Cancellations due to weather would be
rather frequent, which points out one of the not-so-obvious facets of
ETOPS -- having an airport within your limping range isn't any good if
it's closed when you need it, so an ETOPS flight plan must pay close
attention to weather forecasts on the ground at the approrpiate times
in the future.

120-minute ETOPS was soon approved, and this made twin-engine flights
across the North Atlantic fairly routine.  Weather cancellations and
deviations from the "optimal" path taken by aircraft with three or
more engines are minimal.  At least some authorities have approved
another 15%, or 138-minute ETOPS, which eliminates the differences for
virtually all conditions.

Few routes required longer ratings than this, and really only one was
in great enough demand to attract anyone's attention.  That one makes
for an interesting trivia question because it is covered by domestic,
not international, regulations.  Specifically, it's between the U.S.
mainland and Hawaii, which is very nearly at the limits of 180-minute
ETOPS.  (I'm not sure about U.S. to Tokyo but I suspect the military
bases in the Aleutians make this a somewhat easier route.)

Various factors figure into ETOPS ratings.  Proven engine reliability
is one as is the engine/airframe combination.  The airline and its
maintenance program must also be certified.  Backup power sources are
important, too, e.g., Airbus had to certify the in-flight startup of
the APU on the A300/A310 while Boeing opted to use an airflow-driven
generator (a RAT, or Ram Air Turbine).  Minimum equipment lists (MEL)
are progressively more restrictive as distance from airport increases,
i.e., one broken radio might not keep you from flying San Francisco to
Chicago while it might well prevent an ETOPS flight.

>Is it possible to fly a B-747 when 2 of the 4 engines failed? Was it even 
>possible when both engines on one wing woudn't work anymore?

Sure.  One example that comes to mind is the United 747 that blew a
cargo door about an hour into a Honolulu to Australia flight in early
1989.  The debris caused the failure of the #3 and #4 engines, but
the plane limped back to Honolulu with just the #1 and #2 engines (the
ones on the left wing).

>I allways prefer a flight on a B-747 when I have to get from Frankfurt to 
>New York if there were also 2 engine jets on that route. Would you think 
>the same way?

Some people would avoid the twins overwater, period.  Personally, and
this is just my opinion, I'd prefer more engines and thus would choose
a 747 (or L-1011) given the choice.  On the other hand, I'd take a 767
with only two engines over the Atlantic before I took a DC-10 or MD-11,
despite their having three engines.

Over the Pacific is another matter.  Quite a few folks who wouldn't
object to vehemently to a twin over the North Atlantic would not even
consider a twin over the Pacific.

--
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