Re: A 777 with four engines.

Date:         06 Aug 98 11:26:36 
From:         faurecm@halcyon.com (C. Marin Faure)
Organization: NorthWest Nexus Inc.
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In article <airliners.1998.1149@ohare.Chicago.COM>,
darren@daz-technology.demon.co.uk wrote:

> On 26 Jul 98 23:57:17 , k_ish <kenish@ix.netcom.com> wrote:

> >IMHO, four engines create slightly more than twice the chance for an
> >engine failure of some sort, and they buy you no added safety factor
> >when it does happen.
>
> This really, worries me. When on a twin and you lose an engine instead
> of flying at 35-39,000ft you're down to 20,000ft and to achieve this
> you've pushed the power forwards on your last engine, normally to max
> continuous from max cruise. Now any probability statistics for infight
> shut down rates go out of the window since your remaining engine is
> working that much harder. This is why we have ETOPS rules which have
> gradually increased from 60mins to 180mins over the last couple of
> decades.

If we were still flying piston-engine airplanes, you would have a
legitimate concern.  The 60-minute rule, which is still on the books, was
written in the 1950s as the result of several fatal accidents involving
twin-engine piston transports that lost one engine and burned up their
remaining engine trying to maintain altitude and reach an airport.  The
probability for engine failure on piston engines goes up fast with the
amount of power being demanded from it.

This is not true of turbofan engines, as long as temperature and N-speeds
are not exceeded.  If you look at the graph representing the chances of
failure vs. power output of piston engines and turbofan engines, the
failure rate line climbs steeply with power output from a piston engine,
but remains relatively flat with turbofan engines.  In other words, a
turbofan doesn't care what power setting you use so long as its
temperature and rotational limits are not exceeded.  Pushing the power up
on the remaining engine of a twin-engine jetliner does not add to the risk
of engine failure as it would if you were flying a twin-engine piston
airplane.

C. Marin Faure
  author, Flying A Floatplane