Re: A 777 with four engines.

Date:         06 Aug 98 11:26:37 
From:         faurecm@halcyon.com (C. Marin Faure)
Organization: NorthWest Nexus Inc.
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In article <airliners.1998.1150@ohare.Chicago.COM>, robm@null.net wrote:

> If we assume that all aircraft are built to remain flyable with any
> single engine out (N+1 redundancy, where N is the number of engines
> required for flight), we can then assume that a twin has, after a single
> engine failure, X chance of having a second engine failure, whereas a
> four engine aircraft (with only three operating) has 3X chance of a
> second engine failure. Hmmmm.
>
> Now, if we consider that, with an N+1 redundancy, a twin must have 200%
> the power required to continue a flight from V1, whereas a four engine
> aircaraft only needs 133%, we can further assume that a four engine
> aircraft will have a lower thrust to weight ratio. Hence, on takeoff,
> the four engine aircraft will spend more time wallowing around at low
> altitude (where a single engine failure, and the possibly subsequent
> momentary loss of directional control might cause an inadvertant
> landing).
>
> What am I getting at? I think I'd rather fly a twin. Just my opinion.

The other thing to consider, although it is not readily measurable, is the
degree of maintenance received by an ETOPS twin.  Certificating the
airplane for ETOPS is only part of the story.  The airline itself must
also be certificated for ETOPS operations.  This means they have to have
operational, maintenance, and data gathering and tracking practices that
meet ETOPS requirements.  Some airlines have applied their ETOPS
maintenance and operational procedures to their non-ETOPS fleet as well,
and the reliability of all their aircraft has gone up as a result.  So in
addition to whatever statistics about twin vs three or four-engine
airplane safety you care to look at, you also have to take into account
the fact that the ETOPS twin is a very well-taken-care-of piece of
machinery.

Based on what I have observed at the ETOPS airlines I've filmed at over
the last year or so, I cannot think of a safer place to be on this planet
that over the middle of the Atlantic or Pacific on an ETOPS twin, Boeing
or Airbus.

C. Marin Faure
  author, Flying A Floatplane