Re: A 777 with four engines.

Date:         29 Jul 98 00:29:33 
From:         Rob Montgomery <murdock@ameritech.net>
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k_ish wrote:
> Most failures will result in a loss of one engine; in this case, the
> added engines buy you nothing in terms of safety factor.  Actually, the
> added engines do increase the probability of a single engine out
> incident.  There are other scenarios that can cause the loss of all
> engine power (misfueling, ingestion of volcanic ash, maintenance error
> on all engines).  In these cases, it doesn't matter how many engines you
> have.
>
> 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.

Now, I don't claim to be an expert, but I think that we can also take
this thought to the next level. If we assume that an engine has X chance
of failure, then, as you state, we can assume that a twin will have 2X
chance of having one engine fail, while a four engine aircraft will have
4X chance of an engine out condition.

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.

-Rob

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robm@null.net