Re: A 777 with four engines.

Date:         29 Jul 98 00:29:32 
From: (Darren Rhodes)
Organization: Daz Technology
References:   1 2 3 4 5 6 7
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On 26 Jul 98 23:57:17 , 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
>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

Now for four engines you lose only 25 percent thrust, not 50 percent,
hence the power increase is lower to maintain an adequate flight
level. This immediately implies an inherent increase in safety.

| Mr. Darren P. Rhodes, BEng. (Hons)        | Aircraft Performance   |
| Email:  | Analysis & Noise       |
| Web:     | Specialist             |