Re: A 777 with four engines.

Date:         19 Aug 98 16:01:35 
From:         James Matthew Weber <>
References:   1 2 3 4 5 6 7
Followups:    1 2 3 4 5
Next article
View raw article
  or MIME structure

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.

Two points.First of all, no current engine really works very hard at
altitude. The big loads on the engines are at takeoff. 100% N1 is a lot
less load on the engine at FL300 than it is at
sea level.

I also seriously doubt that statistical evidence supports the assertion
that 4 engines are safer than 2 in a currently certified commercial
aircraft. How many crashes of twins have we seen where the number of
engines had anything to do with the crash? I can only think of one, Lauda
Air over Thailand...

Let me point out a counter argument. More engines do mean more in flight
shutdowns, which means more attention to that shut down and a distraction
from the real business of flying the aircraft. This may well make 3 and 4
engine aircraft LESS safe, especially given that such failures have become
so rare.

In a light aircraft, this is a different story, but  the truth be know in a
light aircraft, you are probably safer with 1 engine than with 2...

I am reminded of a comment a friend made about the CAA in Australia. They
used to require light twin pilots to demonstrate landing skills with engine
out on a regular basis, until it was pointed out to them that far more
pilots were getting killed being trained and practicing to deal with the
problem, than the problem was causing...

James Matthew Weber
Service Delivery Manager

Diyar United Trading and Contracting Co.
P.O. Box 44240
Hawalli 32057
State of Kuwait
PH +965 434 0560 x 230  FAX +965 431 5107 Mobile +965 971 2069