Re: Re[2]: ETOPS

From:         mdw@ccu1.auckland.ac.nz (Woodhams)
Organization: University of Auckland
Date:         21 Oct 96 02:51:38 
References:   1 2 3
Followups:    1
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"Peter Mchugh" <PMCHUGH@mail.hq.faa.gov> writes:

>DLawler (dlawler@aol.com) wrote:

>> will be a problem with one or more of the engines, and that consequently
>> 3+ engine airplanes are no more safe in that respect than 2 engine
>> airplanes.

>     Seems to me the question is one of what is being optimized...safety or
>     aircraft availability.  If one wants to have fewer possible failure
>     points reduce redundancy...but if we'd like to keep the aircraft aloft
>     when failures occur, redundancy seems appealing...n'est pas?

If one engine

This would be true if the minimum required number of engines were the
same for both types of plane. (E.g. you need only one hydrolic system,
so in this case, increasing redundancy does increase safety.)

The basic argument is that a tri-jet that looses two engines is just
as dead as a twin that looses two engines, but assuming equal engine
reliability and that engine losses are independent, loosing two
engines on a tri-jet is three times as likely as loosing two engines
on a twin. (There are three ways to lose two engines on a tri-jet:
loose 1 and 2, 1 and 3 or 2 and 3. There is only one way to loose two
engines on a twin.)

If some fixed number of engines (e.g. 1) were sufficient to safely fly
any plane, then more engines would indeed be safer. (A tri-jet is more
likely to have one running engine than a twin, and a four engine plane
even more so.) This is the case for (e.g.) hydrolic systems - you only
need one, so more redundancy means more safety.

Of course, my argument above is simplified. A twin with no engines is
not necessarily dead (Gimli Glider). A tri on one engine is better
than a twin on none. (If the crash rate of engineless twins is worse
than three times higher than the crash rate of single engined tri-jet,
the argument fails.) The safety of twins with one engine out should be
considered (I read in a discussion of light planes that the second
engine is there to take you from where the first engine fails to the
crash site.) The engines on a twin may be more reliable, as they are
less often used at full thrust.

The hypothesised extra safety of a twin over a tri-jet does not come
for free: you need 33% more available thrust on the twin to allow a
single-engine-out takeoff.

Disclaimer: I know something about probability. I know very little
about airplanes.

Michael W.