Re: 2 engines vs 4 engines planes

From:         drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Organization: Boeing Commercial Airplane Group
Date:         01 Nov 93 14:05:08 PST
References:   1 2
Followups:    1 2
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In article <airliners.1993.679@ohare.chicago.com>,
Pete Coe <ditka!sgiblab!uunet.UU.NET!ucsd!frosty.rational.com!pete> wrote:
>acm@SkyNet.GUN.de (Dietmar Hanke) writes:
>
>>Is it possible to fly a B-747 when 2 of the 4 engines failed? Was it even 
>>possible when both engines on one wing woudn't work anymore?
>
>Hey! 747's have been known to fly with no engines.  There was that British
>Airways 747 that flew into a volcanic ash cloud, and for 30 minutes or so
>became the world's heaviest glider.   
>
>That incident, and others (e.g. Eastern Tristar losing all engines due to 
>incorrect maintenance), leads me to think that as far as engines go, two 
>is never enough.  The statisticians can argue the point as long as they 
>like, but the fact is that more has to be safer.

Your own anecdotes show that no matter how many engines the airplane had,
it would not have been sufficient.  As many of you may recall, when an
airplane flies through a cloud of volcanic ash (which does not show up on
radar by the way), it takes all of the engines it has with it through that
cloud, none are excluded.  I might also point out that a faulty maintenance
procedure affects all of the engines, again regardless of their number.

To throw a bit more light on the commercial aviations concern with safety,
I would like to mention that many airlines immediately replace one of the
engines on a brand new airplane to avoid problems such as mentioned above.

A bit of corporate propaganda here, the first person to meet the BA 747
(which landed in Manila, I believe) was the Boeing service engineering
representative.

>  I doubt that the two
>above incidents would be covered by ETOPS regulations, because they
>were not a fault of the engines, or airframe design.  But in both
>cases, the flight crew only managed to save the day because they
>could get _some_ power from _all_ the engines (as opposed to all 
>the power from one of the engines). If either had been a twin, we
>would have lost passengers to another 'unfortunate incident'.

No, those two incidents could not have been covered by ETOPS because ETOPS
is concerned with Extended range Twin engine OPerationS.  ETOPS will not
protect you from volcanic ash (there is an industry wide effort to track
and report volcanic ash clounds, but their results have not been published
yet, as far as I know).  It will protect you from bad maintenance, however.

Also, I fail to see how the absolute number of engines mounted on the
airframe would have prevented those scenarios.  The 747 carries 4.  Only
the An-225 has more (6).  Your understanding of the hydraulic systems is
somewhat less than desireable.  All current production engines windmill and
thereby produce standby hydraulic power.  The number of hydraulic pumps on
the airplane is driven by power requirements and fault-tree analysis, it is
not some fortunate accident that the 747 was able to generate enough
hydraulic power to maintain control until the flight crew could restart the
engines - it was designed that way.  It is a regulatory requirement to
design that way.

>At the time of the BA 747's little problem, my father was project manager
>for 747's at BA.  Up to that incident BA had been considering removing the
>air turbines from the planes as they were not used.  At least that idea
>got canned.  Incidentally, that plane has never been the same since.

The 747 has never had ram air turbines.  The system is not designed to need
one.  What exactly do you mean by 'that plane has never been the same
since'?  

>Sorry about the rambling.  I just think twin engined aircraft are a bad
>idea, 

Yes, I gathered that.

>and while I consider them acceptable for short/medium haul routes,
>I think that Long over water ones are just an accident waiting to happen.

Over the entire history of turbine powered airliners no twin engined
aircraft has had a accident stemming from both engines shutting down for
two unrelated reasons.  Ever.  I pray it never will happen, but I think
many of the worst years are behind us.  Things only seem to be getting
better, ie, more reliable.

>The 777 gives me nightmares.  How many people do we have to kill before
>the airlines stop this crazy quest.

It might be useful to ask how many have already been killed in the quest
for ETOPS.  

The answer is none.  Not a single person.

>  Although I am myself a professional
>in the aviation industry, I consider myself to be well informed.

I would dispute that.

>  I have
>also clocked up well over 500,000 miles in the air.  In that time I have
>had one engine lost to a bird strike (Conway), two in flight precautionary
>shut down's (RB211's), and one aborted take off (RB211 again) due to engine
>malfunction.  I don't know how atypical these numbers are, but the incidents
>have been frequent enough for me to actively avoid the 767 on trans-atlantic
>flights.  Now my statistics all end up being British engines, but that is 
>because I usually fly B.A.  I am sure the American manufacturer's figures
>will be equivalent.

It is difficult to say how typical your experiences might be without some
idea as to how many takeoffs and landings that putative 500,000 miles might
encompass.  Industry data are typically generated by hours or by cycles
(takeoff and landing being a single cycle).  But anecdotal data are no
substitute for a serious study of the issues.

>Rant mode off.
>
>--
>
>-- Pete Coe
>-- Rational
>-- Object-Oriented Products


-- 
Terry
drinkard@bcstec.boeing.com
"Anyone who thinks they can hold the company responsible for what I say has
more lawyers than sense."