Re: 2 engines vs 4 engines planes

From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works
Date:         30 Oct 93 22:33:48 PDT
References:   1 2
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Pete Coe writes:
>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 fact is that more has to be
>safer.

The fact is that in neither of the cases you cite is there any great
reason to beleive that more engines would have been better.  Both were
common mode failures, and except for a stroke of luck, such a case by
definition isn't a function of number of engines.

>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.

The pedants might point out that neither would have been covered by
ETOPS regs since neither involved twins.  Beyond that, the Eastern
flight wouldn't have been ETOPS even if it was a twin since if memory
serves it was operating from Miami to San Juan.

>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).

I don't know about the BA flight, but this simply isn't true for the
Eastern incident -- they limped back, and by the time they were within
sight of land again they only had the #2 engine.  In another case much
like the BA 747, involving a KLM 747-400 and again a volcano, they got
one, maybe two engines fired up to bring them back.

>If either had been a twin, we would have lost passengers to another
>'unfortunate incident'.

If the Eastern flight had been a twin subject to ETOPS regulations,
the incident probably would not have occurred at all because the regs
would prohibit the same mechanic from doing the maintenance on all of
the engines, thus removing the common cause.

>I have also clocked up well over 500,000 miles in the air.  In that
>time I have had [3 engines lost and an aborted take off].  I don't
>know how atypical these numbers are ...

I've probably got around 750,000 miles behind me.  No engine problems
after push back of any sort in all those miles, though plenty of
problems that were dealt with at the gate.  I don't know how typical
or not these numbers are either.

>Now my statistics all end up being British engines ...

Only about a half-dozen of my flights have been with British engines,
but the failure rates just aren't that different.

--
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