Re: Development of MD-11?

Date:         03 Feb 98 19:53:05 
From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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>Back when Boeing was going to build a new version of the 747, the engine
>maker suggested that it could use a mix of B777 size engines on the inner
>pylons and B747 size engines on the outer pylons.  Boeing insisted on a
>new engine of intermediate size.

For that particular example, it probably wouldn't have been very
practical anyway, at least with the GE90 and probably the others.  If
you've ever seen the pictures of GE's 747 testbed with a GE90 mounted
on the #2 pylon, you'll recall that even with a special pylon and
presumably a relatively light fuel load (not much wing droop), there
is very little clearance between the GE90 and the tarmac.

>It seemed to me that if an airline already operated B777's or Airbus
>A330's with big engines, and B747, B767, MD11, A300, A310 or anything
>else with engines of that size, then they could order a B747-500 or 600
>with engines which they already used and maintained, whereas with the
>intermediate sized engines they would have to stock a whole new set of
>spare parts.

That's fine if they happen to use the same airplanes in the same places,
but that may not be the case.  United uses their 747-400s across the
Pacific, whereas their 777s fly Atlantic and Latin routes.  If they
were to have a hypothetical 747-500/600 stuck in Sydney with a problem
in a PW4084, it wouldn't be much consolation that they hard parts and
expertise in London (and in the US).

For another example, look at Cathay Pacific.  They use 747s and A340s
for their really long range routes; the 777s are primarily for shorter,
high-density routes.

>One disadvantage of mixed engines is that I suppose all engine-out
>performance figures would have to be figured for the worst case, with
>one of the larger engines inoperative. This could result in a lower
>maximum take-off weight for the same airframe.

I'm not sure it's quite that clear.  The bigger engines would presumably
be mounted inboard for weight reasons (as you suggested, though moving
them outboard would solve the ground clearance problem) and, for engines
of the same thrust, an inboard engine failure isn't as serious as an
outboard engine failure due to the shorter lever arm resulting in less
yaw effect.  Obviously that doesn't hold if the thrust differential is
great enough, but it would take more analysis to say with confidence
that the failure of a larger inboard engine would be the worst case.

Karl Swartz	|Home
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