Re: What's in a model number? (MD-95 Structural Specs)

From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
Date:         16 Nov 95 03:48:40 
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>OK, so where's the new model?  Airbus & Boeing would consider this kind of
>change at best a new series number.  For these manufacturers a change in
>model number indicates pretty major differences in configuration,
>particularly Boeing where there isn't any comonality in engine configuration
>between models.

I'm not sure I'd use Boeing as a paragon of logic in model numbers, at
least not at too fine a level, and considering engines in particular
can get to be a bit silly.  Consider the following:

    Despite different designations, a 757-222 and 757-232 are quite
    similar, just built for different airlines (United and Delta,
    respectively).  Same general config, same PW2037 engines, etc.

    The more similarly numbered 757-222 and 757-223 are considerably
    more divergent, since the latter (built for American) has Rolls-
    Royce RB.211-535 engines.

    You can't tell which engine a 757-24A(PF) has, since UPS switched
    from the PW2040 on the earlier ones to the RB.211-535 on the later
    ones, but they're all the same model.

I don't really mean to pick on the 757 here -- there are lots of
examples of Boeing designations that don't uniquely identify the
engine (a 747-238B could be either JT9D or RB.211, a 767-332 could
be either CF6-80A2 or PW4060, ad nauseum).

I don't quite understand your comment "there isn't any comonality in
engine configuration between models" with regard to Boeing.  If you
mean different series within a type, the 737-300, -400, and -500, all
of which can use the same CFM56-3C1 engine (amongst other variants)
contradict your point.  Across types, the JT8D-15 and -17 are used on
both the 727-200 Advanced and 737-200 Advanced.  The newer 767s and
the 747-400 even share the same nacelle and pylon for engines from
the same manufacturer.  Doesn't seem to me like engines have much to
do with it.

Airbus designations do encode the engine choice in a fairly reasonable
manner.  For example, an A320-211 has CFM56 engines while an A320-231
has IAE V2500 engines.  An A320-232 also has V2500s, albeit a different
model, hence the different designations.

>I'd have thought changes like the removing the ventral airstair and improving
>the avionics didn't even justify a change in series number.

There's considerably more to the MD-95 than that -- new engines, many
revised and updated systems from the MD-90, etc.  I'm not sure it's
all that much less of a change than the MD-11 or 747-400 were compared
to their predecessors.

>Is there any particular reason why MDD didn't take the DC-9-30 series
>or one of the MD-80s and re-engine it?

Because a simple re-engined DC-9-30 wouldn't have had all the other
improvements that the MD-95 has besides engines, while a re-engined
MD-80 (absent any other changes) would have been too big and too
expensive for the market at which the MD-95 is targeted.

--
Karl Swartz	|Home	kls@chicago.com
		|Work	kls@slac.stanford.edu
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Moderator of sci.aeronautics.airliners -- Unix/network work pays the bills