Re: ValuJet Order: What's Difference MD-95 and DC9-50?

From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
Date:         31 Oct 95 13:36:20 
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>what is the difference between the MD-80 and the MD-90, or for that
>matter, the MD-83 that TWA has been adding to its fleet recently?

I'll admit to being less familiar with the myriad DC-8 and DC-9
variations than I am with the Boeing and Airbus lines, but I think
I can come pretty close, despite some inconsistent references.

Here's a table with all the basic numbers, followed by some details.
Seating is for a typical, mixed-class configuration and maximum in
"sardine class".  MGTOW and engine choice are likewise for a typical
aircraft, and thrust is per engine.  Weights and thrust are in pounds,
range is in miles.

model      seats   w.span   length    MGTOW   engines   thrust  range
-----      -----   ------   ------    -----   -------   ------  -----
DC-9-31   103-110   93'5"   119'5"    98,000  JT8D-7    14,000  1,340
DC-9-32   103-110   93'5"   119'5"   108,000  JT8D-11   15,000
DC-9-51   122-135   93'5"   133'7"   121,000  JT8D-17   16,000  1,260
MD-81     142-155  107'10"  147'10"  140,000  JT8D-209  18,500  1,630
MD-82     142-155  107'10"  147'10"  149,500  JT8D-217  20,000  2,176
MD-83     142-155  107'10"  147'10"  160,000  JT8D-219  21,000  2,618
MD-87        -130  107'10"  119'1"   125,000  JT8D-217  20,000  2,405
MD-88     142-155  107'10"  147'10"  149,500  JT8D-217  20,000  2,176
MD-90-30  153-172  107'10"  152'7"   156,000  V2525-D5  25,000  2,610
MD-95-30  104-115   93'4"   122'5"   114,000  BR715     18,500  1,547

First, an MD-81 is properly a DC-9-81 since that's what's shown on
the certificate.  MD-81 is just a marketing name.  In contrast, the
certificate for the MD-88 shows it as such, though it's still a
DC-9 derivative.  For the -82 and -83, as I recall, the certificate
still shows DC-9-82 (83) but MD-82 (83) is a legal alias.  Finally,
the MD-90 is considered a new type -- the FAA decided that at some
point the plane had grown beyond being a DC-9.  Picking the MD-90
as the break was somewhat arbitrary, but it was clearly no longer a
DC-9 in their view.

The DC-9-81 (aka MD-81) grew out of the DC-9-50, with a 14' 3" fuselage
stretch and a 14' 5" increase in wingspan.  New JT8D-200 series engines,
a significant improvement over the older JT8Ds, provided from 2,500 to
5,000 lbs of additional thrust per engine (up to 21,000 lbs each) and
allowed an increase in MGTOW of 19,000 lbs to 140,000 lbs.

The MD-82 uses the higher thrust engines and a slight MGTOW increase
(149,500 lbs) for greater range and better hot-and-high performance.
With a glass cockpit and other interior improvements, the MD-82 became
the MD-88.

The MD-83 has structural improvements, allowing a 160,000 lbs MGTOW,
and greater fuel capacity, which combine to provide significantly
greater range of around 2,600 miles, vs. 1,600 for the MD-81.

The MD-87 is a shortened version, with a fuselage 4 inches shorter
than the DC-9-31, but the same wing and weights as the MD-81.  The
heavier engines, as compared to the DC-9-31, mean the CG and thus the
wings are further back.  Consequently, the rear doors were eliminated
and the tail height was increased.

The MD-90 started out as a re-engined and stretched MD-80.  The model
currently in production is the MD-90-30, the middle-sized version of
three MD-90 models originally defined.  It uses the MD-83 fuselage,
stretched with a 57 inch plug ahead of the wing to balance the heavier
V2500-series engines, and a strengthened MD-83 wing.  The taller tail
from the MD-87 is used, and the flight deck is based on that of the
MD-88.  At 156,000 lbs, MGTOW is only slightly shy of the MD-83.  The
V2525-D5 engines, identical except for packaging and accessories to
the V2500-A1 offered on the A320, provides 25,000 lbs of thrust for
normal operations, with a cockpit-selectable boost to 28,000 lbs for
hot-and-high operations.  McDonnell Douglas claims the MD-90 is the
quietest large jetliner, and the only one ready for proposed Stage 4
noise requirements.  Finally, the MD-90 incorporates a number of other
improvements, ranging from improved brakes to a vacuum lavatory system
with single-point servicing.

The MD-95 returns to the wing of the DC-9-30 and -50.  Fuselage length
is a bit greater than the DC-9-30 -- some info I have from MD claims
122' 5.4" (a 3' stretch) but with a seat count that's significantly
lower than the 129 claimed for ValuJet's order.  That may be due to
tighter pitch and minimal galley space on the VJ aircraft.  Two flight
deck designs were offered, a basic version similar to the MD-80/90 and
an advanced version with a small number of large flight displays,
providing a "clean" look not unlike the new Airbus products or the 777.
(Presumably ValuJet selected the basic design though the advanced one
is probably still available as an option for other customers.)  The
BMW Rolls-Royce BR715 engine, rated at 18,500 to 21,000 lbs of thrust,
was selected by ValuJet.  Pratt and Whitney's proposed MTFE (Mid Thrust
Family Engine) was also offered prior to launch, but with the launch
order using the BR715 all MD-95s will use that engine.  Finally, many
of the MD-90's improvements (brakes, lavatory, etc.) are incorporated
in the MD-95.


--
Karl Swartz	|Home	kls@chicago.com
		|Work	kls@slac.stanford.edu
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