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The ultimate guide to the DC-9 family.
Author: David Hingtgen
Submitted by: David Hingtgen   Date: 04-02-2010
Comments: (5)  

Ultimate DC-9/MD-80/MD-90/MD-95 Guide

By David Hingtgen

Unlike the 707 and DC-10 guides, this one will go in chronological order. Roughly. It's the easiest way I think, due to how various features were introduced. Also, due to how many times it will be mentioned, the wing/body fairing will henceforth be referred to as the "WBF".



DC-9-10

Much like "MD-80" actually refers to a group of planes (MD-81/82/83/87/88) and there is no specific "MD-80","DC-9-10" also refers to a group of planes--the DC-9-11, -12, -14, and -15.All -11's and -12's have been converted to -14's and -15's by now. They are all externally identical, max takeoff weight is the only difference.Various cargo versions also exist, all of which are externally identical.

The original DC-9 type. 104ft, 5in long with an 89ft, 5in wingspan.Being the first variant, not much to say about it until we compare it to the -30, but here's few things.

Most of the DC-9-10's unique features involve the wing. First, are the leading edge fences, often called the potato chip fairings. (Possibly potato *crisp* fairings in the UK). All members of the DC-9 family have a long vortilon under the wing, covering most of the chord length, but only the -10 series has a separate leading edge fence that also goes up onto the upper surface. Visible in the photo above, it's just below and to the left of the X in the registry number. The large vortilon is just below the small leading edge fence. BTW, vortilon=VORtex-generaTIng pyLON.

Second, the wing has no high-lift devices on the leading edge. No slats, kruger flaps, droops, etc.

Third, the wing trailing edge is not straight on the series 10.Just a bit inboard of the tip, the trailing edge sweep angle increases. Hard to see from many angles, but this shows it well enough:

(red line added to help spot trailing edge kink)

Fourth, unlike all other DC-9's, the horizontal stabilizers have no anhedral, they are perfectly flat and level. Just trust me on this.

Though most people believe the DC-9's to have double-slotted flaps, the inboard-most sections are actually triple-slotted. See here:

(as on many planes, the upper-most slot is very thin and hard to see, it's little more than a black line--the "obvious" slot is the second one, and the inboard section has a second obvious slot, the third slot overall)

While most DC-9-10's only have a single overwing exit on each side, some have two. (Notably Hawaiian's) For those with only a single overwing exit, it is in the same location as the forward of the two overwing exits on any other DC-9.

Although it is a VERY common option, not all DC-9's have ventral stairs.Especially early DC-9-10's. Most -10's don't have them, nearly all -30's have them, all -40's and -50's have them.

Finally, all DC-9's except those built as freighters have an integral set of retractable stairs under the L1 door:



DC-9-31 and DC-9-32

(The -20 was far from the next version after the -10, chronologically, the -31 was the 2nd major variant)

Anyways, the DC-9-31 will establish a new "standard" and will be the reference from which most all other variants will be compared to. But first, we have to establish how it's different from the -10.

The most obvious difference is the length.It is not however, a simple stretch. DC-9 fuselage frames are 19 inches long. All stretches/shrinks are multiples of that. Except for DC-9-10 vs. DC-9-30. It's 179 inches. That doesn't even work out nicely to "something and-a-half" frames. It's 9.421 frames.9 of the frames are explained easily enough. 6 frames added ahead of the wing, 3 frames added aft of the wing. So what's the .421 frames?That is where MDC actually rearranged the back end of the plane.If you look at a DC-9-10 and a DC-9-30/40/50, you will notice the engine nacelles do not match with respect to their relative location to the windows.In simpler terms, the engines are actually mounted in a different location, regardless of fuselage length.

.421 frames is 8 inches, which is how far aft the pylons were effectively moved. On a DC-9-10, looking perfectly from the side, the nacelle "covers up" about 1.5 windows. On a -30/40/50, the nacelle only covers up 1 window. But the main aft fuselage stretch (3 frames) occurs right behind the wing, nowhere near the pylons nor aft windows. So what happened?The whole aft fuselage was effectively re-arranged. Basically, MDC inserted something less than half a frame aft of where the fuselage starts to taper. They actually added a frame that is not a simple tube and does not match any other fuselage frame. It's 8 inches long, and is basically a spacer, moving the tail and pylons aft relative to the main cabin. It's quite clear in a good drawing of DC-9 fuselage frames/stations that the pylons of a -30/40/50 are mounted about one-half frame aft of the cabin itself, compared to a -10. Thus, a -30's nacelles only covers up 1 window, instead of 1.5

Look here and compare the locations of the nacelles to the aft window:


DC-9-10, covers about 1/2 of the last visible window


DC-9-30, doesn't cover any part of the last visible window at all.

No amount of fuselage stretching or shrinking in the main cabin would affect the location of the nacelles/pylons to the windows. (And it's not the angle of the photo) Only rearranging the area immediately between the pylon mounts and the aft-most window would do that. And it's obviously less than a full frame. Thus, the location of the 8-inch long, .421-frame segment in a DC-9-30.This extra .421 of a frame, and the resulting pylon/cabin re-alignment is kept throughout all future DC-9's, MD-80's, etc.It is basically a permanent change to the basic physical arrangement of the plane's aft section, not a mere "stretch". Also, any DC-9 longer than a -10 always has both overwing exits per side.

And all that was just to explain the stretch!Now on to the other changes from a -10 to a -30.

First, a quick and easy change, to make up for the last one. Stabilizer anhedral. The -31, and all future models, have the stabilizers angled down 3 degrees from horizontal.

Next, wing changes.

DC-9-30's add leading edge slats. It is not merely replacing the DC-9-10's leading edge with a moving one. The leading edge is actually extended forward, increasing the chord. But not parallel to the original leading edge, that'd be too simple. It was increased most at the root, tapering to no extension near the tip. That means the sweep angle changed. Have a look:

The new leading edge is also noticeably sharper in cross-section. DC-9-10's have rather blunt leading edges. Also, the span was increased at the tip. But of course, it was more than a simple tip extension. There was a span increase just outboard of the ailerons, and the tip was reshaped. It was flared, very similar to how a 707-320's "straight" tip was replaced by the -320B's triangular tip.The DC-9-10 to DC-9-30 tip had an almost identical shape change.Also, the trailing edge kink near the tip of the -10 was eliminated. Wingtip and kink changes easily visible in the drawing above.Finally, the leading edge fence was removed.(but not the underwing vortilon)

Also of note:due to the increased chord at the wing root due to the slats, the WBF (wing-body fairing) only extends about 1 window ahead of the wing, as opposed to 2 windows ahead in the -10. There was no change to the fairing's shape/size, it is merely the larger wing covering up a bit more of the WBF.Overall DC-9-30 stats:119ft, 4in long with a 93ft, 4in wingspan. DC-9-31 and -32 vary only in weights, engine power, etc. There are also cargo variants of the -32, including the -32CF and -32F, as well as customized -32's built for the US Navy and Air Force, being C-9's and VC-9's respectively. Finally, -31's can (and have) been converted into -32's, as well as -32's into -31's.



DC-9-40

Chronological order, remember?

There is only one version of the -40, the -41.It was designed for SAS, and only purchased by SAS and TDA (Toa Domestic Airlines, later JAS). SAS generally kept theirs, but most of TDA's eventually migrated to the USA.

The first thing I'm going to talk about are the changes made to the DC-9-40's wings. For basic aerodynamic reasons, the wing incidence was increased by 1.25 degrees. That in itself is impossible to see, but it did result in needing a new WBF (wing body fairing).

To compare,we'll first look at the typical, earlier DC-9 WBF:

Of the three photos above, the top two are DC-9-10's and the bottom is a DC-9-32. Note the bare metal area in the center of the WBF. That is because unlike most airliners, there is not a single large WBF covering the plane's belly. There are two distinct WBF's. One for the left wing, and one for the right wing. They are somewhat bean-shaped when viewed from directly below. They do not touch, and the bare metal area between them isthe lower fuselage skin itself. Simply put, the WBF on a DC-9-10, -31, and -32 does not cover the belly. Almost, but not quite. Also, the WBF's are perfectly even with the bottom of the belly skin. They do not project below the fuselage itself at all, they are flush with it at their lowest point.

Now, on the -40, the larger WBF required by the increased wing incidence DOES cover the belly. There is one large WBF, instead of one for each wing. Also, it is MUCH deeper and obviously covers and extends well below the fuselage itself. Here's some photos of the newer, larger WBF:

The top and bottom photos show it best I think, that it is much deeper and not separated into two halves. That is the new, larger WBF, and you should learn to recognize it.

Next, the well-known difference of a DC-9-30 and a DC-9-40, the fuselage stretch. A DC-9-40 is 4 frames longer than a -30.2 frames added ahead of the wing, and 2 frames behind. With DC-9 frames being 19 inches, that's 38 inches per plug, 76 inches overall. Quite hard to tell from a -30, even with a side view. Count windows to be sure. From this point on, all stretch* DC-9 variants have the one-piece, deep WBF. In summary, a DC-9-40 is a DC-9-30 with a new WBF and a fuselage stretch. Total length is 125ft, 7in.



DC-9-33 and DC-9-34

Well, technically there are no DC-9-33's, they all have cargo doors and are thus -33F's, -33CF's, or -33RC's. If you see a DC-9 without a cargo door, it's not a -33. The -33 and -34 are basically the high-weight long-range versions of the DC-9-30. Normally that's not enough to merit being included as a separate entry, but the -33 and -34 are physically different from the -31/-32. Since they were developed after the DC-9-40, they have the -40's increased wing incidence and new WBF. Thus, they are visibly physically different from the -31 and -32, and much deserving of a separate entry.

So yes, it is possible the tell most of the DC-9-30's apart. If has the two-piece, shallow WBF it must be a -31 or a -32. DC-9-31's never have cargo doors, so if it has one and a two-piece WBF it must be a -32. If it has the deep WBF but lacks a cargo door, it must be a -34. If it has a deep WBF and a cargo door, it is most likely a -33, but there are some -34's with a cargo door. Overall though, the overwhelming majority of DC-9-30's are -31's and -32's.

Compared percentages of total DC-9-30 production, including the ones built for the US military which are basically -32's:

DC-9-31: 36.6%

DC-9-32: 58.3%

DC-9-33: 3.3%

DC-9-34: 1.8%

DC-9-33's exist to be freighters using their higher weights, while DC-9-34's use their higher weights for extra fuel and are basically the DC-9-30ER. Ozark certainly used them as such, for flights to the US West coast from St. Louis.

The -33 and -34's came about very late in the DC-9's development history. The -34's in particular were among the very last DC-9's ever built, with a large percentage of them being produced even after MD-80 production had begun. There was a considerable gap between the -33 and -34 being introduced, but since they are basically the freighter and pax versions of the same design, they are here together. Also, I wanted the "early" and "late" -30's to be close to each other in the guide, with the -40 being the link between them, since a -33/34 is basically a -31/32 with the -40's aerodynamic improvements but not the -40's stretch.(Otherwise the -33 would be discussed after the -20 and the -34 after the -50 and the guide's order would be more confusing than it already is)

Bonus! Early and late belly comparison pics! I only had to go through 5,500 photos (about 1,000 a night) to find a "matching" set of the right planes at the same angle, distance, etc.-31 on left, -51 on right.



DC-9-20

Much like the DC-9-40, there is only one version, the DC-9-21, as well as being designed for SAS. Only ordered by SAS. Basically it is a -10 with a -30's wings. But despite being designed after the -40, it does not have the later style of WBF, so it's like a short -31 and not a short -34. It has the tailplane anhedral of a -30. Same length as a -10 (104ft, 5in) with the same span as a -30 (93ft, 4in).



DC-9-50

Longest of the "standard" DC-9's, but actually older than the -34. Again, only one variant, the -51. It has two changes from a -40: It is longer, and it has nose strakes.

Compared to a -40 it is 5 frames longer overall. 3 frames ahead of the wing, and 2 frames aft of the wing.

Compared to a -30, it is 9 frames longer overall. 5 frames ahead of the wing, and 4 frames after of the wing.

Overall length is 133ft, 7in.

Finally, it has nose strakes, mounted above and to the rear of the nose gear. Highlighted by the sun in the photo above. Any DC-9, MD-80, etc that has nose strakes has ones that are identical to the DC-9-50 which introduced them. Here's a view showing how far they project into the airstream:

Now for a feature common to many DC-9's, but best introduced here.Originally, DC-9 thrust reversers were mounted so the reverser buckets deployed perfectly vertical. But this caused some minor problems, so Air Canada came up with a modification to angle the direction of the exhaust. Upper reverser bucket angled in, lower bucket angled out. 17 degrees. The modification was very popular, and many DC-9's were retrofitted, and even McDonnell Douglas incorporated it into the production line, starting with the DC-9-50. So all -50's should have the angled buckets, and any DC-9 of any series built after the -50 started production should have them as well. This would include all -34's. Also a nice view of the triple-slotted inboard flap section on the one with the angled reverser.

You can easily tell if a DC-9 has straight or angled reversers even when they're not in use, just look from behind to see how the reverser actuator tracks (the "spikes" on the exhaust) are mounted--evenly horizontal, or with the outboard one noticeably higher than the inboard one. Of course, this DC-9 seems to have a straight reverser on the left engine, and an angled one on the right. Engine swaps, etc. So there's really no guarantee of any particular DC-9 having any particular reverser type after it leaves the factory.

Yet another "common on many, but introduced on the -50" thing is the nosewheel debris deflector. Fairly prominent on MD-80's, but it's on some DC-9's as well. Same as the reversers, it's on all -50's and any DC-9 built after the -50 entered production. Here's a collage of pics showing nose wheels without and with the deflector installed.



Miscellaneous DC-9 features

These are just things I'd like to point out about the DC-9 before we get to the MD-80, not really particular to any variant.

There are two door sizes for the main gallery door, the forward door on the right side. The small one is vastly more common, if you see a DC-9 with the larger one, it's probably ex-Eastern.

Small door, top level with top of passenger windows:


Large door, top well above top of passenger windows:


And a comparison pic using NW. You can find every single DC-9 variation that exists in NW's fleet.:) Small door on left, large on right.


Another variation is the pressure relief vent, aka the dump vent. It's used to vent the last remaining bit of pressurized air out to equalize pressure just before landing. Thus you always see it open when they're on approach. Most model manufacturers like to print this as a black dot with a line through it, but it's not always round, and trying to print such a large hole has never worked well. It's almost utterly invisible when closed, which is how it is most of the time anyways.

Round, open:


Square, open:

From what I can tell, the square version is a modification only NW does.

Here is an extreme close-up of an MD-80's round one, hopefully proving once and for all that it is not some big black "spot" on the fuselage, it is in fact a moving part and what you're really seeing is the hole it creates when it's opened. It rotates inward 90 degrees

Next, just a little thing. Fairing for the rudder tab actuator. Starboard side only, very bottom of the rudder. DC-9 rudders are normally hydraulically controlled, but the manual reversion requires the tab.


Another little thing, the probe for the artificial rudder feel, on the fin leading edge.


Finally, I'm going to go over all the various intakes and vents at the back end of the plane, as they are the subject of many questions, and even more wrong answers.

First, the intake at the base of the fin leading edge. It is the intake for the air conditioning packs. NOT THE APU.

So where is the APU intake? It's a small slot directly in front of where the ventral air stairs are, on the underside.


And so the next question would be where's the APU exhaust?Starboard side, the round opening just above the pylon with the soot stains streaking out from it. It does project outwards, it is not flush with the fuselage.

Thus leading into the next question--so what's the other opening? That would be the starboard exhaust for the a/c packs. There is also a matching one on the port side. (But there is only one APU exhaust, starboard).

Large photo showing many of the features previously discussed, also shows how the APU exhaust sticks out.

The louvers/grills on the rear fuselage seem to be unconnected to any specific system, as far as I can tell they are simply there to passively vent the unpressurized section of the rear fuselage, around the rear stairs.



MD-81, MD-82, and MD-83

The second generation of the DC-9 family. There is no specific plane called the MD-80. Much like "DC-9-30" means the -31, -32, 33, and -34,"MD-80" means the MD-81, -82,- 83, -87, and -88. Also, officially, technically, legally, etc, MD-80 is only a marketing term and not the true name on the type certificate nor builder's plate. The MD-81, MD-82, MD-83, and MD-87 are officially called the DC-9-81, DC-9-82, DC-9-83, and DC-9-87. The MD-88 is officially the MD-88 however. In formal documents like the FAA type certificate, it is usually written like "DC-9-81 (MD-81)" without the quotes. Because just about everyone calls it the MD-80, not the DC-9-80. Though AA seems quite fond of calling it by the early marketing name of DC-9 Super 80, omitting the DC-9 part. And to this day, the 3-letter code for the MD-80 is S80. The MD-81, -82, and -83 are similar enough to each other that they are often converted into other types. MD-81's have been changed into -82's, -82's into -81's, and -82's into -83's.

Now that the naming part is out of the way, on to physical differences. Overall MD-80's will be compared to the DC-9-50. They incorporate many of the -50's features, such as nose strakes, angled reversers, and nose wheel deflectors. They generally have the forward air stairs and always have the rear air stairs.

First, the simplest part, the fuselage stretch. Compared to a DC-9-50, and MD-80 has 8 frames (12ft, 8in) inches) added ahead of the wing, and 1 frame (1ft, 7 in) aft of the wing.

The pylons, while different from earlier DC-9's due to the new engines, are mounted at the same location on the fuselage, based on the pylon leading edge. While we're on the subject, DC-9 and MD-80 engines are mounted so they're angled 3 degrees up at the intake. The pylons of the MD-80 are very similar to the DC-9's, they are mainly narrower to bring the engines in closer to the fuselage.

The engines are obviously new and larger. JT8D-200's. Basically a re-fanned version of the DC-9's JT8D. Note that the reversers on MD-80 engines are only angled 15 degrees, as opposed to a DC-9's 17 degrees. However, ALL MD-80's have angled reversers.

Also, the engine nacelles have small strakes on them, very similar to the nose strakes.


To accommodate the stretch, with a fuselage nearly 50% larger than the original DC-9's, the wing needed to be enlarged. Basically, McDonnell Douglas took the DC-9-30's wing, sliced them off the fuselage, sliced off the wingtips, added root extensions to the fuselage, added the main part of the wing back onto the new roots, added parallel-chord sections near the wingtips, then attached the original wingtips back on to the new out wing sections, and finally revised the trailing edge flaps to make it all work. In other words, buried within the MD-80's wing is an almost complete original DC-9-30 wing, you just have to know where to look. Illustrations and explanations to follow.

First, the wing root extensions. 5ft 3in each, for a 10ft 6in increase in overall span.

Second, the wingtip extensions. Normally, if you removed the wingtips, then added to the wingspan, the wingtips would no longer fit back on, due to the wing's taper/sweep. The tip would be too large, since the extended wing would now taper further to a narrower chord. However, for the MD-80, the wingtip extension was of parallel-chord design. In other words, all of the ribs in the new part were identical, and the wing's tapering stopped. Thus the original wingtip still fit on, despite being further outboard than it originally was. Or, described in yet another way--going from root to tip, for any plane with a swept/tapered wing, each wing rib is smaller than the previous one. But for the MD-80, the last rib before the tip repeats itself a few times, thus the original wingtip still fits. The new section is aligned with the leading edge and maintains the LE sweep, and thus there is a kink in the trailing edge, since the wing stops tapering and can no longer match the trailing edge sweep of the rest of the wing. Each tip extension is 2ft, added 4ft to the overall span.


You can see quite clearly where the new section was added in this pic:


Here is a nice diagram showing the root and tip extensions. The vast majority of the center of the wing, as well as the very tip itself, are the DC-9-30's wing. The extension near the tip, and the root, are the new parts. Trailing edge flaps were modified, most notably near the root. Double-slotted everywhere, no triple-slotted section. MD-80 span is 107ft, 10in.

Well, a modified wing means a modified WBF!Generally very similar in shape and design to the later, deeper WBF of the DC-9, but slightly larger in all dimensions. There is also a new section added to it at the leading edge, very similar to the leading edge WBF added to late DC-10's. It goes well forward of the leading edge, up and over the wing. See here:

A heavier plane with repositioned wing roots means new main landing gear. Very similar to the DC-9's, just a bit larger and stronger. Semi-random fact: DC-9 main gears do not retract perfectly inwards. They swing forward when retracting, with the wheels ending up about 20 inches forward of where they are when lowered. However, an MD-80's gears do move at a perfect 90 degree angle. DC-9 main tires are normally 40 inches in diameter on 18 inch rims, while an MD-80's are 44.5 inches on 20 inch rims. Both have 26in nose tires.

Next, tail changes. Horizontal stabilizer span was increased with a simple root extension of 20 inches on each stab.

Here's a photo of the real thing, you should be able to see the join where the extension meets the original stab, look between the red lines. Easiest to see near the leading edge.


The extension is fairly hard to see at any distance, but there is another thing that is much easier to see.

DC-9 elevators are not themselves controlled directly by the control cables. It is the control tabs mounted on the elevator trailing edge that actually move the elevator, and those tabs are what are actually controlled by the pilot. They are an aerodynamic control--think of them like an elevator for the elevator. Pilot pulls back on the yoke, elevator tab moves down, which causes aerodynamic forces to move the elevator up. Rudder is the same way--rudder tab left, rudder moves right. Control tabs always move opposite of the control surface.

Now, a DC-9 has two tabs on each elevator. The inner one is the control tab, linked directly to the yoke. The outer one is a geared tab--when the elevator moves, the geared tab moves as well, to help the control tab. So it always moves with the control tab, but is not connected to it in any way. Geared tabs are also known as servo tabs. (Unless they make it harder to move the control surface instead of easier, at which point they are anti-servo tabs)See here:


MD-80's add a third tab, the anti-float tab.On approach, MD-80 control tabs tend to "float" upwards, which moves the elevator down. Hard to flare the plane when the elevator wants to go down. So, McDonnell Douglas added a third tab that fights its tendency to float, thus, the anti-float tab. See here:

Also note the fairing over the control tab's actuator. DC-9's have it too.

So that is honestly about the easiest way to tell DC-9 and MD-80 stabs apart--look for a 3rd tab.

While we're talking about tabs, might as well cover the last control surface. DC-9 and MD-80 ailerons are controlled the same way as the rudder and elevators. A small tab, with a fairly visible fairing covering the actuator. All members of the DC-9 family have identical ailerons.

Tab moves down, aileron moves up. The small tab is a trim tab, not a geared tab. Like the rudder,it is normally hydraulically powered but uses tabs for manual reversion.

And the last thing associated with the tail, the enlarged vertical stabilizer. The original DC-9 h.stab takes up the entire top of the v.stab, so with the MD-80's h.stabs being enlarged at the root, the top of the v.stab needed to be enlarged to accommodate them. Only the upper part of the v.stab was enlarged, and only near the leading edge, creating a kink in the fin's leading edge. It was enlarged JUST enough for the MD-80's larger stabilizer to fit.


Original DC-9 fin.


MD-80 fin with enlarged upper forward section.


Finally, the two types of tailcones. The original tailcone is the same as all DC-9's, and is generally called the pointed tailcone. Photos of that style are all throughout this guide. The newer type is longer and flat. It has several names, with screwdriver, flat, and beavertail being the most common. Here's a photo of it:

Now, the MD-81 was the original version of the MD-80. Your basic MD-80, 134 built. The MD-82 was introduced soon after the -81, and was the most popular version. It is basically an MD-81 with more powerful engines for higher weights. 575 built. The MD-83 was essentially an "MD-82ER", with a higher MGTOW, more power, and optional auxiliary fuel tanks.260 built.

And I can't tell any of them apart. The tailcone means nothing. Generally, MD-80's built from 1979 to 1987 had the pointed style, and those from 1988 to 1999 had the screwdriver style. But MD-81's, 82's, and -83's were all built during both periods. MD-81's were still being built as late as 1994. And the tailcones are changed all the time. The screwdriver style offers lower drag, and some airlines retrofitted their fleets for that reason. Others just got new tailcones to replace damaged ones. For an example on why you should never try to identify an MD-80 by its tailcone, let's follow a specific plane over a few years.

Here's N560AA in January 2001. Pointed tailcone.

Here's N560AA in July 2001. Screwdriver tailcone.

And here's N560AA in 2003. Pointed tailcone again.

Finally, MD-80's add deflectors to the mainwheels, to supplement the nosewheel deflector.(Bigger intakes means greater risk of ingestion) They are mounted behind the wheels, not in front. See here:



MD-87

Basically a short MD-80, but of course there's more changes than a simple fuselage shrink. Not very popular, despite its high performance. Only 75 built.

Might as well start with the fuselage change. It is most often described as having a fuselage equal to a DC-9-30 or DC-9-40, but it is in fact longer than both of those, and is closest to a DC-9-50. It is 13 frames shorter than an MD-80. 8 frames removed from ahead of the wings, 5 frames aft. Overall length 130ft, 5 inches. It is 2 frames shorter than a DC-9-50, but very differently proportioned. Compared to a DC-9-50, it is 2 frames longer ahead of the wing, but 4 frames shorter in the rear. So visually, it has a longer nose and shorter rear than a -50.

Next is the v.stab extension. The MD-87's fin is overall 10 inches taller than the standard MD-80's. It is not a simple, thin sheet of metal added on top, it is as thick as the rest of the v.stab and has he same contours when viewed from above. It is no different than if it was added to the bottom of the fin. Here's some comparison photos.


1st and 3rd planes are MD-87's, standard MD-80 in the middle.


This is an MD-90 but it shows well that the extension on top is as thick as the rest of the v.stab.


Also an MD-90, but shows how the extension is perfectly blended in to the top of the original v.stab near the front, but becomes visibly "added on" as you move to the rear, as there’s a groove.

Next, MD-87's always have the screwdriver tailcone. It was the plane that introduced it, and it was soon incorporated into the production line for all MD-80's.

MD-87's have very different strakes on the engine nacelles. Much longer, and moved forward, touching the intake lip.


This is the standard, short strake on MD-80's.


This is the much longer strake on an MD-87. Thanks to SAS for painting them a different color than the nacelle.


Another angle, showing how it curves around near the intake lip.

Finally, possibly the most nitpicky and subtle thing in this entire guide. (Even more so than early vs. late DC-9 WBF). The pylon root leading edge fairing!Up until the MD-87, MD-80 pylons simply attached directly to the fuselage, same as a DC-9's. But the MD-87 introduced several drag-reducing features, such as the screwdriver tailcone, and pylon fairing. Both of these features were soon incorporated into the production line, and thus all MD-80's built after 1988 have them, and all planes designed afterwards (MD-88, MD-90, etc) have them as well.


This is the standard MD-80 pylon without a fairing.


This is an MD-80 pylon with the fairing. It's a very tiny fairing, blending the leading edge of the pylon into the fuselage. Here are some more pics showing it up close. It's much easier to see if the pylons are bare metal (as they often are) since the fairing is composite and must be painted.

Yes, I could only ever find pics of the #2 pylon's leading edge.

Finally, MD-87's introduce the low-drag flap hinge fairings. However, even I cannot tell them apart. I do believe it is nothing more than "they fit together slightly better, reducing gaps between parts by .02 inches". (MDC likes to do that, simply reduce the gaps between parts to reduce drag--LOTS of that occurred on the MD-11's drag-reduction program)



MD-88


Essentially an MD-82 with an upgraded cockpit. Physically identical to the MD-81/82/83. Always has all of the drag-reducing features the MD-87 introduced, like the screwdriver tailcone and pylon fairing. 150 built in all, 125 of them were for Delta. First eight were built as -82's with pointed tailcones, then modified into -88's.



MD-90

Two versions, the MD-90-30 and the MD-90-30ER.Strangely, it seems MD90-30 is the preferred designation (no hyphen between MD and 90) though the type certificate still says MD-90-30.ER's simply add a small auxiliary fuel tank along with a higher MGTOW. 114 MD-90's built total. Generally assume the MD-90 is based on the MD-83, and uses MD-83 parts where possible.

Simple changes first. Fuselage stretch.3 frames ahead of the wing compared to an MD-80. Overall length 152ft, 7in.

Next, new main wheels for the heavier weight. 1 inch larger than an MD-80's.

Easily seen is their MD-87-style v.stab extension.

The most obvious change is the new engines. IAE V2525's, quite similar to an A320's. MD-90-30ER's have the higher power V2528's.

Note the new strake on the new engines. FYI, the MD-90 prototype had a longer strake that went all the way to the trailing edge of the exhaust.

New engines means new pylons, and the MD-90's are very different from previous ones. They are very wide at the rear, the trailing edge is straight instead of angled, and their cross section is very much like a wing's. They curve up noticeably at the trailing edge. Also, no pylon fairing at the leading edge. Some photos:

This last photo also shows another MD-90 feature well--the APU is new, and has a new exhaust. On the DC-9 and MD-80, the round APU exhaust was directly below the square A/C exhaust--but on the MD-90, it has been moved up and aft, to be behind the A/C exhaust.

Continuing with intakes/vents, MD-90's had quite a few systems/accessories changed from the MD-80. This lead to new intakes on the starboard fuselage. First, the one just above the nose gear door. Thankfully, this one is often outlined in red by the airlines themselves, so I don't have to draw my red lines to point it out.

And the one a bit ahead of the WBF, which isn't outlined by the airlines:

And in what is certainly the neatest feature of any Douglas twin-jet, the MD-90's extra set of elevators.

The MD-90's weight/balance and proportions are such that the elevators may not be enough for pitch control in all conditions, so the MD-90's pylons have their own set of elevators on the trailing edge. They work in conjunction with the elevators, but only activate when the elevators are nearing their limits of travel. The MD-90 prototype did not have these, but all production MD-90's do. You can also see the engine's exhaust cone deep within the nacelle in this photo, lower right. You can also see the exhaust guide vanes, as well as

the blades of the final turbinestage.

A small change was moving the forward overwing exit one frame forward. All other DC-9 family members have a one-window gap between overwing exit doors, but the MD-90 has a two-window gap.

Finally, MD-90's introduced a new type of flap hinge fairing. Here is a set of standard DC-9/MD-80-style flap hinge fairings:

Note the rounded bottom edges.

Here are the new style, with noticeable "prongs" projecting rearwards from the bottom:



MD-95

Basic (and only) version is the MD-95-30, MD-95-10 never came to be.

Quite the mix of parts, I'll start at the front and move back.

Forward fuselage: Basically a DC-9-30 with a 3 frame stretch ahead of the wing. That is the only fuselage stretch, so that makes it 1 frame shorter than a DC-9-40, but with different proportions. Almost identical to the 707-138 vs. 720 situation. A DC-9-40 is 1 frame shorter up front, but 2 frames longer in the back, than an MD-95. MD-95 front must be longer so as to counter the weight of the much larger engines.

Wings:basically a DC-9-34's wings, with the later WBF. However, the front of the WBF is modified. Somewhat like an MD-80's, but definitely a new design. More bloky, less blended.

It also has the new style flap hinges with the prongs.

It has the new APU exhaust location of the MD-90, but no new nose/fuselage intakes.

MD-95's do not have rear air stairs, and also do not have their APU intake in that area. Only member of the DC-9 family to not have the APU intake in front of where the stairs go. It has it "mirrored" from the exhaust--left side of rear fuselage, above the pylon. Only it's in front of the A/C exhaust instead of behind it. This is another "the airlines outline it in red so I don't have to" thing.

Moving up, the pylons are all-new, but resemble a DC-9's more than anything else.

The engines are obviously new, BMW/RR BMR715's. They also have a new, large strake.

Despite all-new engines and nacelles, the reversers are still angled like later DC-9's.

Moving up to the tail, it has the VERTICAL stabilizer of an MD-87/90, but the HORIZONTAL stabilizer of the DC-9-30. A lot of people think the entire tail is an MD-87/90's, but it has the smaller stabs and elevators of a DC-9.

And thus we come to the end of the plane, and the end of the DC-9 family. Of course, I have one last little thing to point out. A fairly rare option available on most members of the DC-9 family is a logo light on the wingtip. It looks like a little bump, just inboard of the trailing edge position lights.

Looking from the other side, you can see it's a light angled and slatted to illuminate the tail logo.


by David Hingtgen

Member Comments :

 comment by: FLYHY posted on 04-02-2010, comment #10080


Thank you so much for providing us with all of these details. I know this will become one of the most popular resources on airlinercafe.


 comment by: KLAS posted on 04-03-2010, comment #10085


VERY well done David! Many thanks for the subtle insights of this great aircraft!


 comment by: Kikiferret posted on 04-04-2010, comment #10087


Thank u very much for this new and very interesting guide David! keep up the great work!

Hope u can extend this Diesel niner / Mad Dog guide with the B717. Yeah, I know, different brand. But same origins.

Saludos


 comment by: sky303 posted on 04-05-2010, comment #10093


David, thanks for the time and effort to produce this article. I am writing a small newsletter article for my local club on Modeling the DC-9/MD series and will add this as a primary point of reference.


 comment by: Stein Meum posted on 04-17-2010, comment #10177


This is a truly excellent article! I have flown the DC-9/MD-80/MD-90 series for 16 years with SAS, and there were many new and unknown facts that I did not know of.
But some detail can be added: The Strut Doors are different on the DC-9 and MD. On the DC-9's, there was a small "trailing edge flap" on the door to help with gear retraction in case of engine failure. This was found unneccessary on the MD.

I flew the -21, -41 and -32F. The DC-9-21 was designed for short field operation on slippery runways. For this purpose it had the 2,5 degree increase in incidence, not the -41. The reason for the different -41 wing root fairing was because of the hydraulically operated, flap follow-up system controlled "Flap/Rudder Throw Limiter".
As the -9 was streched beyond the -30, there turned up a directional stability issue when flaps were extended from retracted to 15 degrees flap setting. Rudder throw was limited to 13 deg. either side. This system is unique for the -41 as the problem was solved by nose strakes on the -51 and -80. This system was located below the wing center section, forward of the wheel well. It protruded below the fuselage and therefore needed the different wing root fairing.
As for the MD-90, the pylon flaps only operated in case of stall recovery and was operated by the hydraulic power augmentation system. It only deflected downwards.

Stein