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Build Process For The DC9, MD80, MD90, 717  
User currently offlinelexkid12300 From United States of America, joined Mar 2011, 88 posts, RR: 0
Posted (2 years 2 months 4 weeks 1 day 22 hours ago) and read 10537 times:

Hi guys,

I did a search but can't find a good answer to this one, so i'd like to see what you all think. I recently took several trips on a few of Delta's DC-9-50 aircraft, and also their MD-88 and MD-90. I've also flown on AirTran's 717 a couple of times, but noticed some differences from a passenger's perspective. Not really in terms of cockpit technology, but more so for the basic construction of each.

Firstly, i know that they're all pretty much related, but to what extent? It appears that the nose and fuselage cross sections of all three are the same, but were they all constructed using the SAME machinery and bonding techniques? I only ask because i have very sensitive sinus' and on most flights i usually have terrible sinus pressure causing my ears to pop. Comparing flights with similar flight times and altitudes, not once have my ears popped on the DC9, but they did on the MD88, MD90 and even more so on the 717. Why would this be? Could it have something to do with the "air-tight-ness" of the plane? Or with the pressurizing equipment? If so, did McDonnell Douglas and Boeing use different skin bonding methods between the years of 1975-1891 with the DC9 and those in the MD88, 90 and 717 in later years? I know Boeing made changes to this process with their 737 after the Aloha accident, which improved the bond of skin and fuselage. I just don't see why my sinus' wouldn't act up on the 35 year old DC9 yet really give me problems on the 10 year old 717. Could it be due to different skins? Did all three derivatives use the same skin?

Next, does anyone know if the chords and major structural parts of the fuselages of all three derivatives are the same? I found a document that gives the recommended amount of cycles aircraft should be allowed to fly in a lifetime, and the DC9 is given 100,000 cycles, with the MD80 only given 50,000 cycles, and for some reason the MD90 and 717 given 60,000 cycles. I understand the number for the DC9 is so high because it's been increased over time since introduction in '65, but why wouldn't they give the same number to the MD80, 90 and 717? Assuming the fuselages are built the same way, one should assume they'd last just as long. And note that the 100k number is given to all fuselage lengths of the DC9, thus length can't be a factor in this 100k estimate. Since the 717 is the same length as one of the DC9's i mean... Here is the document- see p345.
http://www.faa.gov/regulations_polic...ttees/arac/media/tae/TAE_AA_T6.pdf

And lastly, the wings. I understand that the wings of the DC9-30 are said to be the same as those used on the 717, but were they both made using the same machinery? Same metals? Materials? I know there are different grades of aluminum (take the issue with the A380 wing components being of poorly manufactured aluminum) but were these changed for the 717? Or could you literally remove the wings from a DC9 and install them on the 717?

I really like the DC9 and all of it's derivatives, and really appreciate the amazing quality and craftsmanship that went into making the beautiful machines. It would be even more amazing though, if they used the same machinery to make the 717 as they did on the original DC9, 50 years earlier! Personally, i feel safer flying on a 35 year old DC9 than i do on most of the new Embraer tin can jets of today... Is that crazy?

Thanks for any input!

23 replies: All unread, jump to last
 
User currently offlineDalmd88 From United States of America, joined Jul 2000, 2544 posts, RR: 14
Reply 1, posted (2 years 2 months 4 weeks 1 day 21 hours ago) and read 10513 times:

Structures wise they are all pretty much the same aircraft. You can find common part numbers for a lot of frames and such from the DC9 through the series. Some stringers and skins have different numbers but they are common types of metal. They are all constructed in the same manner. None of this has to do with your ears popping. I bet it has to due with the pressurization controllers being different.

As for the wing, the DC9 and the 80/90 share a common wing. On the 80/90 they just extended the center tank wing box outboard about 6 feet on each side. They then added a new inboard slat and made new flaps.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17030 posts, RR: 67
Reply 2, posted (2 years 2 months 4 weeks 1 day 18 hours ago) and read 10439 times:

Very closely related as Dalmd88 says.

MD-81 through MD-87 are marketing names. They are in fact DC-9-81 through DC-9-87. You can see it on the plaque inside the entry door.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineDL_Mech From United States of America, joined Feb 2000, 1937 posts, RR: 9
Reply 3, posted (2 years 2 months 4 weeks 1 day 16 hours ago) and read 10403 times:

Quoting Dalmd88 (Reply 1):
I bet it has to due with the pressurization controllers being different.

IIRC, NWA replaced the DC-9 Outflow Valves and Controllers with ones made for A320's.



This plane is built to withstand anything... except a bad pilot.
User currently offlinePITingres From United States of America, joined Dec 2007, 1137 posts, RR: 13
Reply 4, posted (2 years 2 months 4 weeks 1 day 1 hour ago) and read 10181 times:

Quoting lexkid12300 (Thread starter):
I know there are different grades of aluminum (take the issue with the A380 wing components being of poorly manufactured aluminum)

Lest the thread go too far off-topic on this one, what you meant to say was "the A380 rib feet being made of an aluminum alloy poorly chosen for the manufacturing and flight loads imposed on them". There are no indications that the aluminum alloy itself was "poorly manufactured".


I have been intrigued in the past by statements that the 717 wing is "identical" to the DC9-30 wing. Identical, really? or same size/shape with (presumably minor) manufacturing differences? or close but not quite the same? Just wondering...



Fly, you fools! Fly!
User currently offlineDalmd88 From United States of America, joined Jul 2000, 2544 posts, RR: 14
Reply 5, posted (2 years 2 months 4 weeks 21 hours ago) and read 10098 times:

Quoting PITingres (Reply 4):
I have been intrigued in the past by statements that the 717 wing is "identical" to the DC9-30 wing. Identical, really? or same size/shape with (presumably minor) manufacturing differences? or close but not quite the same? Just wondering..

I have not worked on one, yet. My guess, knowing Douglas is the first choice. Pretty much identical. I know the vertical is the same as on the MD90 which has only a few changes from the MD80/DC9. The biggest change for the 717 is the apu intake was moved so it doesn't suck in hydraulic fluid from the bottom of the airframe.


User currently offlinelexkid12300 From United States of America, joined Mar 2011, 88 posts, RR: 0
Reply 6, posted (2 years 1 month 3 weeks 4 days 14 hours ago) and read 8899 times:

OK guys, i have another question about this topic.

I recently heard in the news that several 717 aircraft have been found with fatigue cracks in the fuselage frames- one plane having only 11,000 cycles and another with 14,000 cycles. And apparently 40 MD80 aircraft have been found with the same cracking at 19,000 cycles as well as one MD90.... But no word on whether these cracks were found on any DC9's.

Apparently there was an AD written a while ago for repairs to the 717/MD80/MD90 but it appears that this didn't apply to the DC9, but my question is why?

I also read that the fuselage frame part that contains the cracking was in the aft area, possibly where the engines are placed. Could this be the reason the DC9 doesn't have the issue.

It also explains why the FAA has certified the DC9 to 100,000 cycles, with the MD80 only given 50,000 cycles, and for some reason the MD90 and 717 given 60,000 cycles. I take it that this estimate will now be changed for the MD88/90 and 717?

Does this mean the DC9 is actually a better built plane than it's later derivatives?

Thanks for the continued help!


User currently offlinesoon7x7 From , joined Dec 1969, posts, RR:
Reply 7, posted (2 years 1 month 3 weeks 4 days 5 hours ago) and read 8810 times:

Quoting PITingres (Reply 4):
I have been intrigued in the past by statements that the 717 wing is "identical" to the DC9-30 wing. Identical, really? or same size/shape with (presumably minor) manufacturing differences? or close but not quite the same? Just wondering...
http://www.boeing.com/commercial/717/...According to the Boeing page it indicates the first ordering client to be spirit airlines. I seem to remember TWA actually took first delivery of an order of 15 units, they only received 11 before the airline went belly up. At the time the 717 was a MD product. I might be wrong on the #'s but I do remember TWA being the launch customer. I also remember that NW decided to rebuild their DC9 fleet rather than invest in new 717's. That proved how robust the DC9 was. Personally I would have thought the B-717 would have been the perfect replacement to any DC9 operators as your on the ground infrastructure didn't need to be changed as well. In the end it did not even sell well as a corporate jet. The 717's lack of success stymied me but I suppose the introduction of the CRJ's to carriers helped curtail sales.
That aside, the 717 is the DC-9 with a glass cockpit, new power plants, (more power but heavier) and various aerodynamic fairings around the tail feathers. MD had a great fabrication technique, the DC9 was a well made aircraft as was the entire MD series. The pressure vessel remained the same but as with any modifications comes fallout. Service life can change from expected fatigue due to stresses of fuselage elongation, heavier power plants, longer wingspan. The interior structures were part# stamped and also the alloy used to fabricate the part was stamped as well.

Quoting lexkid12300 (Reply 6):
Does this mean the DC9 is actually a better built plane than it's later derivatives?


Having chopped up Boeing narrows, B-747's and MD , DC9 products I would have to say that structurally the Douglas products were technically superior in construction and materials. Very little in the way of exterior plastics (composites). The British Hawker 700/ 800/ 900 series were the same as well. all metal...tough as nails, built to last.


User currently offlineDL_Mech From United States of America, joined Feb 2000, 1937 posts, RR: 9
Reply 8, posted (2 years 1 month 3 weeks 3 days 7 hours ago) and read 8592 times:

Quoting lexkid12300 (Reply 6):
I also read that the fuselage frame part that contains the cracking was in the aft area, possibly where the engines are placed. Could this be the reason the DC9 doesn't have the issue.

Could it be the P-Dome Doubler? If the 717 P-Dome is similar, it could require the same DC-9 repair.



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This plane is built to withstand anything... except a bad pilot.
User currently offlineViscount724 From Switzerland, joined Oct 2006, 25170 posts, RR: 22
Reply 9, posted (2 years 1 month 3 weeks 3 days 6 hours ago) and read 8573 times:

Quoting Dalmd88 (Reply 1):
As for the wing, the DC9 and the 80/90 share a common wing. On the 80/90 they just extended the center tank wing box outboard about 6 feet on each side. They then added a new inboard slat and made new flaps.

All the wings and certain other parts for the DC-9/MD-80/90/717 family (also DC-10 and MD-11) were built in Canada at their facility adjacent to YYZ airport. That factory, demolished a few years ago after 717 production ended, had a long history. It was originally de Havilland Canada/Avro Canada. Lancaster bombers were built there during WWII, and the first flight of a jet airliner in North America took place there in August 1949 (13 days after the first flight of the Comet 1). Only one prototype of the Avro Canada C-102 Jetliner flew before the program was cancelled.


User currently offlineDalmd88 From United States of America, joined Jul 2000, 2544 posts, RR: 14
Reply 10, posted (2 years 1 month 3 weeks 3 days 2 hours ago) and read 8539 times:

Quoting DL_Mech (Reply 8):
Could it be the P-Dome Doubler? If the 717 P-Dome is similar, it could require the same DC-9 repair.

I don't think so. For the MD88 the frames I would suspect are sta1418 which is directly above the y-duct, and I think the 1590 frame which is the last one before the tailcone mount frame. Back in DL's HMV days I did a lot of 1418 repairs. Back then it might have been a SB repair. We did it on just about every plane that came through HMV. Once those were done the cracking moved to the aft frame. I don't know if this was a common repair on the DC9s. I would bet it has to do with the heavier, higher thrust engine mounted on the MD80/90.

We did have some P-dome frame problems also, but they were not fatigue cracking issues. We did replace many lower t-cords due to corrosion and self inflicted damage. I personally had very little to do with that job. Did it once or twice and then decided the overtime pay was not worth the aggravation of the task.


User currently offlineLMP737 From , joined Dec 1969, posts, RR:
Reply 11, posted (2 years 1 month 3 weeks 1 hour ago) and read 8238 times:

Quoting Viscount724 (Reply 9):
All the wings and certain other parts for the DC-9/MD-80/90/717 family (also DC-10 and MD-11) were built in Canada at their facility adjacent to YYZ airport.

I remember watching them do the wing/center tank join in BLDG 15 at Long Beach. Those were the days, sad to see what was once the commercial aircraft builder to the world just a footnote in aviation history.

Quoting Dalmd88 (Reply 5):
I have not worked on one, yet. My guess, knowing Douglas is the first choice. Pretty much identical. I know the vertical is the same as on the MD90 which has only a few changes from the MD80/DC9. The biggest change for the 717 is the apu intake was moved so it doesn't suck in hydraulic fluid from the bottom of the airframe.

The first time you crawl into the tailcone it's going to feel like you are in a cave. No airstairs and supporting structure like on the MD-80/90. No pack cooling fans like in the 80/90. And most importantly no Augmentation Valves like on the MD-80.

Quoting soon7x7 (Reply 7):
Having chopped up Boeing narrows, B-747's and MD , DC9 products I would have to say that structurally the Douglas products were technically superior in construction and materials

From a structural stanpoint they probably are the toughest. Unfortanelty it also meant carrying around more wieght.


User currently offlinelexkid12300 From United States of America, joined Mar 2011, 88 posts, RR: 0
Reply 12, posted (2 years 1 month 2 weeks 5 days 12 hours ago) and read 8056 times:

Does anyone know why the 717's are showing structural cracks so soon?

User currently offlineDalmd88 From United States of America, joined Jul 2000, 2544 posts, RR: 14
Reply 13, posted (2 years 1 month 2 weeks 5 days 7 hours ago) and read 8031 times:

Quoting lexkid12300 (Reply 12):
Does anyone know why the 717's are showing structural cracks so soon?

Not unusual. The fleet is going through it's first Heavy check cycle. That is when you find this kind of stuff. Many of these areas are not real accessible when all of the panels, insulation, and systems components are installed. Aircraft are designed with the knowledge that these types of damage will occur and not be found right away. I think it's called redundant load path or something like that.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 14, posted (2 years 1 month 2 weeks 5 days 4 hours ago) and read 7993 times:

Quoting Dalmd88 (Reply 13):
Aircraft are designed with the knowledge that these types of damage will occur and not be found right away. I think it's called redundant load path or something like that.

Redundant load paths was part of an older design philosophy called "fail safe" that came into widespread usage back around the turn of the jet age. It's no longer the primary philosophy, although it is used for certain structures where it still makes sense. The Dan Air 707 crash is 1977 is sort of a canonical example of failure of fail safe design: http://en.wikipedia.org/wiki/1977_Dan-Air_Boeing_707_crash#Cause

Fail safe was an improvement over "safe life", which was supposed to predict the life of parts and then replace them before they reached their life. This is still used on some very high stress parts today (landing gear, turbine discs) but only when it absolutely must be used. The F-111 wing failures in the late 60's kind of killed off "safe life" as an overall design philosophy and begat the primary approach today: "damage tolerant".

Damage tolerance is all about fracture mechanics, which basically assumes you have a crack in every part just below detection size at the start of the the airplane's life. You project the crack growth forward in time (based on the loading and geometry) and figure out how long it will take for the crack to get critical (i.e. the part will fail). Then you cut that time down by some factor (often 2/3) and inspect the part at that interval. If it still doesn't have cracks at the first inspection you "reset the clock" and inspect again later. If it does have a crack you do the appropriate repair or monitoring (depending on its size, location, etc.).

Damage tolerance assumes cracks are present and manages them for the life of the asset; if you do the first heavy inspection and don't find any cracks at all then the structural guys screwed up and the airplane is way over designed or you got incredibly lucky on the manufacturing side when the thing was first built (and that's a non-repeatable kind of luck).

Tom.


User currently offlineTrijetsRMissed From United States of America, joined Oct 2006, 2350 posts, RR: 7
Reply 15, posted (2 years 1 month 2 weeks 22 hours ago) and read 7743 times:

Quoting Dalmd88 (Reply 1):
As for the wing, the DC9 and the 80/90 share a common wing. On the 80/90 they just extended the center tank wing box outboard about 6 feet on each side. They then added a new inboard slat and made new flaps.

To add, the MD-80 wing includes root extensions to the fuselage and an added parallel-chord section near the wingtips. The wingtip extension of two feet results in a "kink" in the trailing edge that was not found on previous DC-9 variants.


Quoting PITingres (Reply 4):
I have been intrigued in the past by statements that the 717 wing is "identical" to the DC9-30 wing.

Technically, it's the DC-9-34's wing with a newer/larger wing-body fairing and upgraded flap hinges from the MD-90.

Quoting soon7x7 (Reply 7):
Having chopped up Boeing narrows, B-747's and MD , DC9 products I would have to say that structurally the Douglas products were technically superior in construction and materials. Very little in the way of exterior plastics (composites).

Composites debuted with the MD-88 and increased in panels with the ensuing MD-90 and 717. Also, later build MD-11s had a greater use of composites than 1991-92 builds. Later build MD-80s of all series, (IIRC, 1992 and on) had the composite panels.

Quoting Dalmd88 (Reply 13):
Not unusual. The fleet is going through it's first Heavy check cycle. That is when you find this kind of stuff. Many of these areas are not real accessible when all of the panels, insulation, and systems components are installed. Aircraft are designed with the knowledge that these types of damage will occur and not be found right away.

  



There's nothing quite like a tri-jet.
User currently offlineGrisee08 From United States of America, joined Mar 2013, 354 posts, RR: 0
Reply 16, posted (1 year 3 weeks 5 days 8 hours ago) and read 4538 times:
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Quoting DL_Mech (Reply 3):
IIRC, NWA replaced the DC-9 Outflow Valves and Controllers with ones made for A320's.

They are actually the same ones currently found on the 717-200, if my information is correct.



You're Losing The Game!
User currently offlineDL_Mech From United States of America, joined Feb 2000, 1937 posts, RR: 9
Reply 17, posted (1 year 3 weeks 4 days 20 hours ago) and read 4396 times:

Quoting Grisee08 (Reply 16):
They are actually the same ones currently found on the 717-200, if my information is correct.

Not sure about the outflow valve, but the Cabin Pressure Controller (CPC) and Cabin Pressure Control Panel (CPCP) are different between the 717 and D95.



This plane is built to withstand anything... except a bad pilot.
User currently offlinehawaiian717 From United States of America, joined May 1999, 3192 posts, RR: 7
Reply 18, posted (1 year 3 weeks 4 days 16 hours ago) and read 4345 times:

Quoting soon7x7 (Reply 7):
According to the Boeing page it indicates the first ordering client to be spirit airlines. I seem to remember TWA actually took first delivery of an order of 15 units, they only received 11 before the airline went belly up. At the time the 717 was a MD product. I might be wrong on the #'s but I do remember TWA being the launch customer.

I have no idea where to got Spirit from. The MD-95 was launched with an order from ValuJet. By the time the first airplane was delivered, Boeing had acquired McDonnell Douglas, ValuJet had acquired AirTran and took the latter's name, and after an evaluation of the program, Beoing decided to proceed with it and rebranded the MD-95 as the 717.

Saying that it "didn't even sell well as a corporate jet" is an understatement. Boeing offered it as the "717 Business Express" and exactly 0 aircraft were sold this way.


User currently offlineLH707330 From United States of America, joined Jun 2012, 754 posts, RR: 0
Reply 19, posted (1 year 3 weeks 3 days ago) and read 4168 times:

OP, as far as physical differences between the different types, I suggest reading this article for more information. You'll see that the 717 is quite the mishmash of different components.

User currently offlineMax Q From United States of America, joined May 2001, 4458 posts, RR: 19
Reply 20, posted (1 year 2 weeks 2 days 10 hours ago) and read 3467 times:

Quoting LH707330 (Reply 19):


OP, as far as physical differences between the different types, I suggest reading this article for more information. You'll see that the 717 is quite the mishmash of different components.

What an amazing site, for a fan of technical trivia I found it to be fascinating



The best contribution to safety is a competent Pilot.
User currently offlineMD-90 From United States of America, joined Jan 2000, 8507 posts, RR: 12
Reply 21, posted (1 year 2 weeks 1 day 11 hours ago) and read 3231 times:

Quoting LH707330 (Reply 19):
OP, as far as physical differences between the different types, I suggest reading this article for more information. You'll see that the 717 is quite the mishmash of different components.

Surely this isn't correct, is it? Almost all DC-9s were built with forward airstairs? Even the final builds?

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


User currently offlineDalmd88 From United States of America, joined Jul 2000, 2544 posts, RR: 14
Reply 22, posted (1 year 2 weeks 1 day 2 hours ago) and read 3137 times:

Quoting MD-90 (Reply 21):
Surely this isn't correct, is it? Almost all DC-9s were built with forward airstairs? Even the final builds?

I bet it's true. All of DL's MD88's had them at one time. They were removed a long time ago, but the door is still there.


User currently offlineLH707330 From United States of America, joined Jun 2012, 754 posts, RR: 0
Reply 23, posted (1 year 2 weeks 1 day ago) and read 3125 times:

Quoting Max Q (Reply 20):
What an amazing site, for a fan of technical trivia I found it to be fascinating

There's quite a handful of other articles up there as well. If you ever have a question about shape changes on planes, the model building guys know best....


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