Northwest And DC-9 Design Life

Tue Dec 28, 1999 7:02 am

Following on from the "Northwest's Ancient Fleet" posting I have been doing some reading of Flight International's Ageing Airliner Census

Many people have said that maintained properly the DC-9 will fly on forever. I'm not convinced and began to look into the design lifespans of the aircraft and came up with some information which would make me uneasy if I were flying on an old DC-9.

The Original DC-9 Design Life was;
40 000 Cycles
30 000 Hours
13 Years

We're well past this by now, but further tests have been done. We all know the design lifes of many types were unrealistically short; Further tests were done to invetigate how the aircraft aged to extend it's flying life

"Test-Supported" DC-9 life
105 000 Cycles
75 000 Hours
20 Years

The average current DC-9 has
61 300 Cycles, 58 300 Hours and is 28 Years old.

Northwest is listed as the largest operator with 182 examples.

What is worrying is the fleet leader. It has
105 486 Cycles
91 423 Hours
34 Years
The survey doesn't state what airline it flys for but I'd say Northwest is a safe bet. Anyway if it's going to have 135 of the things in service after 2007, it's reasonable to assume most, if not all, Northwest's DC-9s will surpass the Test Supported Hours/Cycles limits.

Who knows what will happen to the integrity of the aircraft beyond it's tested limits. We all remember the sad Aloha accident. This really is "flying into the unknown" I would be VERY uneasy about flying on a very old aircraft beyond it's tested limits. There could be any number of unforeseen problems which could arise and threaten flight saftety without being detected until it's too late.

Just out of interest, the 757 has a Design life of 100 000 Cycles and 40 years. Much more of a realistic figure than the original one for the DC-9

If anyone wants Design Life/Test Supported Life/Fleet leader info I have it for most major types. I just posted the DC-9 info because it's relevant to the other thread.

RE: Northwest And DC-9 Design Life

Tue Dec 28, 1999 7:31 am

Jet Setter: Have you ever worked on or in a Douglas built aircraft. Probably not.

Having flown and worked on DC-4s,DC-8s and MD80s as well as dozens of other makes and models of aircraft I can assure you that there are no aircraft more sturdily built than Douglas products. As long as aircraft are properly inspected and maintained their useful lives should be unlimited.

You must remember that the Aloha accident took place before the aging aircraft inspection program was in place and that it was a Boeing not a "Doug".

Remember anything that was built by humans can also be rebuilt.

Sincerely, MD80DRVR
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Joined: Tue Aug 24, 1999 6:54 am

RE: Northwest And DC-9 Design Life

Tue Dec 28, 1999 7:34 am

If memory serves, NW is retiring their DC-9's at 104k cycles. Most are the -10,-15 series. Some of which are setting in Texas. The ARJ's are replacing these.


RE: Northwest And DC-9 Design Life

Tue Dec 28, 1999 3:23 pm

Last year while working for an airline with a few old 9's, it was anounced that the DC-9 was tested past 200,000 cycles and could probably last longer. Maintaining the aircraft in good condition is key of course.
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RE: Northwest And DC-9 Design Life

Tue Dec 28, 1999 4:03 pm

Read "Airframe" by Crighton. He really hypes the durability of the D/MD aircraft.
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Joined: Sat Dec 25, 1999 10:20 am

RE: Northwest And DC-9 Design Life

Tue Dec 28, 1999 4:26 pm

In regards to design life of the DC-9 or any aircraft for that matter, its how it is maintained and operated. With all the new non-destructive testing that can be done now days, there is virtually nothing that could not or would not be detected and repaired.

With the Aloha accident, that airplane had a large number of cycles and most airlines and manufacturers didnt know what cycle fatigue would do to an airplane. Well, obviously we do now and inspection procedures have been put in place to prevent this from occuring again.

Northwest, I'm sure, has spent lots on there enginering department developing an inspection plan to ensure there DC-9's are maintained to the highest standards. I wouldn't hesitate one bit to climb aboard one of there oldest and go for a ride.

I have faith in the system and I have worked aircraft overhaul and especially on the DC-9, enough to know, its a sound machine. But I still prefer the old three-holer Boeing any day!
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RE: Northwest And DC-9 Design Life

Tue Dec 28, 1999 4:30 pm

Jet Setter once the maintenance department at Northwest are doing what they were trained to do efficiently and properly you have nothing to worry about. I would have no problems travelling with a reputable airline with a high standard maintenance department if the aircraft were 50 years old and had a design life of 10 years and 1 cycle.

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RE: Northwest And DC-9 Design Life

Tue Dec 28, 1999 6:23 pm

Anyway when you are in an airplane do you know you have much more to worry about that the design life of the aircraft? In my A&P mechanics course I have learnt enough that I should be petrified of flying ever again.

Think about this, your cruising at 36,000 ft, in a tube held together by a bunch of aluminium rivets (and it doesn't take much to start aluminium corroding once the protective oxide layer is removed) with a greater pressure inside of the tube than outside. What do you think the natural reaction of that tube is?


RE: Northwest And DC-9 Design Life

Wed Dec 29, 1999 1:07 am

The average DC9 life of Northwest is 28 years old, right? But the average NWA DC9 also has 68,000 cycles. The reccomended is 105,000 cycles and 20 years. I think the amount of cycles matters less than the average numer of years. NWA is retiring them after 104,000 cycles, so they are doing LESS than reccomended.

Jack M
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Joined: Mon Dec 17, 2012 6:44 am

RE: Northwest And DC-9 Design Life

Wed Dec 29, 1999 2:22 am

NW has spend millions on engineering and implementing several DC-9 structural upgrades designed to lengthen the service life by as much as 20 years. Not that NW will fly them that long. A major beefing up of the aft pressure bulkhead, belly skin replacements, and skin lap mods just to name a few. As a NW mechanic who has seen first hand the maintenance upgrades our DC-9's have gone through, I can honestly tell you that I have never had a problem placing my loved ones on them.

Thanks For The Ressurance

Wed Dec 29, 1999 2:40 am

I'm no mechanic and was just repeating facts I read in a magazine.
It appears ageing aircraft need much more work on them to continue flying safely, what I mean is some structures are more likely to be replaced/beefed up as opposed to just patched up.

I was in no way criticising NW's maintenence by the way. I only mentioned them because they have the largest DC-9 fleet! If there was something wrong at NW, whith the number of planes you have, Korean Air's problems would seem minor!

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