United777atGU
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If A Plane Flies For A Long Time...

Mon Oct 23, 2006 2:32 pm

Hey guys. This is my first post in this forum. I don't really know how to word this correctly, and I hope that no one takes this as disrespect toward other people as well--you'll understand in one second:
A hypothetical situation: A plane flies every day, despite being fuel inefficient, for 20+ years (e.g. NW DC-9, AA MD80, UA 763, etc). Can it continue to fly another 20+ years? What about 40? Can it fly 100 years (with maintenance maintenance / D checks / what do you call overhaul maintenance?)? How long can you fly a plane before it will literally give out mid-air? I guess most of the conditions depend on the state of the engine, but the state of the aircraft is just as important, obviously. So, how long can a plane fly before giving out? Now you see why I said I don't mean to offend anyone? I hope this makes sense and that it doesn't offend anyone. I hope it's not a dumb question, either. I have to ask someone, somewhere.
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Starlionblue
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RE: If A Plane Flies For A Long Time...

Mon Oct 23, 2006 2:40 pm

Interesting question.

There is no point in time at which the plane becomes unflyable. Eventually every part will be replaced as per check schedules.

However, as time goes by, operating and maintaining the plane becomes more and more expensive. At some point, you reach an economic limit, not a technical one. This is not only due to fuel, but due to the availability of parts and know-how. Imagine how hard it will be in 100 years to find someone with knowledge on how to maintain a Connie.
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Mir
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RE: If A Plane Flies For A Long Time...

Mon Oct 23, 2006 2:46 pm

It's actually a pretty good question. I don't really have an answer for you, but I can say that I'm not at all interested in being the one to find out!  Smile

-Mir
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EMBQA
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RE: If A Plane Flies For A Long Time...

Mon Oct 23, 2006 2:58 pm

There are DC-8 and DC-9's that are pushing 40 years old.....Boeing 767's that are getting to be 20 years old.....some 747-200... 30 years old. With proper maintenance they can keep flying for ever. It just takes money and the question is... when does it cost more to keep the older planes flying.
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jben
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RE: If A Plane Flies For A Long Time...

Mon Oct 23, 2006 3:20 pm

You can keep aircraft running for a very long time... but, as others have said, htere is a point where it becomes uneconomical to maintain an aircraft. It also depends on how essential the aircraft is, and what it's job is.

For instance, think about the B-52H and the C-5. Some of those birds will probably be close to 90+ years when (the airforce has stated they will be retiring the B-52 sometime around 2050) they are eventually retired. But, they're not the same aircraft they came off the production line as. Both have been extensively upgraded over time... in particular the C-5M, which should make it viable for decades. Why keep them? Because they're worth more to than they cost to maintain. One of the major reasons the F-14 got the boot was because of the maintenance, with some needing 30-60+ maintenance hours per flying hour.

Also consider the DC-3... the last one rolled off the production line in 1946 (which went to Sabena). There are still several hundred in revenue service today. Why? Because Douglas built one amazing aircraft. Some of these airplanes are so ridiculously over-engineered that given the right care and feeding, they will fly for a very very long time.
 
United777atGU
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RE: If A Plane Flies For A Long Time...

Mon Oct 23, 2006 3:25 pm

Quoting Mir (Reply 2):
It's actually a pretty good question. I don't really have an answer for you, but I can say that I'm not at all interested in being the one to find out!  

I know that's right!!! I'll take a DC-9 before any centenarian plane!

So if I bought a 777 (not that I ever, ever could...well, maybe, you never know!!) I could keep it a life-time?

I figured that economics would be the sole reason to retire a plane out of a fleet. I guess if you were SRB, it wouldn't matter about the money, 'eh?
Thanks for your responses... I appreciate it.
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speedracer1407
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RE: If A Plane Flies For A Long Time...

Mon Oct 23, 2006 4:15 pm

Quoting Starlionblue (Reply 1):
There is no point in time at which the plane becomes unflyable. Eventually every part will be replaced as per check schedules.

However, as time goes by, operating and maintaining the plane becomes more and more expensive. At some point, you reach an economic limit, not a technical one.

I wonder, though, if there is SOME part that cannot be replaced....like a pressurized fuselage. Engines, parts of wings, various systems, the cockpit and avionics, etc can all be replaced, even in a way that's economically feasible. But it's my understanding that a pressurized aircraft's ultimate longevity is dependent on the longevity of its ability to maintain pressurization. Perhaps i'm overlooking something, or uninformed on the issue, but it seems like the fuselage of a pressurized aircraft is one of the only parts that cannot be replaced, yet is also subject to cycle-dependant fatigue. taking proper steps to resist corrosion and repairing cracks before they propegate are ways to maximise the longevity of a pressure vessel, but surely there's some temporal limit to that structure's ability to safely operate without a complete rebuild, and a complete rebuild of the fuselage surely makes the plane a "new" one. Am i missing something, like a fundamental understanding of metal fatigue?

O
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TheJoe
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RE: If A Plane Flies For A Long Time...

Mon Oct 23, 2006 4:25 pm

Quoting Starlionblue (Reply 1):
There is no point in time at which the plane becomes unflyable. Eventually every part will be replaced as per check schedules.

This is a good point. Every mechanical part will be replaced as they become time expired. I believe that one of the bigger issues in maintenance is corrosion control and fatigue in the primary structure. There's only so many time you can bend a big aluminium beam before it starts to get a bit old and tired! This depends entirely on how well the aircraft is designed. You can replace engines, pumps, flight controls, etc quite easily but when the primary structure of the aeroplane starts to fail due to fatigue and corrosion, it's time to think about buying a new aeroplane!

I've seen parts of wing spar replaced, bits of aeroplane re-skinned, huge patches installed and as everyone has been saying, it is an economical and operational decision more than anything. If the company decides that by incorporating mods, adding doublers, grinding corrosion and replacing fatigued parts of structure they will get another 10 years out of their aeroplanes, they probably will invest the money. Aeroplanes don't make any money for a company by sitting in a hangar! So when the maintenance dollar per flying hour and maitenance hours required per flying hour get too high, time to think about replacing the fleet...
 
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HAWK21M
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RE: If A Plane Flies For A Long Time...

Mon Oct 23, 2006 5:26 pm

If Scheduled checks are carried out on time & Components replaced when due.The Aircraft can fly on.
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oly720man
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RE: If A Plane Flies For A Long Time...

Mon Oct 23, 2006 11:38 pm

Quoting United777atGU (Thread starter):
How long can you fly a plane before it will literally give out mid-air?

Depends what it's doing. In the extreme case of structural failure due to fatigue or corrosion it's more likely to be a military or fire fighting aircraft that gets thrown around rather more than a passenger plane and is in more hazardous environments.

Very sad....
http://www.youtube.com/watch?v=zGSwWVh5TAI


For passenger aircraft, incidents of this nature are seemingly after repairs, and sometimes a long time afterwards

http://english.people.com.cn/200212/25/eng20021225_109073.shtml
(CI B742 maybe 22 years after damage)


http://www.japanlaw.info/lawletter/jan87/fax.htm
JAL 123, repaired 1977, crashed due to matal fatigue in 1987

Otherwise, bad luck

http://en.wikipedia.org/wiki/Aloha_Flight_243
Corrosion after suspected manufacturing fault.

Generally though, if items are replaced as necessary, a plane can go on indefinitely.
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JAGflyer
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RE: If A Plane Flies For A Long Time...

Tue Oct 24, 2006 12:21 am

Where is the airline's "odometer"? They say they have a certain amount of landings, hours, miles, etc? Where do they find this info?
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320tech
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RE: If A Plane Flies For A Long Time...

Tue Oct 24, 2006 1:51 am

Where is the airline's "odometer"?

Airplanes track take-off and landing times in their flight management computers. This information is sent to the operations centre by ACARS (a data link system), where it is compiled. Of course, it also gets recorded in the aircraft journey log.

Older airplanes, like DC-3s, have their flight hours and cycles recorded in log books only. It's not uncommon for log books to be missing (a log might only cover a few month's worth of flying, so a DC-3 could have dozens or hundreds of journey logs), but the totals are carried over, so flying times should still be reasonably accurate.

I wonder, though, if there is SOME part that cannot be replaced....like a pressurized fuselage.

Exactly right. But these major structures - wings and fuselage - are inspected more frequently and in greater detail as time goes by and flying hours accumulate. This is one of the reasons why maintenance gets more expensive as an airplane ages. Manufacturers figure out where cracks are likely to occur, and come up with repair schemes for them. It's possible to replace fairly large sections of skin, frames, splice in new sections of spars, etc.

In this way, an airplane can safely fly to the end of its economic life.
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FredT
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RE: If A Plane Flies For A Long Time...

Tue Oct 24, 2006 2:18 am

An aircraft has a fatigue life which is not, contrary to some posts above, infinite. It is given as a number of cycles (flights) or a number of flight hours, whichever comes first.

When the fatigue life is used up, the aircraft is no longer airworthy.

Manufacturers are required to prove the calculated fatigue life. This is done by having test aircraft undergo a large amount of accelerated simulated flight time in hydraulic rigs simulating the stress of take off, landing and flight. You also pressurize the aircraft for each flight. These tests go on for years, and are usually ended by a destructive test preceding which you saw through parts of the structure to indicate the damage tolerance of the aircraft.

These fatigue test airframes are at all times to be well ahead of the lead aircraft in the fleet in both cycles and hours, in order to find any fatigue problems in the test rig first. In a perfect world, any emergency ADs concerning structure would come from this testing and not from aircraft in operation.

Manufacturers of aging fleets may at times go to great lenghts to extend the fatigue life of the aircraft. It all comes down to wether it is economically feasible or not.

It is probably possible to get aircraft excepted from the fatigue life limits for e g display purposes, but I doubt you could get an aircraft into revenue service once it is past the end of its fatigue life.

Engines live different lives, which are a lot shorter. While the maximum time on wing figures for engines have gone up tremendously over the last decades and is at times nothing short of stunning, it does not come close to the fatigue life of airframes.
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DeltaGuy
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RE: If A Plane Flies For A Long Time...

Tue Oct 24, 2006 2:34 am

Quoting Speedracer1407 (Reply 6):
I wonder, though, if there is SOME part that cannot be replaced....like a pressurized fuselage.

Good point- it is large components like this, and damage or sheer wear to them that'll make the a/c be a writeoff, and be more valuable in spare parts- hence why we're seeing 757/767's taken apart for their parts.

As is the case with the jet engines I work on, and all aircraft in general, is aircraft Nxxxxx really still Nxxxx after you've replaced all of the parts? An aircraft after 30-40 yrs is going to have well over half of it's original factory parts removed and replaced with newer stock- the only original relics are going to be major assemblies (fuselage, wing), and even those are subjected to repair.

I was taught back in Air Force aircraft tech school that, say you have an engine that has every single part replaced at once, save for the date plate...it's still engine XXXX. Same with the aircraft- so an a/c that rolled off the line 40 yrs ago may only be 5 yrs old by some measure of the parts.

DeltaGuy
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320tech
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RE: If A Plane Flies For A Long Time...

Tue Oct 24, 2006 2:49 am

An aircraft has a fatigue life which is not, contrary to some posts above, infinite. It is given as a number of cycles (flights) or a number of flight hours, whichever comes first.

Older airplanes have a designed life of so many flying hours, airframe cycles, etc. But newer ones don't. We have A320s with almost 60,000 flying hours, and 30,000 cycles, and no end in sight. It will be an economic decision, not a life cycle one, as to when to scrap these aircraft.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
 
SlamClick
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RE: If A Plane Flies For A Long Time...

Tue Oct 24, 2006 4:22 am

Here is something that might go toward your answer:
http://www.readbookonline.net/readOnLine/1157/
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lehpron
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RE: If A Plane Flies For A Long Time...

Wed Oct 25, 2006 11:40 am

Do you mean like what airplanes nowadays do, fly and land and fly and land for years and years? If so, then everyone else may have answered your question. Or, do you mean nonstop for however long, as if it had an infinite fuel supply but didn't weight anymore than it did on takeoff? For that I'd say a few years, less than 5. There will be a point where a part in the plane will give out, disabling or crippling it's operation. More and more commcerical airplanes are being designed to require less maintenence than before, but not maintenence free. There will be parts to replace.

Just like an automobile, how long/far can it be driven until it needs a change in oil or air in tires?
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Starlionblue
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RE: If A Plane Flies For A Long Time...

Wed Oct 25, 2006 3:11 pm

Quoting Lehpron (Reply 16):
Or, do you mean nonstop for however long, as if it had an infinite fuel supply but didn't weight anymore than it did on takeoff? For that I'd say a few years, less than 5

If you had "infinite" fuel, the next bottleneck would be engine lubrication. You would need to land within 2-3 days. The VC-25s, for example, can be refueled in the air but are limited to 72 hours.
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Dalmd88
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RE: If A Plane Flies For A Long Time...

Thu Oct 26, 2006 3:19 am

On modern airliners all parts can be replaced. It all comes down to how much do you want to spend. Even the pressurized fuselage parts that you all are worried about. I've replaced the frames, stringers and entire skin sections on many airframes. The skins were replace not because of cycles but wear from debris hit the belly of the aircraft.

Modern aircraft can in effect become the George Washington cherry tree axe if the operator wishes to spend the money. "This is the axe George used, the handle and the blade have been replaced though."
 
2H4
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RE: If A Plane Flies For A Long Time...

Thu Oct 26, 2006 3:47 am




Quoting Starlionblue (Reply 17):
If you had "infinite" fuel, the next bottleneck would be engine lubrication.

So, if someone devised a way to replenish engine oil in-flight, what would the next flight-critical bottleneck be?



2H4


Intentionally Left Blank
 
zanl188
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RE: If A Plane Flies For A Long Time...

Thu Oct 26, 2006 7:10 am

Quoting 2H4 (Reply 19):
So, if someone devised a way to replenish engine oil in-flight, what would the next flight-critical bottleneck be?

The size of the blue room holding tank....
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AC320tech
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RE: If A Plane Flies For A Long Time...

Thu Oct 26, 2006 7:55 am

As everyone is stated, it can fly for a long time, its just the amount of money the airline is willing to pump into the aircraft in question.

Fed Ex in Canada here only has to fly the 727 with 3 cans on the upper deck to break even on that flight. So imangine if that flight was full? The airplanes MTC can be pretty much paid for.

AC retired their 747-100's and -200 because they aircraft was becoming to expensive to maintain. 70000+ hours and wing spar fixes, thats when it starts to get a bit expensive.
 
access-air
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RE: If A Plane Flies For A Long Time...

Fri Oct 27, 2006 12:24 am

Werent the Concorde and Viscount airframes limited to 30 years of pressure cycle use??? I read that some place......

I know that Fokker has cleared the Fokker F.27 up to as high as 90,000 cycles....In 1995 I flew on Mesaba's last Fokker F.27 flight as a guest, 15 September 1995.. I got the crew to sign my log book, the total hours since it was originally delviered to Ansett in 1965 an VH-ANK, was 53,371 and 3 mins...The total landings/cycles were at 54, 058 That day..Not bad for something was 1 year older than myself.... This Fokker N278MA c/n 10280, went on to fly for Eagle Canyon/Scenic I saw it at Opa Locka in January 2005 engineless, but now is in service with Air Panama I believe....

I have also ridden on a Northwest DC9-14, N3312L ( originally with Delta) , that was also built a few months before I was born and it was pushing the 90K cycles mark... I really wish Boeing would bring back the DC9-10 series but update it with RR Tays....The shorty 9s were one of my favourites...

Access-Air
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JAXFLL
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RE: If A Plane Flies For A Long Time...

Mon Oct 30, 2006 5:18 am

Quoting Speedracer1407 (Reply 6):
like a pressurized fuselage

This can be replaced. I've been a part of replacing sections of fuselage skin. Like was said earlier, it all depends on how much money you want to spend.

On F-18s for example, the Navy is doing a huge "center barrel" replacement program. Basically the Navy is putting the F-18 in a jig and taking out the center section and replacing it. That section was the life limiting section, so once it's replaced the aircraft has "new life."

I've also seen many wing spars, wing planks, center wing spars and planks replaced. Most of the time it comes down to economics - is it worth it to replace whatever is damaged or cannibalize the aircraft.

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