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Constructing An Aircrafts Lifespan

Thu Jun 15, 2000 6:34 pm

(I posted this also on the general aviation forum)

For every technical product engineers also have to "construct" the average lifespan, if for a car
or ship or aircraft. A Mercedes is constructed to live twice as long as an Alfa Romeo for example.

What about aircraft? I know the B747 is constructed to fly a minimum of 60.000 hours/20 years.
Quite short for such an expensive product - and it belies reality. At the moment it looks like 2
out of 3 Jumbos will even see their 30th birthday!

The B767 is constructed to fly 30 years plus, 50% longer than the Jumbo. That really astonishes me
because I don´t see a reason here.

Can someone tell me why? And does someone have figures of other aircraft, the A340 for example?

RE: Constructing An Aircrafts Lifespan

Thu Jun 15, 2000 7:08 pm

Remember the DeHavilland Comet or the L-188 ELECTRA?

When you think of a car´s life-cycle, there´s seldom a car breaking in two, most of them die in accidents or have an uneconomical-to-repair engine damage one day.

Aircraft DID break up.

So, when You look at Your history book, You´ll see that the concept of the 747 is much older than e.g. the 767. Metallurgy, science etc. have developed since then, so facts like "fatigue" are better calculatable. The earlier aircraft had "lower-quality" metals and no or few compound plastics of GFK at all.
But, if You see and oldtimer like the Ilyushin-18 turboprop, its cycle is 50years or approx. 50.000 hours. Which car would You drive for 50 years ?
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RE: Constructing An Aircrafts Lifespan

Thu Jun 15, 2000 10:03 pm

Service life is one of the trade-offs when you are designing an aircraft. The design goal for the 707 was 60,000 hrs / 20,000 landings. You have to set this so that the engineers know how tough to make the structure. An aircraft with a 120,000 hr goal will be much heavier and much more expensive. Heavier airframe = lower payload. The manufacturer must figure out how long the airlines intend to use the aircraft and then design to that.

Even when you've reached the original design goal, that doesn't mean you have to park the aircraft. You can continue to operate it until it costs too much to inspect and repair. But you can expect a lot more problem areas to develop after you've crossed that age.
aaron atp
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Joined: Sun Mar 19, 2000 1:17 pm


Fri Jun 16, 2000 8:06 am

Well, I know naval architecture, so I'll try to relate what I know to aeronautical engineering.

When designing a vessel, you look at forecast stress levels. We usually build ships to withstand a 25 year stress. A lot of factors go into the figure, which means the 25 year stress for the Queen Mary is different that that of a Bertram 54. Sure the bertram goes offshore, but it isn't a transoceanic design, and won't meet the same conditions or stresses as a ship. It's just a statistical concept for generalized areas of operation.

After you design a ship to withstand it's prerequisite stress levels, you can forecast a service life for that vessel. Depending on the margin by which it can withstand those stresses and other factors, you may give it a 30 year service life.

Let's say two nearly identical vessels suited for the same purposes are proposed for a 85' motor yacht. One company builds a handlaid fiberglass hull and superstructure and the other company builds an aluminum hull and structure and both have comparable value. The owners of these vessels decide they want to operate their boats on the gulf of mexico, instead of the atlantic. These boats were built to survive crashing through the cape, but now they are destined for more gentle seas...

thirty years later a marine appraiser inspects both vessels, and finds that while the fiberglass hull is in perfect condition, the aluminum hull has sustained severe metal fatigue with associated microscopic cracking. While neither hull ever met its 25 year stress, one is destined for the scrapyard, while the other was sold with considerable profit.

Why did one outlast the other? Because the 25 year stress played a major role in determining the life of the fiberglass hull, but was less important in determining the life of the aluminum hull; even though both had comparable lives. Just imagine that the fiberglass hull could not have survived it's 25 year stress twice, which the aluminum hull would have. The life of the metal hull was influenced more by its salt water environment than its ability to survive a rogue wave.

I think with a little imagination, you can understand my point: The service life of two different aircraft may depend on varying factors. Factors other than the statistical 25 year stress it was designed to withstand.

In addition, as Meyrowings said, advances in metallurgy and testing methods allow us to better estimate the remaining life an aircraft has. This may vary between aircraft of the same make and model. The maintenance department will use ultrasound, x-ray specs, UV dyes, and other methods to inspect each aircraft in the fleet to determine its condition. Given that info, management decides which aircraft are not worth retrofitting and updating; and retire those aircraft accordingly. I suppose it's just rather arbitrary.


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RE: Constructing An Aircrafts Lifespan

Fri Jun 16, 2000 5:25 pm

Especially to Aaron with the excurse to the High Seas.

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