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What Would Give First?

Posted: Mon Dec 13, 2010 9:11 am
by Faro

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Wholly theoretical question just out of curiosity.

If a veteran airliner like the above 767 was not retired from service but instead flown indefinitely (with representative payloads/missions) what would ultimately make it unflyable? Would it be impairment of some primary or secondary structure or problems with deep embedded wiring, chemical weathering of fuel tanks or something else?

Re primary/secondary structure, what would be the most likely trigger of impairment/failure? Encounters with heavy turbulence, heavy landings or just plain-vanilla operational cycling leading to nominal fatigue failure?

We may assume for the purposes of the question that maintenance is carried out normally for all equipment/components except for major structural repairs of the fuselage, wings and empennage.


Faro

RE: What Would Give First?

Posted: Mon Dec 13, 2010 3:37 pm
by 411A
If regular line maintenance is carried out, normally corrosion (many times in the landing gear attach fitting areas) and stress cracking of the fuselage skin will be the first noticable defects.

RE: What Would Give First?

Posted: Mon Dec 13, 2010 3:43 pm
by tdscanuck
Quoting faro (Thread starter):
If a veteran airliner like the above 767 was not retired from service but instead flown indefinitely (with representative payloads/missions) what would ultimately make it unflyable?

The cost of making required structural repairs for fatigue damage at each maintenance check would grow to exceed the replacement cost of the aircraft.

Quoting faro (Thread starter):
Re primary/secondary structure, what would be the most likely trigger of impairment/failure? Encounters with heavy turbulence, heavy landings or just plain-vanilla operational cycling leading to nominal fatigue failure?

Most likely plain-vanilla operational cycles. Fatigue lives are calculated using a statistical loading profile that includes some proportion of turbulence, heavy landings, etc. Unless you're doing a statistically significantly higher number of abusive maneuvers, it would just be overall cycles piling up that would get you.

Tom.

RE: What Would Give First?

Posted: Mon Dec 13, 2010 6:47 pm
by tsugambler
Quoting tdscanuck (Reply 2):
The cost of making required structural repairs for fatigue damage at each maintenance check would grow to exceed the replacement cost of the aircraft.

So, assuming one had both the desire to perpetually maintain an aircraft and an infinite cash flow, could one keep an airplane flying indefinitely?

RE: What Would Give First?

Posted: Mon Dec 13, 2010 9:18 pm
by Faro
Quoting 411A (Reply 1):
If regular line maintenance is carried out, normally corrosion (many times in the landing gear attach fitting areas) and stress cracking of the fuselage skin will be the first noticable defects.
Quoting tdscanuck (Reply 2):
Quoting faro (Thread starter):
Re primary/secondary structure, what would be the most likely trigger of impairment/failure? Encounters with heavy turbulence, heavy landings or just plain-vanilla operational cycling leading to nominal fatigue failure?

Most likely plain-vanilla operational cycles. Fatigue lives are calculated using a statistical loading profile that includes some proportion of turbulence, heavy landings, etc. Unless you're doing a statistically significantly higher number of abusive maneuvers, it would just be overall cycles piling up that would get you.

Funny I would have thought that that the first bits to fail/impair would be linked to movable flight surfaces, things like stabiliser/rudder/aileron attachments points, flaps and the like. Also bits subject to significant static and vibration loads like engine pylons. I guess the loads imposed by pressurisation cycles on the fuselage walls must take their toll in the long run. Corrosion of the landing gear attachment points is also surprising; would have thought that rain/snow/slush/mud is substantially pH-neutral.

BTW, can an encounter with extreme turbulence or a very heavy landing permanently 'bend' a wing by a couple of degrees or so?

Faro

[Edited 2010-12-13 13:33:31]

RE: What Would Give First?

Posted: Mon Dec 13, 2010 9:40 pm
by b78710
Quoting tsugambler (Reply 3):
So, assuming one had both the desire to perpetually maintain an aircraft and an infinite cash flow, could one keep an airplane flying indefinitely?

until they stop making/overhauling the spares

RE: What Would Give First?

Posted: Mon Dec 13, 2010 11:59 pm
by Braniff747SP
Quoting tsugambler (Reply 3):
So, assuming one had both the desire to perpetually maintain an aircraft and an infinite cash flow, could one keep an airplane flying indefinitely?

In theory. But, it would get to a point that, so much would be replaced or fixed, It would not be original anymore...

RE: What Would Give First?

Posted: Tue Dec 14, 2010 3:26 am
by BreninTW
Quoting faro (Reply 4):
BTW, can an encounter with extreme turbulence or a very heavy landing permanently 'bend' a wing by a couple of degrees or so?

Yes, a wing can be bent permanently. CI had a 747SP that was involved in an incident where the pilots lost situational awareness. The aircraft entered a steep dive -- it's believed to have gone supersonic -- when the pilots recognized what was happening, the recovery action was so forceful that it caused the aircraft to lose part of its horizontal stabilizer and the wings were permanently deflected upwards a few inches at the tips.

From Wikipedia: http://en.wikipedia.org/wiki/China_Airlines_Flight_006
Copy of NTSB report: http://www.rvs.uni-bielefeld.de/publ...CS/ComAndRep/ChinaAir/AAR8603.html

RE: What Would Give First?

Posted: Tue Dec 14, 2010 5:24 am
by tdscanuck
Quoting tsugambler (Reply 3):
So, assuming one had both the desire to perpetually maintain an aircraft and an infinite cash flow, could one keep an airplane flying indefinitely?

Yes.

Quoting faro (Reply 4):
Funny I would have thought that that the first bits to fail/impair would be linked to movable flight surfaces, things like stabiliser/rudder/aileron attachments points, flaps and the like.

Those things tend to have limit loads that are a lot higher than their fatigue loads, so fatigue may not be their dominant design factor.

Quoting faro (Reply 4):
Also bits subject to significant static and vibration loads like engine pylons. I guess the loads imposed by pressurisation cycles on the fuselage walls must take their toll in the long run.

Engine pylons, like hinges, tend to have really high limit loads that may override fatigue considerations (although sonic fatigue is a big deal for nozzles). The fuselage gets a huge proportion of its overall load from pressurization, and it happens consistently on every cycle, which is why fatigue is so dominant there.

This is just a general statement for typical airliners though...certain oddball aircraft may be limited in other parts.

Quoting faro (Reply 4):
Corrosion of the landing gear attachment points is also surprising; would have thought that rain/snow/slush/mud is substantially pH-neutral.

Corrosion isn't a pH thing, it's an electrochemical thing. Unless you're getting de-ionized water (not common in mother nature), it's conductive enough to cause galvanic corrosion, mostly because aluminum corrodes so easily. Deice fluid can also be a pain for some metals, especially cadmium.

Quoting faro (Reply 4):
BTW, can an encounter with extreme turbulence or a very heavy landing permanently 'bend' a wing by a couple of degrees or so?

Yes.

Quoting Braniff747SP (Reply 6):
But, it would get to a point that, so much would be replaced or fixed, It would not be original anymore...

True, although you're probably never going to replace 100%. There are rumours of JT8D engines running around where the only original part is the data plate.

Tom.

RE: What Would Give First?

Posted: Tue Dec 14, 2010 5:45 am
by KELPkid
Quoting tdscanuck (Reply 8):
True, although you're probably never going to replace 100%. There are rumours of JT8D engines running around where the only original part is the data plate.

I know that, on piston engines, if you send the engine back to the manufacturer for overhaul (in a core-exchange arrangement), the engine gets a new serial number (and that the new serial number denotes that it is a factory-remanufactured powerplant). If your mechanic does the overhaul, you keep the same serial number (and you save many many dollars   ). Well, provided the crankcase is resuable...but if it's not, then I suppose you're in the market for a new engine anyways   So many parts on an aviation engine are interchangeable, though, that I'm sure you end up with some engines which have been overhauled so many times that the only part which is original to the engine is the crankcase (similar to the JT8D situation which you mention).

RE: What Would Give First?

Posted: Tue Dec 14, 2010 9:14 am
by Faro
Quoting tdscanuck (Reply 8):
Quoting faro (Reply 4):
Corrosion of the landing gear attachment points is also surprising; would have thought that rain/snow/slush/mud is substantially pH-neutral.

Corrosion isn't a pH thing, it's an electrochemical thing. Unless you're getting de-ionized water (not common in mother nature), it's conductive enough to cause galvanic corrosion, mostly because aluminum corrodes so easily. Deice fluid can also be a pain for some metals, especially cadmium.

Very interesting, I would have thought that the protective paints applied to the wheel wells are pretty rugged and would take care of galvanic currents. Is corrosion also the subject of testing 'to destruction' (or impairment) like wing ultimate strength tests?

Quoting tdscanuck (Reply 8):
The fuselage gets a huge proportion of its overall load from pressurization, and it happens consistently on every cycle, which is why fatigue is so dominant there.

Does fuselage static testing go all way up to fatigue cracking or is it modelled beyond a certain number of hours/cycles? How many hours/pressurisation cycles would you need to get skin fatigue cracking? Will this be dramatically different with CFRP fuselages?


Faro

RE: What Would Give First?

Posted: Tue Dec 14, 2010 2:11 pm
by tdscanuck
Quoting faro (Reply 10):
Very interesting, I would have thought that the protective paints applied to the wheel wells are pretty rugged and would take care of galvanic currents.

They are, for the places that are painted. But you can't paint everything, like bearing surfaces, and you don't paint some things (like electrical connector bodies).

Quoting faro (Reply 10):
Is corrosion also the subject of testing 'to destruction' (or impairment) like wing ultimate strength tests?

Not that I'm aware of, although on any damage tolerant design it's covered by the fact that you assume any single failure can happen (you don't necessarily assume why it failed).

Quoting faro (Reply 10):
Does fuselage static testing go all way up to fatigue cracking or is it modelled beyond a certain number of hours/cycles?

The regs are, as I recall, that the fatigue test frame has to stay two or three times ahead of the actual fleet in hours & cycles. So it's actually tested well beyond the fleet life.

Quoting faro (Reply 10):
How many hours/pressurisation cycles would you need to get skin fatigue cracking?

Depends on which aircraft you're talking about. If the design was done right, you should expect to see some non-critical cracking as you approach the design life.

Quoting faro (Reply 10):
Will this be dramatically different with CFRP fuselages?

It should be, since CFRP has very different fatigue properties than aluminum.

Tom.