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kanban
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767 Wing Skin Cracks

Mon Oct 10, 2011 6:06 am

The attached article talks about "The FAA is proposing heightened scrutiny of the wing skin after cracks as large as a half-inch (1.3 centimeters) were found on either side of a fastener hole on a plane that had 18,900 flight cycles and 89,500 total flight hours. The 767 is a wide-body plane typically used on international flights. "

I question that the cracks are half and inch wide ... but they maybe that long..

http://www.bloomberg.com/news/2011-1...-expand-jet-checks.html?cmpid=yhoo
 
ha763
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 7:12 am

Didn't AA find cracks on the wing skin of some 767s that had winglets installed?
 
SEPilot
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 10:31 am

Cracks in the wing skin, while not something to be ignored, are not as serious as cracks in the fuselage skin, as the wings are not pressurized. The biggest danger would be if the wing skin cracks were a result of structural cracks in the ribs or spars that are not visible.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
AA737-823
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 12:18 pm

Quoting SEPilot (Reply 2):
Cracks in the wing skin, while not something to be ignored, are not as serious as cracks in the fuselage skin, as the wings are not pressurized.

Tell that to the passengers who are watching out their window as fuel streams out of the wing and into the stratosphere....
 
bennett123
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 12:29 pm

Would that happen if just the skin was cracked.
 
tdscanuck
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 1:32 pm

Quoting AA737-823 (Reply 3):
Tell that to the passengers who are watching out their window as fuel streams out of the wing and into the stratosphere....

Structural cracks are nearly airtight...the amount of fuel going through them would be tiny; probably invisible, certainly not significant enough to freak people out. Besides, if the crack is where people can see it then it's on the top surface and may not have fuel against it at all.

Quoting bennett123 (Reply 4):
Would that happen if just the skin was cracked.

With a *really* big crack, yes, but very unlikely for a 1/2" crack at a fastener hole. Depending on which fastener, the crack might be fully behind sealant anyway.

Tom.
 
wn700driver
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 1:51 pm

Quoting tdscanuck (Reply 5):
Besides, if the crack is where people can see it then it's on the top surface and may not have fuel against it at all.

Would the pressure differential from the venturi effect of all that air rushing over the wing "suck" fuel out the top?
Base not your happiness on the deeds of others, for what is given can be taken away. No Hope = No Fear
 
JAAlbert
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 2:24 pm

So is 18,900 cycles a large number? What's the maximum cycles recommended for a 767??

The article says that some of AA's planes are in excess of 24 years, wow.
 
474218
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 2:40 pm

Quoting wn700driver (Reply 6):
Would the pressure differential from the venturi effect of all that air rushing over the wing "suck" fuel out the top?

Very unlikely for several regions:

o Cracks in structure as thick as wing skins may not extend completely through the material.

o Since the cracks are adjacent to fasteners, the substructure (rib, spar, fitting, etc.) attached to the skin by the fasteners, may (or may not) be cracked.

o Fuel should never reach the upper wing skin. The fasteners through the upper wing skin are not brush coated with sealant, just installed with sealant on the fastener shanks. .

[Edited 2011-10-10 07:43:30]
 
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 2:41 pm

Quoting JAAlbert (Reply 7):
So is 18,900 cycles a large number? What's the maximum cycles recommended for a 767??

Cycles are of primary importance for fuselage cracks, as each cycle constitutes a pressurization and depressurization, which are the most significant loads on the fuselage. But it is not the case with wing skins; as the wings are not pressurized. The life of wings is less predictable than pressurized fuselages, as the loads they are subjected to are very much dependent on how the plane is used, and particularly on how much turbulence it has encountered. About 20 years ago a Piper Cherokee had a wing fail and crashed; the postmortem showed fatigue failure of the main spar. An AD was issued to inspect all Cherokees after 5,000 hours (the crashed plane had about 7,000) which was quite expensive as it required the wings to be removed, and severely affected Cherokee values. After several years none turned up with any signs of fatigue, and the AD was rescinded because it was determined that the crashed Cherokee had been used for pipeline inspection for its whole life, and therefore had been continually flown at very low level where turbulence was almost constant. I think the AD still exists for "unusually severe" operations, but it no longer applies across the board to all Cherokees. In the case of the 767, the hours are more significant than the cycles, and 90,000 hours is a significant number. There haven't been all that many aircraft that have flown beyond 100,000 hours, and the ones I have heard of have been either 747's or DC-9's.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
babybus
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 3:02 pm

I've seen loads of aircraft with cracks on the wing in my time on the ramp. The answer appears to stick duct tape on it.

However, I would think that although this is merely a wing crack, the trouble is any weakness in the structure will lead to weaknesses somewhere else in the frame.

With wings taking so much bending it might be good to get it sorted out as soon as possible.
and with that..cabin crew, seats for landing please.
 
doug_or
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 3:35 pm

Quoting Babybus (Reply 10):
I've seen loads of aircraft with cracks on the wing in my time on the ramp. The answer appears to stick duct tape on it.

Cracks in the wing skin or cracks in the fairing?
When in doubt, one B pump off
 
B6JFKH81
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 3:36 pm

Quoting Babybus (Reply 10):
The answer appears to stick duct tape on it.

We utilize something called "Speed Tape" which is actually Aluminum Tape that is very strong, not duct tape. The most common one from my inventory is the 425 series which we procure from 3M:

http://solutions.3m.com/wps/portal/3...tyTape/ProdInfo/Spec6/AlumFoil425/
"If you do not learn from history, you are doomed to repeat it"
 
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kanban
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 4:20 pm

As I recall, the fix is to drill a stop hole at the end of the crack to keep it from continuing and glob some sealant under if necessary. Speed tape, while used frequently can trap corrosives in the crack if it wasn't properly cleaned and Alodined which will lead to more extensive repair later.
 
redflyer
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 4:41 pm

Quoting Babybus (Reply 10):
However, I would think that although this is merely a wing crack, the trouble is any weakness in the structure will lead to weaknesses somewhere else in the frame.

Could be wrong, but given that the crack was on either side of a fastener hole, the issue is not so much about the structural integrity of the wing or any part of it (at least at the moment), but rather the manufacturing process that was apparently flawed enough that allowed the crack to appear. This would indicate there are other fastener holes that may have suffered from the same manufacturing flaw, and may also be exhibiting cracks, which taken together could pose a safety issue.

Quoting SEPilot (Reply 9):
There haven't been all that many aircraft that have flown beyond 100,000 hours, and the ones I have heard of have been either 747's or DC-9's.

I had the honor of flying on one of NW last DC-10's a few years back. The old gal had somewhere around 118k hours on her. May she rest in peace, wherever fine beer in cans are sold.  
My other home is a Piper Cherokee 180C
 
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 4:56 pm

Quoting redflyer (Reply 14):
I had the honor of flying on one of NW last DC-10's a few years back. The old gal had somewhere around 118k hours on her. May she rest in peace, wherever fine beer in cans are sold.

I stand corrected. But I think she is more likely in your car's cylinder head. 
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
SJUSXM
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 6:51 pm

Quoting JAAlbert (Reply 7):
The article says that some of AA's planes are in excess of 24 years, wow.

That's the small subfleet of 762's that does the transcons. The 763's are all younger, under 20 at least...
AT7, ER3, ER4, ER5, CR7, E70, E75, F100, M82, M83, 722, 732, 738, 752, 762, 763, AB6, 320, 321, 772, 77W
 
wn700driver
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 7:14 pm

Quoting 474218 (Reply 8):

Hmmm, I wasn't thinking of cracks that didn't go all the way through the surface. That changes things for obvious reasons.
Base not your happiness on the deeds of others, for what is given can be taken away. No Hope = No Fear
 
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kanban
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 7:25 pm

Quoting redflyer (Reply 14):
but rather the manufacturing process that was apparently flawed enough that allowed the crack to appear.

I'm not sure how one arrives at that assertion... please elaborate.
 
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 8:34 pm

Quoting kanban (Reply 18):
but rather the manufacturing process that was apparently flawed enough that allowed the crack to appear.

I'm not sure how one arrives at that assertion... please elaborate.

Not sure what you're asking me to elaborate on. If the in-service life expectancy of an aircraft is, say, 100k hours or 20k cycles (just random numbers I'm using for demonstrative purposes), then I would venture to guess that fatigue cracks should not appear on major components of the aircraft within those time/cycle parameters. If they do then the reasons have to be understood. Cracks that appear to emanate from a fastener hole would indicate the hole was not properly drilled for a number of reasons. (I suppose the sheet metal may not have been correctly milled either, or that the rivet was not properly riveted/bucked, but I'm assuming those are not the cases here.)

That is exactly what happened on the WN 737 that blew a hole in its fuse a few months back flying over my great state. I seem to recall the rupture emanated from around some improperly drilled fastener holes that lead to fatigue cracks which eventually led to the "zipper" affect in flight.
My other home is a Piper Cherokee 180C
 
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 8:47 pm

That it has taken this long for this (seemingly minor at this point) problem to crop up certainly makes the old bird look quite sturdy...   No doubt that an AD will be issued requiring inspection and/or repair. Does anyone know if the panel in question is aluminum or composite?
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 10:08 pm

Quoting Babybus (Reply 10):
I've seen loads of aircraft with cracks on the wing in my time on the ramp. The answer appears to stick duct tape on it.

Which Aircraft type are you rreferring to that had wing cracks stuck with duct tape & considered Airworthy?.
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 10:20 pm

Quoting redflyer (Reply 19):
If the in-service life expectancy of an aircraft is, say, 100k hours or 20k cycles (just random numbers I'm using for demonstrative purposes), then I would venture to guess that fatigue cracks should not appear on major components of the aircraft within those time/cycle parameters.

The life expectancy is dependent upon regular inspection and repair. Any aircraft that gets that many hours will have had numerous repairs, including cracks, and will likely have had some structural elements and skins replaced at some point. That is what heavy checks are all about.

Quoting redflyer (Reply 19):

That is exactly what happened on the WN 737 that blew a hole in its fuse a few months back flying over my great state. I seem to recall the rupture emanated from around some improperly drilled fastener holes that lead to fatigue cracks which eventually led to the "zipper" affect in flight.

Just because one plane had a skin failure that resulted from a manufacturing defect does not mean that whenever you have a crack it is the result of a manufacturing defect.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 10:38 pm

Quoting SEPilot (Reply 22):
The life expectancy is dependent upon regular inspection and repair.

The overall (read final) life expectancy is dependent upon regular inspection and repair, but I thought OEMs establish minimum lifespans for their hardware to ensure they maintain integrity to a certain number of hours/cycles?

Quoting SEPilot (Reply 22):
Just because one plane had a skin failure that resulted from a manufacturing defect does not mean that whenever you have a crack it is the result of a manufacturing defect.

True, but what else would account for cracks developing on either side of a fastener hole? I'm sure if a piece of metal is worked long enough that cracks will develop somewhere eventually, but if they develop around a fastener hole wouldn't that indicate the hole (or fastener) was the weak point on the component when in fact that point should be just as strong, if not stronger, than the surrounding area?
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kanban
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 10:57 pm

One odd part about this conversation is we have no idea where on the wing the crack is located.. It's obviously in an area the authorities consider worth watching. The wing skin panels and stringers are assembled using NC riveters, so most drilling, prepping, and riveting is beyond the human error factor. The skins themselves are not hand profiled, but also NC produced. There is man work required when the upper, lower and inspar ribs are joined. So without knowing where on the wing and in the assembly process this fastener is located, it's hard to assess blame.
 
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 11:02 pm

Quoting redflyer (Reply 23):
The overall (read final) life expectancy is dependent upon regular inspection and repair, but I thought OEMs establish minimum lifespans for their hardware to ensure they maintain integrity to a certain number of hours/cycles?

The life expectancy of any part cannot be precisely predicted; it is only an estimate, as two seemingly identical parts will fail at different points because of microscopic structural differences. Also, the loads each part on different aircraft sees will vary widely due to variation in severity of use. Hence, the aircraft are designed to be damage tolerant; they should not fail catastrophically due to any cracks that should appear between inspections (which also means that design should prevent cracks from propagating beyond acceptable limits. Again, this is why heavy checks are mandated, which involve inspecting every structural component and skin for cracks, among other checks. If you designed an aircraft so that no part would crack in 90,000 hours it would be way to heavy to fly.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
redflyer
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 11:18 pm

Quoting SEPilot (Reply 25):
The life expectancy of any part cannot be precisely predicted; it is only an estimate, as two seemingly identical parts will fail at different points because of microscopic structural differences. Also, the loads each part on different aircraft sees will vary widely due to variation in severity of use. Hence, the aircraft are designed to be damage tolerant; they should not fail catastrophically due to any cracks that should appear between inspections (which also means that design should prevent cracks from propagating beyond acceptable limits. Again, this is why heavy checks are mandated, which involve inspecting every structural component and skin for cracks, among other checks. If you designed an aircraft so that no part would crack in 90,000 hours it would be way to heavy to fly.

I agree, and maybe we're talking two sides of the same coin. However, we are not talking here in the case of the 767 wing skin cracks the life expectancy of a part; we are talking about a crack that developed on either side of a fastener hole. That would indicate the hole was the weak spot on the component. Shouldn't the hole in fact be the strong point, or at least as strong as the surrounding area (with the fastener in place, of course)?

Also, there should be some expectation with regards to life expectancy of any component (at least that is how I understand it). I can't imagine an OEM pushing iron out the door without some expectation or warranty regarding minimum hours/cycles. In fact, the articles I've read indicate that the failures occurred sooner than expected - "expected" would indicate the part failed before its intended design life-span:

Quote:
American Airlines and other U.S. carriers may need to inspect their Boeing Co. 767s twice as often after one operator found “significant crack sizes” had developed sooner than expected.
[Emphasis added]

http://www.businessweek.com/news/201...-767-spur-expanded-jet-checks.html

[Edited 2011-10-10 16:19:14]
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474218
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RE: 767 Wing Skin Cracks

Mon Oct 10, 2011 11:59 pm

Quoting redflyer (Reply 26):
That would indicate the hole was the weak spot on the component. Shouldn't the hole in fact be the strong point, or at least as strong as the surrounding area (with the fastener in place, of course)?

Any time you remove material, such a drilling a hole you weaken original part. The fastener does not restore the strength, it attaches another part that restores the strengths.

Quoting redflyer (Reply 26):
In fact, the articles I've read indicate that the failures occurred sooner than expected - "expected" would indicate the part failed before its intended design life-span:

OEM's use static and fatigue testing of airframes and parts. By finding out where these test articles fail they can come up with a expected point where the parts will fail in service.
 
tdscanuck
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RE: 767 Wing Skin Cracks

Tue Oct 11, 2011 12:47 am

Quoting redflyer (Reply 19):
If the in-service life expectancy of an aircraft is, say, 100k hours or 20k cycles (just random numbers I'm using for demonstrative purposes), then I would venture to guess that fatigue cracks should not appear on major components of the aircraft within those time/cycle parameters.

You guess incorrectly. On a modern damage tolerant design, you *expect* cracks to appear before you reach the life expectancy. What you don't expect is for a crack to go undetected and reach critical length before the next inspection. In this case, exactly what you'd expect happened...a below critical length crack got caught during inspection.

Quoting redflyer (Reply 23):
The overall (read final) life expectancy is dependent upon regular inspection and repair, but I thought OEMs establish minimum lifespans for their hardware to ensure they maintain integrity to a certain number of hours/cycles?

The OEM's do establish minimum lifespans but that is predicated on you doing all the inspections and repairing what you find per the established procedures. If you don't do the inspections and repairs, the airframe will *not* make it to its design life.

Quoting redflyer (Reply 23):
True, but what else would account for cracks developing on either side of a fastener hole? I'm sure if a piece of metal is worked long enough that cracks will develop somewhere eventually, but if they develop around a fastener hole wouldn't that indicate the hole (or fastener) was the weak point on the component when in fact that point should be just as strong, if not stronger, than the surrounding area?

Drilling a hole in anything instantly ups the stress around the hole by a factor of three beyond what's in the bulk material away from the hole. You combat this, somewhat, by thickening the metal near the holes but you don't usually see that go all the way up to 3x thickness and there are complicating factors from the proximity of other holes and edges. Long story short, if you've got holes in a piece of metal there's a *very* high probability that that's where you're going to find the first cracks.

Quoting redflyer (Reply 26):
I can't imagine an OEM pushing iron out the door without some expectation or warranty regarding minimum hours/cycles.

Airplanes have warranties, but finding cracks in damage tolerant airframes that are fairly deep into their design life is normal and not necessarily covered by warranty. It would only be warrantable if it were a manufacturing or design defect (as opposed to normal statistical distribution of fatigue damage).

Tom.
 
redflyer
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RE: 767 Wing Skin Cracks

Tue Oct 11, 2011 3:55 am

Quoting tdscanuck (Reply 28):

Thanks for the clarifications, Tom. I was waiting for an engineering expert like you to jump in and give the definitive answer.  

A follow-up question:

Not necessarily related to this issue of the skin cracks on the 767, but when stressing a wing on a new model to 1.5x the design load, when the wing finally "snaps" (as we've all seen in various videos), does it always snap along a rivet/joint line?
My other home is a Piper Cherokee 180C
 
tdscanuck
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RE: 767 Wing Skin Cracks

Tue Oct 11, 2011 5:58 am

Quoting redflyer (Reply 29):
Not necessarily related to this issue of the skin cracks on the 767, but when stressing a wing on a new model to 1.5x the design load, when the wing finally "snaps" (as we've all seen in various videos), does it always snap along a rivet/joint line?

Failure at fasteners is a tension phenomenon...in compression, the hole "squashes" onto the fastener and you have much better load sharing. You don't grow fatigue cracks in compression, only in tension, so fastener cracking is overwhelmingly driven by tension cycles.

The ultimate load test is a one-time deal...fatigue isn't really a factor here. The critical piece in static loading is typically compression buckling; in long/thin structures in bending, buckling is almost always more limiting than tensile strength. The most common failure is compression buckling of the upper wing skin stringers. The stringer may have local stress concentration due to fasteners and that might be the critical buckling location but you're not likely to see it happen along a line of fasteners.

Tom.

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