Hardtailjohn
Registered User
Marion, MT
And that had nothing to do with a strut. Total longeron corrosion failure. But yes, for sure scary! Now, back to the thread....
John
Super Cub Structural Failure
- Thread starter Steve Pierce
- Start date
And that had nothing to do with a strut. Total longeron corrosion failure. But yes, for sure scary! Now, back to the thread....
John
BC12D-4-85
SPONSOR
Fairbanks, AK.
The pilot Dave Wiley maintained the Taylorcraft. The strut fitting was covered with fabric and the normal lower drain hole was.....well decide for yourself. Rust never sleeps and can fail to hold hands. Some edits shown. His and passenger's life ending story.
Gary
Gary
Attachments
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mountainflier
Registered User
Steve Pierce,
I have the NTSB Materials Laboratory Factual Report back from Washington D.C.. The rear spar fractured about 4 inches inboard the rear strut attach point, and also a foot and a half inside of that fracture point. The inboard spar fracture started with a hairline crack that started with a stainless steel #4 sheet metal screw that was screwed into the top forward spar cap to fasten the flap bay skin to the rear spar. It cracked all the way through the spar cap and only had to go 1/2 inch past the top spar cap to get to the lighting hole that was drilled out of the spar to save weight. I counted 22 lighting holes drilled out each spar that the original builder said he saved a total of 7 pounds.
I have the NTSB Materials Laboratory Factual Report back from Washington D.C.. The rear spar fractured about 4 inches inboard the rear strut attach point, and also a foot and a half inside of that fracture point. The inboard spar fracture started with a hairline crack that started with a stainless steel #4 sheet metal screw that was screwed into the top forward spar cap to fasten the flap bay skin to the rear spar. It cracked all the way through the spar cap and only had to go 1/2 inch past the top spar cap to get to the lighting hole that was drilled out of the spar to save weight. I counted 22 lighting holes drilled out each spar that the original builder said he saved a total of 7 pounds.
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Steve Pierce
BENEFACTOR
Graham, TX
Thanks for the insight. Useful information. Light is good but a little engineering to go with it is important. Hope you two are ok without any prolong issues from this.
Perhaps another reason not to use stainless PK screws as well. They don’t specify if the crack propagation started from corrosion, but stainless is farther from aluminum on the galvanic chart than carbon steel. Corrosion will start faster with stainless and aluminum than carbon steel and aluminum. That corrosion could cause the crack propagation to accelerate.
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mountainflier
Registered User
dgapilot,
No corrosion associated with the #4 sheet metal screw fracturing the rear spar on the inbound fracture. On the outboard fracture of the rear wing spar near the rear wing strut attach point the NTSB Materials Lab Report described the fracture under the microscope as mixed intergranular separation and dimple rupture.
No corrosion associated with the #4 sheet metal screw fracturing the rear spar on the inbound fracture. On the outboard fracture of the rear wing spar near the rear wing strut attach point the NTSB Materials Lab Report described the fracture under the microscope as mixed intergranular separation and dimple rupture.
mountainflier
Registered User
behindpropellers
Registered User
NE Ohio
Interesting. Looks like those lightening holes were punched.
RVBottomly
PATRON
Asotin County Washington (KLWS)
I've been pondering this for a while and figured I'll ask. Does anyone have any idea how the crack would have developed without the lightening holes?
It seems like the crack would have occurred anyway. If so, wouldn't it eventually have the same result, even without the holes?
Or did the holes increase the likelihood of the crack starting?
skywagon8a
MEMBER
SE Mass MA6
My take is the bottom of the spar cap failed at the large lightning hole not by cracking but by a tension overload. The section of the wing outboard of the lift strut exerts an upward force which is hinged at the strut attach location. Thus the tension loads are concentrated on the spar lower cap just inboard of the strut attachment at the lightning hole alone, rather than distributing the load throughout the entire spar. Had the hole not been there, the loads on the lower spar cap itself would have been lower since they would have been dissipated throughout the entire web.
I'm not sure about the outer break. It could have been a result of the inboard break and just a secondary failure?
Having an education in this as well as failure analysis back ground.
Short answer, Yes the drilled screw holes where they were are a problem.
Personally I never liked the screws in the spar caps. At least these spar caps. Wrong thing to do to a high stress region, but these do not rear their head often. To me the fastener should be into the web of the spar, or the cap increased to accept the disrupted load path. But that is generally just not truly necessary.
This area of the rear spar is one of the highest loaded sections of a Cub and has required reinforcement when utilized at higher gross weights. This has been known for a long time and is accepted practice.
Now to me, someone comes along and wants to build a light Cub, and with little to no experience in material properties or stress analysis decides to lighten one of the highest stress regions of the structure.
He lightened a region the factory adds reinforcement too.
In this case, having a screw hole lined up with the greatly reduced cross section of the spar is a guarantee of failure. Had the screws been in the region of full web, between the added holes, maybe it would not have failed at the loading it did. Or as soon as it did in the count of stress cycles.
In engineering logic, if you are going to remove material from the spar web, do so in reduced load regions, particularly bending loads. These would be outer span of the structure and with smaller holes on the inner span, but just not in the region of the highest load, the strut attachments.
And the case in hand here, do not add stress risers in the weakest spots.
And yes, I expect these spars would have failed in time.
They do, and did.
Steve Pierce
BENEFACTOR
Graham, TX
Good thing about original, un-modified Piper spars, there are literally tens of thousands out there that are as much as 80 years old, used, abused by both pilots and mechanics along with the elements and they are still providing great service.
aktango58
FRIEND
18AA
Very good observation Steve!
With no engineering background, and only learning through being around others and seeing failures, I am appreciative of the simplicity yet durability of the 1930's through 1940's designed aircraft. It is absolutely amazing how many of the popular light and working aircraft were designed prior to 1960... and continue to be used every day.
Just a quick look at the number of ADs issued on a given design can be a clue to the robustness of the design. Compare either PA-18s, PA-20s, and PA-22s to PA-28s. I think you could add all the ADs on the -18, -20, and -22 and still have less total ADs than on the PA-28-140!
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Hardtailjohn
Registered User
Marion, MT
Would a drilled and tapped hole for a machine screw be a better option than the PK screw, or would there be any appreciable difference?
John
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skywagon8a
MEMBER
SE Mass MA6
Interesting. Patey's test shows that had that lightning hole not been in the broken spar, perhaps that accident would not have happened. Patey's failure happened outboard of the strut attachment. By making that lightning hole inboard of the strut attachment the spar was weakened. Likely without any understanding by the maker of the hole, of what the result could be.
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