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Topic-icon The logic behind. the wing spar AD?

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18 Jan 2022 19:19 #3351

Hi Bryan;
For what it's worth, I have remotely tracked some of these bolt removal/eddy current inspections and have not heard of bolt hole problems caused by bolt removal, but it's surely possible.
Perhaps that's the reason 1345A goes into greater detail about the bolt removal process--support the wings to take the shear load off the bolts, etc.
I don't know what percent of the inspected wings have failed the bolt hole inspection. but I do know that some owners have gone ahead and gotten the eddy current inspection done on both wings for peace of mind.
I've reached out to a couple of the eddy current companies in an attempt to get the results of its failure rate.
Steve

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17 Jan 2022 15:02 #3349

The only logic I can see is that it is a fishing expedition as the FAA appears to be establishing a present and future data set to determine the extent of risk. BTW, read between the lines on the latest SB1345A, so what changed between SB1345 and SB1345A? IMO, much damage is being done on the removal of bolts that have been installed for decades. Cadmium coated steel bolt friction fit against aluminum, what could go wrong?

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12 Jan 2022 21:14 #3347

H S-Arvind;
There is the AD (2020-24-05)for the "factored hours" calling for an eddy current inspection of the bolt holes.
There is also two other important inspections. I have attached the service information about these inspections.
In my opinion, when the FAA started looking at the PA 28 wings they found corrosion on the aft inboard portion of the main spar, and on the forward and aft stub spars where they bolt to the fuselage.
Although some are inspecting the aft side of the main spar by removing the fuel tank and using a borescope to view that area, there is a kit to install an inspection plate in the wing aft of the main spar.
The stub spars have steel inboard ends that are rivets to the aluminum spars. It's pretty common to see rust on those steel parts.
In order to look at the aft one, some airplanes require an inspection panel to be installed in the baggage compartment floor.
I've attached some service bulletins.
Steve

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10 Jan 2022 01:45 #3341

Got it. Thank you. So this AD is all about metal fatigue under high stress conditions rather than corrosion, which could be an additional contributing factor.

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10 Jan 2022 00:13 #3340

The AD is more about metal fatigue than corrosion. Metal fatigue occurs when metal is bent/stressed (think wing bending moments).

Take a soda can pull tab and bend it back and forth until it breaks. That is metal fatigue. So operation at low altitude in training or powerline type operations tend to be in more turbulence and more landings than high altitude cruise. Both contribute to metal fatigue in the wings.

I'm sure there are others that can address this more accurately than me but that is the basic theory. If you want to check for corrosion (always wise) I would not use the "FAA exemption" from the AD to stop you from doing a complete check for corrosion. Two different things! Make sure you check for corrosion and also comply with the AD as needed.

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09 Jan 2022 21:13 #3339

Hi:
I am in the process of evaluating a well maintained PA20-180 with 3400 non-flight school use for purchase.
Newbie here trying to understand the FAA logic behind the wing spar AD.
It seems to me that corrosion is a function of time. You can slow down corrosion significantly using inhibitors but any amount that's pre-existing has weakened the structure from its original rated specifications?
It appears that the FAA exempted any aircraft that hasn't reached 5000 factored hours, regardless of the amount of corrosion present? This seems to be fairly risky. Would appreciate any insight into whether I should pay for a full boroscope with 10X magnification?
Thank you.

Last edit: 09 Jan 2022 21:13 by S-Arvind.

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