Narrative:

I was initially contacted to perform several specific non-destructive testing (ndt) inspections on the left wing lower skin panel/risers/attach fittings just outboard of the #2 pylon as a precautionary measure by the customer who was attempting to determine the source of a fuel leak. Two existing repairs are installed on the lower wing skin. When I initially arrived to review what the customer was requesting; the tank had been drained and purged; and the lower wing skin cleaned off. The repairs were installed because of cracking previously found in two of the panel risers. The risers run spanwise inside the tank. The risers are milled into the wing planks and typically initially crack at the top of the riser and move down toward the plank. Once the repair is installed; the crack is only detectable at the top edge of the riser since repair angles are installed on the forward and aft sides of the riser which form a 'U' shaped cross section and are attached between the risers. Additionally; there are two layers of repair doubler installed on the lower side of the wing. At the locations of the repairs; the customer requested an x-ray inspection. I explained that the typical crack that occurs in the riser would not be possible to image because the orientation of the cracking in relation to the x-ray beam. He asked if the crack moved down the riser into the horizontal portion of the wing plank if we could detect it. I explained that we could not guarantee detection of such a crack; but it might be seen. The primary reasons for not seeing such a crack are: 1) the total doubler stack-up thickness is roughly three times the thickness of the original skin. A crack limited to the skin thickness would not produce a significant density change on the film. 2) cracking caused by stress corrosion is usually intergranular and may not form a straight line. It would more likely follow the grain structure; making it more difficult to see on an exposed film. 3) when the aircraft is sitting on the ground; the sides of a crack in the wing plank would be under compression; while it would tend to 'open up' during flight as the sides would tend to pull away from each other as the top of the wing produced lift. I informed the customer that the best method for detecting such cracking would be to remove the repairs and accomplish a high frequency eddy current (hfec) inspection of the planks adjacent to the risers. He requested that we conduct the x-ray inspection and I made it clear we could not guarantee the results and so stated on the written report we supplied after the inspection was complete. I did not see any crack indications on the film; and I made no judgments or statements regarding the airworthiness of the repaired areas. We also accomplished some other ndt tasks in the general area which required subsequent visits to check open fastener holes with eddy current. During one of the subsequent visits; I learned from statements other employees made that the existing repairs were typically made after a fuel leak was detected in the plank; therefore I suspect a crack must have run into the plank area. Additionally; an employee mentioned that the fuel leak they were looking for had been seen from another aircraft during flight. Although the checks we did were precautionary on the request of the customer; I believe the repairs should have been removed for a conclusive inspection. There was an issue with one of the open holes that was inspected with eddy current also. After one of the holes was oversized; equally spaced ridges or flutes were caused by the ream and were noted as consistent lift-off indications in hole bore. There was not a rejectable indication in the hole after the oversize; however the customer was informed that the 'lift-off' due to the geometry of the hole bore; could conceivably mask an eddy current indication.

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Original NASA ASRS Text

Title: A Non-Destructive Testing (NDT) Inspector reports that an X-ray shot inspection of a wing plank through a previous doubler repair on a Lockheed C-130 aircraft; was not adequate to determine if a crack had migrated from the top of the plank riser down into the horizontal section of the wing plank. Fuel was seen leaking from the left wing in flight.

Narrative: I was initially contacted to perform several specific Non-Destructive Testing (NDT) inspections on the left wing lower skin panel/risers/attach fittings just outboard of the #2 pylon as a precautionary measure by the Customer who was attempting to determine the source of a fuel leak. Two existing repairs are installed on the lower wing skin. When I initially arrived to review what the customer was requesting; the tank had been drained and purged; and the lower wing skin cleaned off. The repairs were installed because of cracking previously found in two of the panel risers. The risers run spanwise inside the tank. The risers are milled into the wing planks and typically initially crack at the top of the riser and move down toward the plank. Once the repair is installed; the crack is only detectable at the top edge of the riser since repair angles are installed on the forward and aft sides of the riser which form a 'U' shaped cross section and are attached between the risers. Additionally; there are two layers of repair doubler installed on the lower side of the wing. At the locations of the repairs; the customer requested an X-Ray Inspection. I explained that the typical crack that occurs in the riser would not be possible to image because the orientation of the cracking in relation to the X-ray beam. He asked if the crack moved down the riser into the horizontal portion of the wing plank if we could detect it. I explained that we could not guarantee detection of such a crack; but it might be seen. The primary reasons for not seeing such a crack are: 1) The total doubler stack-up thickness is roughly three times the thickness of the original skin. A crack limited to the skin thickness would not produce a significant density change on the film. 2) Cracking caused by stress corrosion is usually intergranular and may not form a straight line. It would more likely follow the grain structure; making it more difficult to see on an exposed film. 3) When the aircraft is sitting on the ground; the sides of a crack in the wing plank would be under compression; while it would tend to 'open up' during flight as the sides would tend to pull away from each other as the top of the wing produced lift. I informed the Customer that the best method for detecting such cracking would be to remove the repairs and accomplish a High Frequency Eddy Current (HFEC) inspection of the planks adjacent to the risers. He requested that we conduct the X-Ray inspection and I made it clear we could not guarantee the results and so stated on the written report we supplied after the inspection was complete. I did not see any crack indications on the film; and I made no judgments or statements regarding the airworthiness of the repaired areas. We also accomplished some other NDT tasks in the general area which required subsequent visits to check open fastener holes with Eddy Current. During one of the subsequent visits; I learned from statements other employees made that the existing repairs were typically made after a fuel leak was detected in the plank; therefore I suspect a crack must have run into the plank area. Additionally; an Employee mentioned that the fuel leak they were looking for had been seen from another aircraft during flight. Although the checks we did were precautionary on the request of the customer; I believe the repairs should have been removed for a conclusive inspection. There was an issue with one of the open holes that was inspected with Eddy Current also. After one of the holes was oversized; equally spaced ridges or flutes were caused by the ream and were noted as consistent lift-off indications in hole bore. There was not a rejectable indication in the hole after the oversize; however the Customer was informed that the 'lift-off' due to the geometry of the hole bore; could conceivably mask an Eddy Current indication.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.