HISTORY OF FLIGHT On February 24, 2014, about 1642 Alaska standard time, the crew of an Douglas C-118A airplane, N351CE, discovered a crack about 18 inches long in the left wing lower skin during a post-flight inspection after landing at Emmonak Airport (ENM), Emmonak, Alaska. The crack was located near wing station 175 inside the number 2 engine nacelle and extended from the forward edge of the skin panel at the forward spar, aft past stringer 19. A temporary repair was installed, and the airplane was ferried to Fairbanks for permanent repair. The airplane was registered to Tatonduk Outfitters, Ltd., dba Everts Air Cargo, and operated as a 14 Code of Federal Regulations, Part 121 Supplemental non-scheduled domestic cargo flight. The flight originated from Ted Stevens Anchorage International Airport (PANC), Anchorage, Alaska, about 1450 Alaska standard time. DAMAGE TO AIRPLANE The flight crew noticed a steady stream of fuel from inside the number 2 nacelle during a post-flight inspection of the airplane. Maintenance personnel discovered the crack in the lower wing skin and noted no damage to the front spar, stringers, or other surrounding structure. Initially, the operator reported that a temporary repair would be affixed to the wing so that the airplane could be ferried to a maintenance base for further evaluation and repair. Several splice plates (doublers) were affixed to the lower wing skin and the airplane was ferried to Fairbanks, Alaska. After initial evaluation, the operator reported that the permanent repair would not remove the cracked wing skin pieces and the NTSB decided that an investigation would be impossible without the fractured wing skin. On May 29, 2014, the operator reported that a permanent repair would be accomplished on the airplane that would entail removing the cracked wing skin so the NTSB initiated an investigation. A section of lower left wing skin about 20 inches long by about 3 inches wide and containing the entire crack was removed from the airplane and shipped to the NTSB in June, 2014. AIRCRAFT INFORMATION The incident airplane, a Douglas C-118A, serial number 44599, was manufactured in 1954 and had accumulated 36,899.7 hours total time when the crack was found. METEOROLOGICAL INFORMATION The crew reported that the flight was conducted under visual flight rules conditions with clear skies and more than 10 miles of visibility. During the flight they reported encountering one bump of moderate turbulence. TESTS AND RESEARCH The fractured piece of lower wing skin was examined at the NTSB Materials Laboratory in Washington, DC. The details of the examination can be found in the NTSB Materials Laboratory Factual Report contained in the public docket for this incident. The total fracture measured about 15.5 inches long and examination of the fracture faces revealed two regions exhibiting features consistent with pre-existing progressive cracking, with the remainder of the fracture exhibiting features consistent with overstress separation. The two progressive regions contained areas of fatigue cracking and areas of mixed mode fracture (fatigue with intermittent overstress regions). Fatigue cracking progressed forward and aft from the rivet hole location coincident with the forward flange of wing stringer 18 and both cracks initiated at the lower edge of the hole. Fatigue cracking also progressed forward and aft from the rivet hole location coincident with the aft flange of wing stringer 18 and both cracks initiated at the upper edge of the hole. There was no evidence of material defects at the four fatigue crack initiation points. The progressive crack forward of the stringer 18 forward hole progressed about 0.07 inch in stable fatigue and about 1.89 inches in mixed mode for a total length of about 1.96 inches. The progressive crack aft of the stringer 18 forward hole progressed about 0.25 inch in stable fatigue and about 0.72 inch in mixed mode for a total length of about 0.97 inch. The progressive crack forward of the stringer 18 aft hole progressed about 0.20 inch in stable fatigue and about 0.85 inch in mixed mode for a total length of about 1.05 inches. The progressive crack aft of the stringer 18 aft hole progressed about 0.03 inch in stable fatigue and about 0.11 inch in mixed mode for a total length of about 0.14 inch. The total length of progressive cracking was about 4.12 inches. The chemical composition, hardness and electrical conductivity of the wing skin were examined, and all were consistent with 7075-T6 aluminum alloy. It was noted that there were several areas of the fracture face that had a brown material consistent with sealant on them. The operator reported that the area of the fracture was covered with sealant before attaching the temporary repair doublers. Two additional small cracks were noted on the skin section submitted but were not opened and examined. One crack was located aft of the stringer 19 forward flange hole and the other was located between two holes near the stringer 18 location. ADDITIONAL INFORMATION The location of the fracture on the incident airplane left lower wing skin was located near wing station 175, just inboard of the number 2 (left, inboard) nacelle, outboard nacelle attach angle. This area of the lower wing skin was only visible by looking up into the main landing gear wheel well. Federal Aviation Administration (FAA) Airworthiness Directive (AD) 80-12-02R1 was originally effective in June 1980 and revised in January 1987 requiring radiographic (x-ray) inspection of the lower wing skins on all McDonnell Douglas DC-6, -6A, -6B, R6D, and C-118 airplanes. The AD required repetitive x-ray inspections of the left and right wing lower wing skins, stringer, and fittings between wing stations 60 to 130 and 167 to 185 from the front spar to the center spar every 1,000 flight hours. The operator performed the most recent AD x-ray inspection on the incident airplane on December 10, 2012, at an airframe total time of 36,288.0 hours, 611.7 hours prior to the incident. There were no discrepancies reported from this inspection. The original x-ray films were provided to the investigation for examination. Under close scrutiny, optimal lighting conditions, and with a known crack location the crack was evident on the x-ray films but was difficult to detect and was estimated to be 4 inches long at that time. The manufacturer reported that the minimum detectable crack size for this type of x-ray inspection is about 2 inches. Based on the condition and resolution of the x-ray films, the manufacturer reported that the crack was highly likely to be missed by the operator. Boeing Commercial Airplanes released Alert Service Bulletin (SB) DC6-57A001 on April 28, 2016, based on the findings from this investigation. The SB details revised inspection procedures for crack detection in the area around wing station 175. The SB calls for high frequency eddy current, low frequency eddy current, and radiographic (x-ray) inspection of the lower wing skin inside the inboard nacelles on all DC-6, -6A, -6B, C-118A, R6D-1, and R6D-1Z airplanes.