# NTSB ENG14IA016: Douglas C 118A, N351CE, Emmonak, AK, February 25, 2014 - Type: NTSB incident investigation - Classification: Incident · No injuries · Incident investigation · Final report - Date: February 24, 2014, 16:42 local (Feb 25, 2014, 01:42 UTC) - Place: Emmonak, AK - Coordinates: 62.791389, -164.504165 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=62.7914&lon=-164.5042&mi=10 - Phase of flight: [Unknown](https://openairdocs.org/ntsb-phases/990) (OpenAirDocs' grouping) - Defining event: [Unknown: Aircraft structural failure](https://openairdocs.org/ntsb-events/337) - Highest injury: No injuries - Injuries: 3 uninjured - Status: Completed, report October 18, 2016 - Report: Approved by Office Director - NTSB office: Aviation Engineering Division - ADs cited in the narratives: [AD 80-12-02 R1](https://openairdocs.org/ad/80-12-02-R1) (OpenAirDocs) - NTSB case key (mKey): 89346 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=ENG14IA016 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/89346/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=89346 - Page: https://openairdocs.org/ntsb/ENG14IA016 ## Probable cause The fatigue failure of the lower wing skin due to aging mechanisms. Contributing to the failure were lower wing skin inspection procedures insufficient to identify the crack. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Wing structure: plates/skins (on wing)](https://openairdocs.org/ntsb-findings/01035730): Aircraft › Aircraft structures · Fatigue/wear/corrosion (probable cause) - [Policy/procedure: adequacy of policy/proc](https://openairdocs.org/ntsb-findings/04021015): Organizational issues › Management · Manufacturer (probable cause) ## Analysis The airplane suffered a crack in the left wing lower skin just inside the left inboard engine nacelle. The crack was discovered by the flight crew during a post flight inspection when they saw fuel streaming from the nacelle area. The incident airplane was manufactured in 1954 and had accrued almost 36,900 flight hours. Examination of the lower wing skin revealed a crack with a total length about 15.5 inches. Two separate regions of pre-exiting progressive fracture were found that totaled about 4.12 inches in length. Fatigue cracking initiated and progressed from two separate rivet holes where a stringer was attached to the wing skin. There were no material discrepancies or other anomalies noted at the fatigue initiation sites. The remainder of the fracture was due to overstress separation. The fractured area of lower wing skin was inspected by x-ray in accordance with an existing Airworthiness Directive 611.7 hours prior to the discovery of the crack. No discrepancies were reported from this inspection. Post incident examination of the x-ray films showed the crack was present but very difficult to detect without close scrutiny, optimal lighting and a known crack location. The manufacturer released a Service Bulletin based on the findings of the investigation to provide more detailed instructions and additional methods to detect cracking of the lower wing skin at the inboard nacelle area. ## Factual narrative 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 withoverstress 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 of36,288.0 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. ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [After landing: Hazardous material leak/spill](https://openairdocs.org/ntsb-events/450) - [Unknown: Aircraft structural failure](https://openairdocs.org/ntsb-events/337) (defining event) ## Aircraft - Registration: [N351CE](https://openairdocs.org/aircraft/N351CE), registered now, S/N 44599, built 1954 (OpenAirDocs) - Reported make and model: DOUGLAS C 118A - Registry model: [Douglas C-118A](https://openairdocs.org/model/douglas-c-118a) (registry code 3021710) (OpenAirDocs) - Registry engine: [Pratt & Whitney R-2800 series](https://openairdocs.org/engine/pratt-whitney-r-2800-series) (registry code 52026, reciprocating) (OpenAirDocs) - Category: Airplane - Serial number: 44599 - Year built: 1954 - Airframe hours: 36900 - Engines: Pratt & Whitney R2800-CB3 253 horsepower reciprocating; Pratt & Whitney R2800-CB3 253 horsepower reciprocating; Pratt & Whitney R2800-CB3 253 horsepower reciprocating; Pratt & Whitney R2800-CB3 253 horsepower reciprocating - Damage: Minor damage - Flight: Part 121 - Operator: Tatonduk Outfitters Limited DBA - Registered owner: Tatonduk Outfitters Limited DBA - Airworthiness certificate: Standard, Transport - Last inspection: Continuous airworthiness, January 20, 2014 - Departed: ANC, ANCHORAGE, AK - Destination: ENM, Emmonak, AK - Flight plan: IFR - Airport: ENM, Emmonak ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 51; injuries: none; flight instructor, airline transport, flight engineer; airplane ratings: single-engine land, multi-engine land; instrument ratings: airplane; medical: class 1, with waivers/limitations; last flight review October 4, 2013; seat: left; restraint: lap only; flight hours: 15,000 total, 7,000 in this make and model, 10,000 as pilot in command, 153 in the last 90 days - Co-pilot; age 41; injuries: none; airline transport, flight instructor, flight engineer; airplane ratings: multi-engine land, single-engine sea, single-engine land; medical: class 1, without waivers/limitations; last flight review May 31, 2013; seat: right; restraint: lap only; flight hours: 12,000 total, 5,600 in this make and model, 5,000 as pilot in command, 180 in the last 90 days, 60 in the last 30 days, 3 in the last 24 hours - Flight engineer; injuries: none; flight engineer, airline transport; seat: center; flight hours: 8 total ## Weather - Conditions: Visual (VMC), Day - Wind: 110° at 16 knots, gusting 20 - Visibility: 10 statute miles - Lowest ceiling: 5,500 ft overcast - Temperature: 30 °F, dew point 19 °F - Altimeter: 29.75 inHg - Observed at: PAEM ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Incident 20140224003089I](https://openairdocs.org/incident/20140224003089I), Feb 24, 2014: Douglas C-118A, Emmonak, AK, Substantial damage. Ptrs number: AL07201401642. After fxga, flt 99, arrived at Emmonak, AK. The crew discovered a fuel leak in the post flight walk around inspection. The leak was… ## Docket (5 files) The files NTSB released with the investigation on October 19, 2016, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=89346. 8.3 MB in all. - Materials Laboratory Factual Report (item 1, .PDF, 15 photos, 3.3 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40445501&FileExtension=.PDF&FileName=Materials%20Laboratory%20Factual%20Report-Master.PDF - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 2, .PDF, 815 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40445502&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Master.PDF - Statement of Party Representatives to NTSB Investigation (item 3, .PDF, 417 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40445503&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation-Master.PDF - Release of Aircraft Wreckage, NTSB Form 6120.15 (item 4, .PDF, 60 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40445504&FileExtension=.PDF&FileName=Release%20of%20Aircraft%20Wreckage,%20NTSB%20Form%206120.15-Master.PDF - Boeing Metallurgy Lab Report (item 5, .PDF, 3.8 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40445505&FileExtension=.PDF&FileName=Boeing%20Metallurgy%20Lab%20Report-Master.PDF From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.