Iliamna, AK, USA
N401LC
LOCKHEED 382G
The airplane suffered a rupture in the forward pressure bulkhead canted web causing a rapid depressurization during flight. The crew declared an emergency and diverted to the nearest airport where they landed safely. The accident airplane was manufactured in 1975 and had accumulated 35,421 flight cycles. Examination of the fractured canted web revealed multiple fatigue cracks that initiated at the forward and aft surfaces of the web adjacent to a vertical line of rivets at the airplane centerline. The fatigue cracking from the aft face propagated through 85%-90% of the thickness and the fatigue cracking from the forward face propagated through the remaining thickness. The fatigue cracking was present near 21 consecutive rivet holes. Dark staining determined to be aluminum oxide was noted on the mating fracture faces in areas that contained the larger fatigue cracks indicating the larger fatigue cracks were present for some time prior to the accident flight. The aluminum oxide contained trace evidence of chlorine. Chlorides are known contaminants that promote corrosion in aluminum alloys. The investigation did not determine the source of the chlorine contamination. Detailed examination of the fatigue crack initiation sites revealed the presence of isolated areas that contained evidence of intergranular cracking. Intergranular cracking and chlorine contamination typically are associated with stress corrosion cracking in aluminum alloys. There were no material discrepancies noted on the fractured web. Areas between fatigue cracks and at the upper and lower ends were due to overstress separation. A fatigue crack striation study indicated that the cracks likely initiated about 5,000 cycles prior to the accident flight. The manufacturer released a Service Bulletin based on the findings of the investigation to provide more detailed instructions and updated intervals to inspect for cracking of the forward pressure bulkhead canted web.
HISTORY OF FLIGHTOn February 12, 2016, about 0745 Alaska standard time, a Lynden Air Cargo, LLC., Lockheed 382G airplane, N401LC, experienced a rapid depressurization during cruise flight about 100 nautical miles east of Bethel, Alaska. The flight crew declared an emergency and diverted to Iliamna Airport (ILI), Iliamna, Alaska, where they landed without further incident. There were no injuries to the 5 crew members onboard and the airplane sustained substantial damage. The domestic cargo flight was operating under the provisions of 14 Code of Federal Regulations Part 121 Supplemental from Bethel Airport (BET), Bethel, Alaska, to Elmendorf Air Force Base (EDF), Anchorage, Alaska. PERSONNEL INFORMATIONThe captain, age 47, held an Airline Transport Pilot certificate with single-engine land, multiengine land, and instrument airplane ratings. He held a type rating for the L-382 airplane. His most recent FAA first-class airman medical certificate was issued on October 23, 2015, without limitations. His most recent FAA proficiency check was dated September 17, 2015, and was performed in a Lockheed 382G airplane. His reported flight time after the accident was a total flight time of 20,528 hours with 3,769 hours as pilot-in-command in the accident airplane make and model. The check pilot, age 51, held an Airline Transport Pilot certificate with single engine land and sea, multiengine land, and instrument airplane ratings. He held type ratings in the DC-3, DC-7, DC-8, and L-382 airplanes. His most recent first-class airman medical certificate was issued on January 25, 2016, with the limitation that he must wear corrective lenses. His most recent FAA proficiency check was dated July 21, 2015, and was performed in a Lockheed 382G airplane. His reported flight time after the accident was a total flight time of 25,503 hours with 8,508 hours as pilot-in-command in the accident airplane make and model. The flight engineer held a Flight Engineer certificate with a type rating for the L-382 airplane. His reported flight time after the accident was a total of 13,300 hours. AIRCRAFT INFORMATIONThe accident airplane, a Lockheed 382G, serial number 4606, was manufactured in 1975 and had accumulated a total of 55,422 hours and 35,421 cycles at the time of the accident. The most recent continuous airworthiness inspection of the airplane occurred on February 8, 2016, at an airplane total time of 55,393.5 hours. METEOROLOGICAL INFORMATIONThe nearest official reporting station was an Automated Surface Observing System at Iliamna Airport, Iliamna, Alaska, located about 90 nautical miles northwest of the airplane at the time of the event. About 8 minutes after the accident, at 0753 AKS, an Aviation Routine Weather Report (METAR) reported: Wind, 260 degrees at 5 knots; visibility, 10 statute miles; sky condition, broken clouds at 12,000 feet; temperature, 0 degrees C; dew point, -2 degrees C; altimeter, 29.32 inches Hg. AIRPORT INFORMATIONThe accident airplane, a Lockheed 382G, serial number 4606, was manufactured in 1975 and had accumulated a total of 55,422 hours and 35,421 cycles at the time of the accident. The most recent continuous airworthiness inspection of the airplane occurred on February 8, 2016, at an airplane total time of 55,393.5 hours. WRECKAGE AND IMPACT INFORMATIONExamination of the airplane revealed a rupture (crack) of the forward pressure bulkhead canted web at fuselage station (FS) 93. The crack was located near a row of 24 vertically aligned rivets at the airplane centerline and intersected one of the rivet holes. The fracture also progressed outboard (to the right side) towards the next line of vertical rivets at the upper and lower ends. Photographs of the canted bulkhead taken immediately after the accident showed evidence of dark staining emanating from the vertical fracture. The staining had a streaked appearance towards the left side of the bulkhead. Four of the radome closure fasteners fractured during the event allowing the radome to partially open in flight. The radome remained attached to the airplane at the upper hinge. The damage to the forward pressure bulkhead was determined to be substantial in accordance with NTSB regulations. The fractured portion of the canted bulkhead was removed from the airplane and shipped to the NTSB for examination. ADDITIONAL INFORMATIONLockheed Martin Aeronautics Company (LMAC) recommended inspection procedures for the canted bulkhead are contained in Service Manual Publication (SMP) 515-C, card no. ST-1, "Detailed Visual Inspection – Exterior Fuselage". The inspection card details a visual inspection of all the fuselage external surfaces with all fairings, access doors, and panels removed but does not specifically provide instructions to open the radome and examine the canted bulkhead. LMAC recommends that the task be accomplished every 6 years. Lynden Air Cargo (LAC) last performed the ST-1 inspection on March 25, 2013, at an aircraft total time of 51,527 hours and 29,831 cycles with no findings. Examination of the airplane maintenance records revealed that the canted bulkhead on the accident airplane had never been repaired and there were no applicable Service Bulletins or Airworthiness Directives. Following the accident, LAC examined the canted bulkheads on their other L-382 airplanes with no findings. LMAC examined the service history records for both military and commercial operators of the airplane model and did not find any other similar occurrences. On March 10, 2016, LMAC issued Alert Service Bulletin (SB) A382-53-67, Fuselage – Inspection of Fuselage Station (FS) 93 Bulkhead for Cracks, providing procedures for a detailed visual inspection of the upper canted bulkhead on all 382, 382B, 382E, 382F, and 382G airplanes. The SB was to be accomplished within 90 days for all aircraft with more than 15,000 flight cycles and repeated at each C Check, not to exceed 3 years or 3,000 flight cycles. TESTS AND RESEARCHThe fractured portions of the canted bulkhead were examined in the NTSB Materials Laboratory. The details of this examination are contained in the Materials Laboratory Factual Report in the public docket for this accident. The rivet hole locations were labeled H1 through H24 beginning with the upper most rivet on the submitted web section. The fracture was located to the right of a vertically aligned row of rivets where a C-channel stiffener was fastened to the aft side of the bulkhead. The fracture curved up and outboard above rivet H1 and curved down and outboard below rivet H23. The vertical portion of the fracture aligned with the right outboard edge of the manufactured head of the rivets along its length on the forward face of the web. Both the forward and aft faces of the web were coated with green paint although the forward face was a lighter green. Material cross sections revealed one layer of paint on the aft face and two layers of paint on the forward face. The mating vertical fracture faces showed evidence of black deposits over a length about 10.5 inches between rivets H7 and H21 prior to any cleaning. Testing confirmed the deposits to be aluminum oxide. The fracture face in areas that contained aluminum oxide also contained evidence of chlorine. Bench binocular microscope and scanning electron microscope (SEM) examination of the fracture faces revealed multiple origin fatigue cracking that emanated from both the forward and aft faces of the web next to 21 consecutive rivet holes (H3 through H23). The fatigue cracking emanating from the aft face of the web propagated through about 85% to 90% of the thickness and those emanating from the forward face of the web propagated through the remaining thickness. The fatigue crack regions adjacent to each hole location were of varying lengths vertically and were separated by areas with micro-void coalescence features typical of overstress separation. The vertical length of each fatigue crack region measured between 0.55 inch and 0.75 inch with the largest fatigue crack near hole H10 measuring 0.75 inch. Detailed SEM examination of the fatigue cracks revealed initiation on both the forward and aft faces at isolated areas that contained evidence of intergranular cracking. The intergranular cracking portion extended between 0.0013 inch and 0.0023 inch below the surface. The fractures located above hole H1 and below hole H23 showed rough texture features on a slant plane consistent with overstress separation. The forward and aft faces were examined to determine their condition. The aft surface of the web exhibited multiple irregular cracks in areas between each rivet hole and the fracture face. Similar cracks were found under the location of all the rivet heads on the aft surface and some of the rivet head locations on the forward face. Isolated areas of corrosion were found on both the forward and aft faces of the web. The manufacturer's drawing specified the web material as 7075-T6 aluminum alloy with a 7072 aluminum alloy cladding layer on both surfaces. During assembly the web was to be covered with Alodine and coated with green epoxy primer. Testing was performed to verify the material composition, electrical conductivity, hardness, and material microstructure along with the thickness of the web, clad layer and paint layers. All were within specifications. A fatigue striation count study, contained in the public docket, was developed for a fatigue crack in the general area of hole H17. The fatigue crack propagated forward from the aft face of the web and terminated between 0.0425 inch and 0.0495 inch from the origin. The terminus of the fatigue crack was intersected by an opposing fatigue crack that originated from the forward face of the web. The accumulated cycles at a distance of 0.0425 inch and 0.0495 inch from the fatigue origin was calculated to be 4,615 cycles and 5,079 cycles, respectively.
The fatigue failure of the forward pressure bulkhead canted web that originated from stress corrosion cracking areas. Contributing to the failure were inspection procedures and intervals inadequate to identify the crack.
Source: NTSB Aviation Accident Database
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