Lockheed 382G-44K-30 N403LC accident near St. Mary'S, Alaska

Probable cause

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 fatigue cracks.

NTSB analysis

The airplane suffered a fracture in the forward pressure bulkhead canted web causing a rapid depressurization during flight. The crew declared an emergency, descended and continued to the destination airport where they landed safely. The accident airplane was manufactured in 1975 and had accumulated 36,034 flight cycles.

Examination of the fractured canted bulkhead 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 largest fatigue cracks propagated from the aft face through 90%-95% of the web thickness prior to intersecting smaller fatigue cracks which propagated from the forward face. The fatigue cracking was present near 21 consecutive fastener holes. Several areas exhibited black/brown and greenish deposits on the mating fracture faces in areas that contained the larger fatigue cracks indicating the larger fatigue cracks were present for some time before the accident flight.

Trace evidence of chlorine was present near the edge of the fracture. Chlorides are known contaminants that promote corrosion in aluminum alloys. The investigation did not determine the source of chlorine contamination.

Detailed examination of the fatigue crack initiation sites revealed the presence of isolated areas of intergranular and branched cracking. Intergranular cracking, chlorine contamination, and a constant or prolonged state of applied or residual tensile stress 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 largest cracks likely initiated from the aft surface of the web about 4,098 cycles prior to the accident flight.

Lockheed published Alert Service Bulletin (SB) A382-53-67 on March 10, 2016, applicable to all model 382, 382B, 382E, 382F, and 382G airplanes based on information from a previous investigation DCA16LA096. The bulletin recommended a detailed visual inspection of the forward side of the FS 93 bulkhead web for cracks and any loose or missing fasteners; however, an inspection of the aft surface was not required. The bulletin specified the initial inspection should be performed within 90 days on any aircraft that had accrued more than 15,000 flight cycles with repetitive inspections recommended every C-check not to exceed 3 years or 3,000 flight cycles.

According to maintenance records, the most recent SB inspection on the airplane accident was completed on November 7, 2022, with no discrepancies noted. In addition, there was no record of any previous repairs to the canted bulkhead web.

Following this accident the manufacturer reported that a new Service Bulletin is in development to add an ultrasonic inspection of the FS 93 canted bulkhead web for cracking. They estimated that the bulletin would be completed and FAA approved by the end of calendar year 2026. In addition, the manufacturer anticipated that a maintenance action to remove and replace the FS 93 bulkhead web after a specified number of cycles would be added to the maintenance program once further engineering analysis was completed.

Text from the NTSB. Open the NTSB docket for DCA25LA061 for the full record.