Lockheed 382G N401LC accident near Iliamna, 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 crack.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for DCA16LA096 for the full record.