DOUGLAS C 118A near Emmonak, AK — 2014-02-25

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.

NTSB narrative

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.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Failure to detect the crack during the last x-ray inspection prior to the incident.

NTSB coding

• Aircraft structural failure · Unknown

Evidence available

• 5 docket documents

Docket documents5

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Release of Aircraft Wreckage, NTSB Form 6120.15form
• Statement of Party Representatives to NTSB Investigationinterview
• Boeing Metallurgy Lab Reportreport
• Materials Laboratory Factual Reportreport

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