BOEING 737 near Charleston, WV — 2009-07-13

Probable cause

Fuselage skin failure due to preexisting fatigue at a chemically milled step.

NTSB narrative

Flight data recorder data revealed that the airplane took off and climbed for about 25 minutes to an altitude of approximately 35,000 feet, at which point the cabin altitude warning activated, and the captain disengaged the autopilot. Postincident examination of the airplane revealed fatigue cracking of the fuselage skin near the leading edge of the vertical stabilizer adjacent to the rupture. The fatigue cracking penetrated the fuselage skin and created an approximate 18-inch by 12-inch flap in the skin that depressurized the airplane. The fuselage skin assembly near the leading edge of the vertical stabilizer was manufactured by bonding two full aluminum sheets together, then selectively chemically milling away pockets (bays) of the inner sheet. Continuous fatigue cracks initiated from multiple origins on the inner surface of the skin adjacent to the step formed at the edge of the chemically milled area and propagated outward. Following the Southwest Airlines (SWA) flight 2294 event, on September 3, 2009, Boeing issued Service Bulletin (SB) 737-53A1301, calling for repetitive external inspections to detect cracks in the fuselage skin along the chemically milled step at stringers S-1 and S-2 right and between BS 827 and BS 847. (The hole from the SWA event was within those boundaries.) If cracks are detected, operators are to contact Boeing for repair instructions. On January 12, 2010, the Federal Aviation Administration issued Airworthiness Directive 2010-01-09, which mandated the inspection requirements in SB 737-53A1301.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Inspection of fuselage skin near vertical stabilizer could have prevented failure.

NTSB coding

• Aircraft structural failure · Enroute-cruise

Evidence available

• FDR / data
• 21 docket documents

Docket documents21

• Flight Data Recorder 10 - Attachment 1 FDR Tabularfdr
• Flight Data Recorder 10 - Factual Report of Group Chairmanfdr
• Maintenance Records 11 - Attachment 12 - N387SW Paint Informationform
• Maintenance Records 11 - Attachment 2 - Maintenance Informationa and Maintenance Check Scheduleform
• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Flight Attendant Statementsinterview
• Pilot Statementsinterview
• Maintenance Records 11 - Attachment 1 - Air Carrier Certificateother
• Maintenance Records 11 - Attachment 10 - Time Limit Components Listingother
• Maintenance Records 11 - Attachment 11 - Crown Maintenance Program Requirementsother
• Maintenance Records 11 - Attachment 3 - Deferred and Open Itemsother
• Maintenance Records 11 - Attachment 4 - Supplemental Type Certificateother
• Maintenance Records 11 - Attachment 5 - Airworthiness Directive and Service Bulletin Listingother
• Maintenance Records 11 - Attachment 6 - Aircraft Flight Logother
• Maintenance Records 11 - Attachment 7 - Weight and Balance Summaryother
• Maintenance Records 11 - Attachment 9 - Major Repairs and Alterationsother
• Materials Laboratory 15 - Study 09-060Sother
• Materials Laboratory 15 - Study 09-060S Appendix 1other
• Maintenance Records 11 - Attachment 8 - Service Difficulty Reports and Mechanical Interruption Summary Reportsreport
• Maintenance Records 11 - Factual Report of Group Chairmanreport
• Materials Laboratory 15 - Factual Report 09-060report

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