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NTSB Cites 'A Chemically Milled Step' In Airplane Depressurization

"Preexisting Fatigue" Named As Probable Cause For A Hole Opening In A Southwest Airlines 737

The NTSB says "fuselage skin failure due to preexisting fatigue at a chemically milled step" is the probable cause for a 18x12 inch hole opening up in the roof of a Southwest Airlines 737 last year, causing the airplane to depressurize. No one was injured due to the incident.

NTSB Identification: DCA09FA065
Scheduled 14 CFR Part 121: Air Carrier operation of SOUTHWEST AIRLINES CO
Accident occurred Monday, July 13, 2009 in Charleston, WV
Probable Cause Approval Date: 8/18/2010 1:52:06 PM
Aircraft: BOEING 737, registration: N387SW
Injuries: 131 Uninjured.

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.


NTSB Photo Exterior Skin

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.


NTSB Photo Interior Skin

The airplane, serial number 26602, was delivered to SWA in June 1994. At the time of the SWA event, the airplane had accumulated approximately 42,500 cycles and 50,500 hours.

FMI: www.ntsb.gov

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