Thursday, August 19, 2010

Southwest Airlines Boeing 737 fuselage skin rupture: NTSB final report

by B. N. Sullivan

NTSB photoThe U.S. National Transportation Safety Board (NTSB) has released its final report regarding the Southwest Airlines Boeing 737-300 that developed a hole in its fuselage during flight on July 13, 2009.  According to the NTSB report, the probable cause of this accident was, "Fuselage skin failure due to pre-existing fatigue at a chemically milled step."

At the time of the accident, the aircraft (registration N387SW) was en route from Nashville International Airport (BNA) to Baltimore-Washington International Airport (BWI), operating as Southwest Flt WN2294.  A hole in the crown of the aircraft's fuselage developed when the skin ruptured just forward of the vertical stabilizer.  When the  hole opened, the cabin depressurized.  The crew diverted to Charleston, WV where they made a safe emergency landing at Yeager Airport (CRW).  No one was injured.

NTSB photoQuoting from the summary of the NTSB report:
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.
Note:  The photos on this page were released by the NTSB In July of 2009.  The first photo (top of this page) shows the section of fuselage skin facing inside the aircraft.  The second photo shows the section of fuselage skin on exterior of aircraft.  Clicking on either photo will take you to a larger version on the NTSB website.

Links to the NTSB report [NTSB ID DCA09FA065]:  SummaryFull Narrative