"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.