Finds Rivet Lines Did Not Line Up Properly Where Fuselage
Opened
The NTSB has updated its investigation of the April 1, 2011
incident involving Southwest Airlines flight 812, which experienced
a rupture in the fuselage while in flight.
NTSB Photo
The Boeing 737-300 registration N632SW, experienced a rapid
depressurization caused by a rupture in the fuselage. The flight
was at 34,000 feet when the depressurization occurred. The flight
crew conducted an emergency descent and diverted the flight to Yuma
International Airport, Yuma, AZ. At the time of the accident, the
aircraft had accumulated 48,740 hours of service and 39,781 cycles
(a cycle is a takeoff and landing). The accident aircraft was
delivered to Southwest Airlines on June 13, 1996.
On-scene inspection by NTSB investigators revealed an
approximately 9-inch wide by 59-inch long rectangular-shaped hole
in the fuselage crown on the left side of the airplane, aft of the
over-wing exit. The 59-inch longitudinal fracture occurred in the
aluminum fuselage skin along the lap joint at stringer-4 left
(S-4L) between body station (BS) 666 and BS 725. At S-4L, the crown
skin overlaps the lower skin forming a lap joint. The two skins are
connected at the lap joint by three rows of rivets (referred to as
lower, middle, and upper row of rivets.) The fracture was through
the lower skin and connected 58 consecutive rivet holes in the
lower row of lap joint rivets. The exterior surface of the skin in
the area of S-4L is painted blue. Evidence of blue paint was also
found inside the joint between the upper and lower skin and on
several areas of the skin fracture surface.
Southwest 737 File Photo
Following an on-scene examination of the accident aircraft, a
portion of the fuselage skin that contained the hole and another
portion of the skin located forward of the hole (total size 116
inches by 19 inches) were excised from the accident aircraft and
transported to the NTSB Materials Laboratory in Washington, DC. The
airplane was then released back to the operator.
At the NTSB Materials Laboratory, microscope examination of the
fracture faces of the ruptured skin revealed fatigue cracks
emanating from at least 42 of the 58 rivet holes connected by the
fracture. Electrical conductivity measurements, hardness tests, and
X-ray energy dispersive spectroscopy elemental analysis of the skin
in the area of the fracture revealed that the aluminum skin
material was consistent with the specified material. The skin was
the specified thickness.
Non-destructive eddy current inspections conducted around intact
rivets on the removed skin section forward of the rupture revealed
crack indications at nine rivet holes in the lower rivet row of the
lap joint. To assess the condition of the intact rivets and the
skin rivet holes, X-ray inspections were performed on the skin
located forward of the rupture location. This inspection revealed
gaps between the shank portions of several rivets and the
corresponding rivet holes for many rivets associated with S-4L.
Upon removing selected rivets, the holes in the upper and lower
skin were found to be slightly offset relative to each other and
many of the holes on the lower skin were out of round.
In this ongoing investigation, the NTSB Materials Laboratory
work is actively conducting additional inspections and examinations
in the following areas:
- Removal of rivets and examination of rivet hole dimensions,
rivet dimensions, and rivet hole alignment between upper and lower
skins.
- Detailed fractographic analysis of the skin fractures emanating
from the rivet holes using optical and scanning electron
microscopes.
- Fatigue striation analysis using a scanning electron microscope
of specific skin fractures to determine the rate of crack
propagation.
- Additional portions of the lap joints from the accident
aircraft.
Following the depressurization accident and on-scene examination
of the accident aircraft, Boeing issued Alert Service Bulletin SB
737 53A1319-00 on April 4 instructing operators of certain Boeing
737-300, 400, and 500 aircraft to inspect the lower row of
fasteners at stringer S-4R and S-4L, from BS 360 to BS 908 for
cracking in the lower skin of the lap joint on airplanes.
The FAA issued Emergency Airworthiness Directive AD 2011-08-51
on April 5 mandating the inspections in the Boeing Service
Bulletin. To date, the NTSB has been informed that 136 airplanes
have been inspected worldwide in accordance with the Service
Bulletin and Airworthiness Directive including all U.S. registered
airplanes covered by the Directive. As stated in a previous
release, four of these airplanes were found to have crack
indications at a single rivet and one airplane was found to have
crack indications at two rivets. These airplanes had accumulated
between 40,000 and 45,000 total cycles. The lap joints from these
areas of the subject airplanes have been removed and will be fully
documented as part of the NTSB investigation.