BOEING 737-300Southwest Airlines Co.

The flight was descending to the destination airport, passing through an altitude of 11,400 feet above mean sea level (msl), when it encountered severe turbulence. Two seat belts became unhooked at seats 21B and 22F, resulting in those passengers contacting the overhead baggage compartments. An infant, who was being held by his mother, hit the overhead area as well. Two additional passengers and a flight attendant were injured by rough contact with the airplane structure during the turbulence encounter. The flight continued and landed without further incident. The flight data recorder disclosed that the airplane experienced a vertical acceleration over a 2-second period between -0.76 and +1.762 G's. Following the incident, the failed restraints were examined. Investigators noted that the keeper in the D-ring attachments for the belts for seats 21B and 22F were bent about 15 degrees from the center of the steel hook, which would allow the D-rings to unhook from the D-ring attached to the seat frame. NTSB Safety Recommendation A-99-11 recommended that the FAA reexamine the design of seat belts installed on passenger seats on air carrier, air taxi, and commercial airplanes to determine the reason some had become unhooked from their seat attachments during turbulence or a hard landing and establish a suitable means of ensuring that the seat belts remain attached to their shackles during all modes of flight. In 1999 the FAA formed an industry Seat Committee that determined the seatbelts that became unhooked during hard landing or turbulence were only on seats equipped with D-ring seat attachments. Because the Seat Committee reported that the seats with D-rings ceased being manufactured in 1992, the FAA issued a Special Airworthiness Information Bulletin (SAIB) NM-04-37, to advise the operators of transport category airplanes to expedite the replacement of D-ring-type seat anchors with an improved design fitting. The incident operator reported that seat 21B was manufactured in November 1996, and seat 22F was manufactured in August 1992. The incident operator’s airplane was a make and model that was listed in the SAIB as possibly having D-ring fitting type seat anchors installed. SAIB’s are advisory in nature and compliance is not mandatory.