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On February 24, 2008, about 1340 Pacific standard time, a Boeing 737-300, N648SW, encountered severe turbulence during descent for landing at McCarran International Airport, Las Vegas, Nevada. The airplane, which was being operated under 14 Code of Federal Regulations Part 121 by Southwest Airlines as flight 2809, was not damaged. Five passengers and one flight attendant (FA) sustained minor injuries, of which two were incurred as a result of the passengers' seat belts separating. The remaining 132 passengers and 4 crewmembers were not injured. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed for the regularly scheduled domestic passenger flight. The flight departed from Ontario, California, at 1251, with an intended destination of Las Vegas. Following the turbulence encounter, the airplane landed in Las Vegas without further incident.
According to information provided by Southwest Airlines, the flight was descending into Las Vegas at an altitude of 11,400 feet above mean sea level (msl) when it encountered severe turbulence. Two seatbelts became unhooked at seats 21B, and 22F, resulting in those passengers contacting the overheard baggage compartments. An infant, who was being held by his mother, hit the overhead area as well. Two additional passengers were injured by rough contact with the airplane structure during the turbulence upset. The captain communicated with Southwest Operations to arrange for paramedics to meet the airplane at the gate in Las Vegas. The flight continued and landed without further incident. Medical personnel met the airplane as requested, and the treated the injured passengers and FA, classifying their injuries as minor.
A review of the flight recorder data provided by Southwest Airlines disclosed that the airplane experienced a vertical acceleration minimum of approximately -0.761 G, followed 2 seconds later by a positive peak of approximately +1.762 G.
A National Transportation Safety Board investigator interviewed the crewmembers and several of the injured passengers. One FA, who was injured during the event, recalled that the seatbelt sign remained on during the entire flight and the captain called the flight attendants several times during the flight to advise them to sit down until the turbulence subsided. The captain would then inform the flight attendants when they could get back up and resume their normal duties. Just prior to the airplane encountering severe turbulence, the captain advised the flight attendants to clean up the cabin early and to take their seats.
The FA further stated that she was in the front of the cabin when the turbulence occurred. While attempting to brace herself, her body lifted up from the floor and her head hit the ceiling. She did not notice any passengers in the forward cabin that sustained injuries from the turbulence, and had only learned about injured passengers in the rear cabin after they were on the ground. After the turbulence the captain called the flight attendants and inquired about the condition of the passengers. The captain informed the passengers that there would be an Emergency Medical Technician (EMT) waiting for their flight for those needing medical assistance. She added that there were no passengers out of their seats when the turbulence occurred.
Another FA stated that about 5 minutes after the captain made the call to clean up the cabin, severe turbulence was encountered. She was just folding down her jump seat with her left arm when another jolt hit; she struck her head on the cabin ceiling. As the turbulence continued, her face and neck hit the galley door. She observed a baby seated on his mother's lap in seat 23F and recalled witnessing the baby hit the ceiling during the turbulence.
The passenger who occupied seat 21B, a seat that had the lap belt unhook, stated that while the airplane was descending to Las Vegas, the seat belt light was illuminated and the flight attendants were preparing for landing. The airplane encountered two episodes of violent turbulence in the vertical direction. He described the turbulence akin to a rapid cycling up and down. He estimated that the turbulence consisted of two episodes, approximately 20 seconds during each, with a lapse of time between them. His seat belt stayed intact during the first turbulence event and became taught. It subsequently became unhooked on the right side of his seat during the second episode. During the turbulence there were negative g-forces and when his restraint unhooked he was thrust up into the air and hit the top of his head on the overhead bin. He sustained a large knot on the top of his head and a sprained neck.
The passenger further stated that a young female's restraint became unhooked, and she was seated in the same row adjacent to his. When the young female contacted the overhead bin it was severe enough to damage a corner of the overhead bin. The passenger recalled that there was no warning of the impending turbulence by the flight crew or the flight attendants. The captain made an announcement after they landed and apologized stating that he did not see any evidence of turbulence on his radar prior to the event.
A passenger in seat 23F was holding her infant on her lap for the duration of the flight. She stated that she was holding her son on her left shoulder, as he had fallen asleep. The airplane felt as if it had suddenly dropped and as her son was launched upward, she reached for his legs. Her arm hit the metal part of the armrest between the seats and she observed all the passengers lift out of their seats. A FA quickly examined her son and found a gash on top of his head. The captain made an announcement about 5 minutes after the turbulence occurred, stating that the airplane had encountered an air pocket that he did not anticipate and there would be emergency personnel meeting their flight in Las Vegas.
TESTS AND RESEARCH
Southwest Airlines provided the following data for the seats and restraints on board the airplane at the time of the incident:
Seat 21B - Part Number: 87647323 (B/E Aerospace Inc. part number 86879043)
Serial Number: 398681
Date of Manufacture: November 1996
Seat 22F - Part Number: 87647324 (B/E Aerospace Inc. part number 86879042)
Serial Number: 352132(R.)
Date of Manufacture: August 1992
The seats were equipped with D-ring shackles (part number 4144000) manufactured by B/E Aerospace Inc.
The incident restraints were both manufactured by Davis Aircraft Products CO. Inc., as part number FDC6400-202-700-2-014. The restraints were comprised of two major sections, one which contained a locking latch and the other had a metal tongue (referred to in this report as female and male, respectively). The hardware utilized to affix these sections to their respective seat side encompassed a D-ring shackle attachment. This attachment was comprised of a steel hook and spring-loaded keeper. The spring-loaded keeper is designed to be secured to the steel hook with a cotter pin and two rivets.
According to Southwest Airlines, the restraints on the airplane did not have serial numbers and were therefore not individually tracked during maintenance. During the last 8Y Check (8 year scheduled inspection that lasts 12-14 days) on July 5, 2007, it was recorded that all seat assemblies are removed and inspected/refurbished. All seat belt assemblies are removed and placed in a box and returned to Davis Aircraft Products to be inspected/refurbished (rewebbed) and then returned to the operator for installation. There were no other maintenance actions on the airplane involving the seat belts at seats 22F and 21B after the 8Y check.
Investigators examined the restraints for seats 21B and 22F and noted that the cotter pins and rivets that secured the keepers to the steel hooks were intact. The keeper for the male restraint for seat 22F and for the female restraint for seat 21B were bent about 15 degrees from the center of the steel hook. Photographs of the damage are contained in the public docket for this accident.
The Safety Board issued recommendation A-99-11 on February 19, 1999, after two passenger seat belts became unhooked when an American Airlines Boeing 727-223 landed short of a runway threshold at Chicago O'Hare International Airport on February 9, 1998. Recommendation A-99-11 recommended that the Federal Aviation Administration (FAA):
"Reexamine the design of seat belts installed on passenger seats on air carrier, air taxi, and commercial airplanes to determine the reason some have 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 December 1999 the FAA formed an industry team to evaluate why the seat belts became unhooked from their seat attachments during turbulence or hard landings. The team examined the accident in the subject recommendation letter and a subsequent accident of an American Airlines flight 1420, an MD-82 in Little Rock, Arkansas, where a seat belt became unhooked during the accident. The industry team determined that on older seats using D-ring type seat attachment fittings:
"Detachment of the seat belt occurs when an out-of-plane load is applied to the seat belt end fittings by the seat belt webbing. As the seat belt attempts to align the end fitting with the load, the spring keeper is depressed and the seat belt can become detached from the end fitting."
The industry team and the FAA concluded that a seat belt became unhooked during a hard landing or turbulence only on seats equipped with a D-ring type seat attachment. The FAA further informed that Safety Board that in January 2001, the Society of Automotive Engineers' Seat Committee and several seat manufactures reported that the D-ring-type seat attachment has not been manufactured since 1992, and on December 22, 2003, the FAA issued a Special Airworthiness Information Bulletin (SAIB) No. NM-04-37 that advised the operators of transport category airplanes to expedite the replacement of D-ring-type seat anchors with an improved design fitting. The Boeing 737 airplane was listed as a possible airplane having the D-ring fitting installed on its seats. SAIB's are advisory documents and compliance is not mandatory.