BOEING 737-7H4SOUTHWEST AIRLINES CO
Laverne, OK, USA · 2012-03-20·N247WN
What happened
NTSB summary, verbatimThe flight had been experiencing moderate turbulence along its route. Once the turbulence had subsided, the seatbelt sign was turned off. The flight attendants had resumed their service when the flight encountered three additional turbulence jolts. The aft galley flight attendant was trying to get seated and strapped into the jumpseat but was injured when another turbulence jolt occurred. She was thrown across the galley and hit her back/ribs on an object. Flight data recorder information showed that vertical accelerations fluctuated between about 0.75 g and 1.2 g; lateral accelerations also fluctuated during this period, between about +/- 0.05 g. An airmen’s meteorological information advisory for moderate turbulence was active for the accident location at the time. A significant meteorological information advisory for occasional severe turbulence was active for an area west of the accident location.
Probable cause
the Board's determinationThe airplane’s encounter with severe turbulence during cruise flight, which resulted in a serious injury to a flight attendant.
Occurrence sequence
1 stepsNTSB coding · CICTT taxonomy- 1 · Enroute-cruiseTurbulence encounterdefining event
Findings
1 causal · 0 contributing- CAUSEEnvironmental issues › Conditions/weather/phenomena › Turbulence › Clear air turbulence — Effect on personnel
Sequence of events
13 timed eventsfrom the FDR factual report · claude-sonnet-5- 05:45Pre-departure/climbp.3Figure 1 basic parameter plot begins, covering entire flight through landing at 10:05
- shortly after departureClimbp.3Aircraft climbed to flight level 380
- 08:24Cruise at FL380p.3Winds aloft speed calculated by the FMC was about 140 knots from about 160 degrees true
- 08:22 to 08:28 (about 5-6 minutes)Cruise at FL380p.3Vertical accelerations fluctuated between about 0.75g and 1.2g; lateral accelerations fluctuated between about +/- 0.05g
- 08:27Descentp.3Aircraft descended from flight level 380 to flight level 340
- 08:27:15Descentp.4Shortly after starting a descent to flight level 340, the fluctuating accelerations decreased significantly
- after descent to FL340 (approx 08:28 onward)Cruise at FL340p.3Fluctuations in vertical and lateral accelerations decreased, fluctuating between 0.997g and 1.004g and +/- 0.01g, respectively
- 08:38:20Cruise at FL340p.3During otherwise decreased vertical and lateral accelerations at FL340, the vertical acceleration varied from about 0.255g to 1.7g within a 1 second period of time
- 08:38:20 (1 second period)Cruise at FL340p.4Elevator surface deflection varied between 2.78 to -5.07 degrees; rudder surface varied between about -1.5 and 1.8 degrees
- about 15 seconds after 08:38:20Descentp.4Altitude hold mode was disengaged and the aircraft began a descent to flight level 320
- about 20 minutes after 08:38:20Cruise at FL320p.4FMC calculated a decrease in winds aloft at flight level 320 to under 30 knots for about 12 minutes; during this period the winds aloft shifted from about 160 degrees true to 340 degrees true
- 08:17:15 through 08:30:45Cruise at FL380p.4Expanded scale plots (Figures 3 and 4) show aircraft at FL380 experiencing fluctuations in vertical and lateral accelerations for about 6 minutes from 08:22 until 08:28
- 08:35:30 through 08:42:00Cruise at FL340/descent to FL320p.4Expanded scale plots (Figures 5 and 6) show the time period around the 0.255g to 1.7g vertical acceleration encounter at about 08:38:20
NTSB analysis
from the final reportThe flight had been experiencing moderate turbulence along its route. Once the turbulence had subsided, the seatbelt sign was turned off. The flight attendants had resumed their service when the flight encountered three additional turbulence jolts. The aft galley flight attendant was trying to get seated and strapped into the jumpseat but was injured when another turbulence jolt occurred. She was thrown across the galley and hit her back/ribs on an object. Flight data recorder information showed that vertical accelerations fluctuated between about 0.75 g and 1.2 g; lateral accelerations also fluctuated during this period, between about +/- 0.05 g. An airmen’s meteorological information advisory for moderate turbulence was active for the accident location at the time. A significant meteorological information advisory for occasional severe turbulence was active for an area west of the accident location.