NTSB Factual Report

HISTORY OF THE FLIGHT

On March 20, 2012, at 0830 central daylight time, a Boeing 737-7H4, N247WN, experienced turbulence during cruise flight near Laverne, Oklahoma. Southwest Airlines operated the airplane as Flight 162 under the provisions of 14 Code of Federal Regulations Part 121. The flight crew, 2 flight attendants, and 137 passengers were not injured; 1 flight attendant sustained serious injuries. The airplane was not damaged. An instrument flight rules (IFR) flight plan had been filed for the flight that departed Tampa International Airport (TPA), Tampa, Florida, at 0653 eastern daylight time. The flight was destined for Phoenix Sky Harbor International Airport (PHX), Phoenix, Arizona.

According to Southwest Airlines, the airplane was in cruise flight over Laverne when it encountered three turbulence jolts. The aft galley flight attendant was injured while trying to get seated and strapped into the jump seat when another turbulence jolt occurred. The flight attendant was able to perform all required safety duties for the remainder of the flight, but reported significant back pain upon arrival to Phoenix. The flight attendant was not transported to a hospital. On March 22, the flight attendant informed the company that she had been diagnosed with a rib fracture.

In statements submitted by the flight attendants, they reported that there was a long duration of turbulence during cruise. When the turbulence diminished, the seatbelt sign was turned off. They got up and then it started to become turbulent again. As the two flight attendants in the back of the airplane attempted to secure themselves in the aft jumpseats, one of them was thrown across the galley and hit her back/ribs on an object.

FLIGHT RECORDERS

Data obtained from the flight data recorder (FDR) showed that the airplane climbed to flight level 380 shortly after departure. About 0824, winds aloft speed calculated by the flight management computer (FMC) was about 140 knots from about 160 degrees true. For about 5 minutes around this time at flight level 380, vertical accelerations fluctuated between about 0.75g and 1.2g; lateral accelerations also fluctuated during this period, between about +/- 0.05g. At 0827, the airplane descended from flight 380 to flight level 340. Shortly after the descent began, the fluctuations in vertical and lateral accelerations decreased, fluctuating between 0.997g and 1.004g and +/- 0.01g, respectively. After 9 minutes at flight level 340, during the period of otherwise decreased vertical and lateral accelerations, the vertical acceleration varied from about 0.255g to 1.7g within a 1-second period of time at 08:38:20. About 20 minutes after this vertical acceleration encounter, the FMC calculated a decrease in winds aloft at flight level 320 to under 30 knots for about 12 minutes; during this 12-minute period the winds aloft shifted from about 160 degrees true to 340 degrees true.

Because this was not determined to be an accident until 2 days after the event, the cockpit voice recorder information was written over by normal use.

METEOROLOGICAL INFORMATION

A Senior Meteorologist completed a weather study. A 250-hectopascal (hPa) analysis chart obtained from the Storm Prediction Center and valid for 0700 central daylight time (CDT) identified a portion of a jet stream stretching north from central Texas into Canada. A jet streak (localized regions of very fast winds embedded within the jet stream) was identified in the vicinity of the accident location with southerly wind magnitudes greater than 150 knots. Calculations by the Rawinsonde Observation Program (RAOB) indicated the potential for significant clear-air turbulence between about 36,000 and 40,000 feet.

An Airmen’s Meteorological Information (AIRMET) advisory for moderate turbulence between FL220 and FL430 was issued at 0525 CDT and was active for the accident location at the accident time. A Significant Meteorological Information (SIGMET) advisory for occasional severe turbulence between FL330 and FL380 was issued at 0526 CDT for an area west of the accident location. This SIGMET was valid until 0926 CDT.

NTSB Probable Cause

The airplane’s encounter with severe turbulence during cruise flight, which resulted in a serious injury to a flight attendant.