History of Flight
On December 1, 2011, at 1953 Pacific standard time, a Boeing 737-7H4, registration N261WN, operating as Southwest Airlines flight 1489, encountered turbulence at flight level 30,000 feet (FL300) over Yosemite National Park, California. The flight was a scheduled passenger flight under 14 Code of Federal Regulations Part 121, departing Albuquerque International Sunport Airport (ABQ), New Mexico, and destined for Metropolitan Oakland International Airport (OAK), California. An instrument flight rules (IFR) flight plan was filed.
The aircraft had been cruising at FL400 and was descending to FL300 when it encountered severe turbulence. All three flight attendants were not seated at the time. One flight attendant in the forward galley was thrown up and down and side to side, with her feet leaving the cabin floor, but she managed to hold onto a counter and eventually secure herself in the forward jumpseat. The two flight attendants in the aft galley were thrown around, striking their heads and backs, and eventually secured themselves in their respective jumpseats, remaining there for the remainder of the flight. One flight attendant sustained serious injuries; two flight attendants received minor injuries. The two pilots and 95 passengers were not injured. Both injured flight attendants were transported to the hospital via ambulance; one was released with minor injuries, and the other was released but later informed of a compression fracture of his vertebrae. No passengers required transport to the hospital.
According to statements from the flight attendants, the flight crew had advised them to secure the cabin early due to expected turbulence later in the descent. Flight crew statements indicated that the flight attendants had been required to be seated multiple times throughout the flight due to turbulence. The captain reported that the descent from FL400 to FL300 was due to moderate turbulence, and they were authorized to stop at any smooth altitude. Continuous turbulence was experienced through FL320. At FL300, the ride improved briefly, but then abrupt and significant severe turbulence occurred for approximately 10 seconds. The aircraft accelerated from .760 mach to .865 mach with vertical pitch change and associated yaw. The crew requested an immediate descent, checked on cabin crew and passengers, were advised of injuries, declared an emergency, and landed at Oakland where medical personnel met them.
The first officer reported that upon departure, flight attendants were requested to remain seated until called. About 45 minutes after departure, the captain cleared them to move around. Additional turbulence prompted the captain to request them to take their seats. At FL400, intermittent moderate turbulence led to a request for a different altitude, and the flight descended to FL300. All passengers had seatbelts on because the seatbelt sign was illuminated, but the three flight attendants were not seated. The first officer could not recall the captain clearing them from their seats again.
Flight attendants stated that the flight crew informed them to secure the cabin 35 minutes early due to projected turbulence. Shortly after, severe turbulence occurred, lifting all flight attendants from the floor and strewing unsecured service items about the galleys.
According to the dispatcher, the flight was planned and filed to descend to FL260 prior to entering the area of the event.
Flight Recorders
The cockpit voice recorder (CVR) was examined but contained no audio pertinent to the incident, consistent with the recording being overwritten by subsequent events. Data from the flight data recorder (FDR) showed that at 1931, the aircraft was in level flight at 40,000 feet experiencing vertical accelerations of about +/- 0.1g. At 1943:05, it descended to 30,000 feet. The flight management computer computed wind speed showed a significant increase at 30,000 feet compared to 40,000 feet. At 1953:21, while level at 30,000 feet, the aircraft experienced vertical accelerations for about 13 seconds, with maximum negative and positive accelerations of -0.18g and +3.52g, respectively.
Meteorological Information
A Senior Meteorologist reviewed weather information. Synoptic conditions were favorable for mountain wave activity over California. The National Weather Service had issued significant meteorological information (SIGMET) and airman’s meteorological information (AIRMET) notices for turbulence over the region. Additional information from the flight crew’s enhanced weather information systems (EWINS) provider (SW45) expected severe turbulence between FL300-420. The event occurred in the region covered by these notices. No visual sign of turbulence was observed other than some lower cap or lenticular clouds over the peaks, but visual meteorological conditions prevailed at the location.