Incident Overview
On December 1, 2011, about 1339 Pacific standard time, United Airlines flight 721, a Boeing 757-222 (registration N547UA), experienced turbulence at flight level 300 during cruise. The flight was operating under 14 CFR Part 121 as a scheduled domestic passenger service from Denver International Airport (DEN) to San Francisco International Airport (SFO). Visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed.
Events Leading to Turbulence
The flight departed Denver at 1215. The planned cruise altitude was FL360, but the flight descended to FL300 at 13:17 PT due to reports of smoother conditions. As the aircraft neared the Sierra Nevada Mountains on the Modesto 3 Standard Terminal Arrival for SFO, reports of turbulence ahead were received. The captain illuminated the seat belt sign and announced for passengers to be seated. The captain directed the purser to have flight attendants wrap up cabin service and stay near their seats. One flight attendant went to the lavatory to brush her teeth after the announcement.
Turbulence Encounter
Approximately 7–10 minutes after the announcement, 2–3 jolts of moderate turbulence were followed by a brief episode of severe turbulence. This occurred 22 minutes before landing, near the ELCAP intersection. The flight attendant in the lavatory was thrown twice in the air and landed awkwardly on her right ankle. The flight continued to SFO and landed at 1401 local time. United Airlines reported the event on December 5, 2011, after confirming the flight attendant had suffered a broken fibula.
Weather and Flight Data
An NTSB meteorologist determined the area was favorable for mountain wave activity due to an upper-level low and strong wind flow over the mountains. GOES-11 satellite imagery showed clear skies with no visual turbulence signs. Pilot reports in the area at varying altitudes included low-level wind shear, extreme turbulence, occasional to moderate chop, continuous light chop, and moderate to severe turbulence with mountain wave activity.
The flight data recorder (FDR) analysis showed that about 1 hour 34 minutes after takeoff, vertical acceleration fluctuated between 0 g and 2 g, peaking at 1.91 g over a 28-second period.
ATC Communications
At 1242, ATC advised UAL721 to expect occasional light chop and that AIRMETs were available. Upon checking in with a new sector, the crew was told their ride would deteriorate in about 20 minutes and to plan for a lower altitude. About 15 minutes later, the flight crew requested a lower altitude. Another sector controller advised of "bad rides over the Sierra Nevada Mountain Range, but that FL300 and FL320 were not bad." The crew reported light chop initially, then about 10 minutes later reported moderate to severe turbulence at FL300 and requested a lower altitude. The flight was cleared to FL270, and the pilot characterized the ride as "bad" but not severe. The flight landed without further incident.