History of Flight
On November 18, 2009, at about 1930 Eastern Standard Time, United Airlines Flight 209, a Boeing 777-200 (registration N209UA), encountered turbulence while en route from Chicago, Illinois, to Dulles International Airport in Sterling, Virginia. The flight originated from O'Hare International Airport at 1819 Central Standard Time. The crew consisted of an airline transport-rated captain, a first officer, and seven flight attendants, with 189 passengers on board. No injuries were reported among the captain, first officer, or seven flight attendants and 189 passengers; however, one flight attendant sustained minor injuries and one passenger sustained a serious injury.
The captain, who was the pilot flying, reported that there were no reports of turbulence or significant weather from other aircraft or air traffic control, nor had United Flight Dispatch provided any airman meteorological information (AIRMETs) or significant meteorological information (SIGMETs). He noted the absence of “watch boxes” warning of adverse weather. The weather radar was on and tilted down one degree at the 40-mile range. While cruising at Flight Level 310 (FL310) at Mach 0.84, the airplane suddenly skimmed the top or passed through a cloud. The captain described the turbulence as moderate, lasting about 10 to 15 seconds. Simultaneously, an overspeed warning sounded and the autopilot disengaged. The captain took manual control and turned on the seat belt sign, which had been off due to smooth conditions. He stated that the airplane was never out of control and that conditions became smooth immediately after the turbulence. He later learned of the injuries and notified Washington Air Route Traffic Control Center. The flight continued and landed uneventfully at Dulles.
The first officer stated that the weather radar showed no returns along their route and that the flight was in clear air above any cloud tops while at FL310. He experienced a sudden jolt of moderate turbulence, followed by the overspeed warning and autopilot disengagement. The airplane was in instrument meteorological conditions with noticeable St. Elmo's fire, which lasted 10 to 20 seconds. After the turbulence, the first officer contacted the cabin crew to check on everyone. The lead flight attendant reported that they would assess and inform the flight deck of any injuries. The first officer subsequently notified FAA Air Route Traffic Control. During descent, the flight deck was informed that a passenger and a flight attendant had been hurt, prompting a request to the dispatcher for paramedics upon arrival.
One flight attendant, who was standing in the rear galley, observed a passenger “rise up and down then fall to the floor” while walking near a lavatory. Another flight attendant fell to her knees during the turbulence, sustaining minor injuries, and described the event as very extreme but brief. After the turbulence ceased, a flight attendant attended to the injured passenger, who had serious injuries. The lead flight attendant then informed the flight deck that medical assistance was needed.
Flight Recorder Data
The flight data recorder (FDR) was recovered and read out by the NTSB Recorder Laboratory. The recorder contained 53 hours of data. The data indicated that while the aircraft was level at 31,000 feet, vertical acceleration changed from approximately 1.25 g to -0.5 g in less than one second, then abruptly increased to 2.0 g within a few seconds, before reducing to minor oscillations between 1.25 g and 0.75 g lasting about 15 seconds. The total duration of the turbulence encounter was approximately 20 seconds from the first disturbance to the last. The FDR also showed that the autopilot was engaged prior to the turbulence and disengaged when the aircraft's vertical acceleration first reached about -0.5 g.
Meteorological Information
The NTSB conducted a meteorological study to assess weather conditions in the area. The National Weather Service's Radar Summary for 1918 Eastern Standard Time depicted an extensive area of rain showers over the region, with radar echo tops between 31,000 and 48,000 feet. Regional radar composite imagery for the period between 1910 and 1940 Eastern Standard Time showed a small area of reflectivity between 5 and 25 decibels in the vicinity of the turbulence encounter.