NTSB Factual Report

HISTORY OF FLIGHT

On November 18, 2009, about 1930 Eastern Standard Time (EST), N209UA, a Boeing 777-200, registered to and operated by United Airlines Inc., as a Title 14 CFR Part 121 domestic passenger flight, encountered turbulence in the vicinity of New Market, West Virginia. The flight was en route from Chicago, Illinois, to Dulles International Airport, Sterling, Virginia. The airline transport-rated captain, first officer, 7 flight attendants, and 189 passengers were not injured. One flight attendant received minor injuries and a passenger received a serious injury. The flight originated from O'Hare International Airport, Chicago, Illinois, about 1819 Central Standard Time.

According to the captain, who was the pilot flying at the time of the accident, there were no reports of turbulence or significant weather from other aircraft in the vicinity or from air traffic control, nor had United Flight Dispatch informed him of any airman meteorological information (airmets), or significant meteorological information (sigmets). He added that there were also no “watch boxes” that warned of possible adverse weather. The radar was on and tilted down one degree at the 40 mile range, when suddenly the airplane skimmed the top or went through a cloud at mach 0.84, while in cruise flight at FL310. He further stated that the turbulence was moderate and that it lasted about 10 to 15 seconds. According to the captain, at the same time the turbulence occurred, there was an overspeed warning and the autopilot tripped off. He said he immediately took manual control of the situation and turned on the seat belt sign in the cabin, which had previously been off because conditions had been smooth. He said that at no time was the airplane out of control, and that after the turbulence, conditions immediately became smooth. He said he later learned that there was an injury in the passenger cabin and that he informed Washington Air Route traffic Control Center. The flight proceeded and otherwise landed uneventfully at Dulles International Airport.

The first officer stated that the weather radar was operating while in cruise flight at FL310 and that no radar returns were observed along their route, adding that the flight was “in the clear, well above any cloud tops.” He further stated that while seated he experienced a sudden jolt of moderate turbulence, followed by the overspeed warning alarm during which the autopilot tripped off. He said at the time that the airplane was in instrument meteorological conditions and there was considerable “St. Elmo’s Fire”, which lasted 10 to 20 seconds. He said he then called back to the cabin and asked if everyone was “all right”, and that the lead flight attendant told him that he would check the condition of everyone and will let the flight deck know if there were injuries. He then communicated with FAA Air Route Traffic Control to let them know about the turbulence. The first officer said that a few minutes later while in the descent to land they were informed that a passenger and flight attendant had been hurt, and he sent a “quick note” to the dispatcher requesting paramedics when the flight arrive at the gate.

One flight attendant, who had been standing in the rear galley when the turbulence occurred noted seeing a passenger “rise up and down then fall to the floor” while walking by the lavatory. Another flight attendant stated that she had fallen to her knees during the turbulence encounter, and received minor injuries, adding that the turbulence was of a short duration but had been very extreme. After the turbulence ceased, a flight attendant attended to the fallen passenger who was found to have received a serious injury. The lead flight attendant then informed the flight deck that one passenger had been injured and was in need of emergency medical care.

FLIGHT RECORDERS

The airplane’s flight data recorder was recovered and sent to the NTSB’s Recorder Laboratory in Washington, DC for readout. The recorder was found to contain 53 hours of data. The data indicated that while the aircraft was level at 31,000 feet, vertical acceleration changed from about 1.25 g’s to about -0.5 g’s in less than one second, and then abruptly became 2.0 g’s within a few seconds, before reducing to minor oscillations varying 1.25 g’s and 0.75 g’s which lasted for about 15 seconds. The duration of the turbulence encounter was about 20 seconds from the first disturbance until the last. In addition, the recorder data showed that the autopilot had been engaged prior to encountering turbulence and that it was disconnected when the force on the aircraft first reached about -0.5 g’s.

METEOROLOGICAL INFORMATION

As part of the investigation, the NTSB conducted a meteorological study to assess weather conditions in the area where the flight encountered turbulence. It was noted that the National Weather Service’s Radar Summary for 1918 EST, depicted an extensive area of rain showers over the region with a few radar echo tops between 31,000 feet and 48,000 feet. In addition the regional radar composite imagery showed that between 1910 and 1940 EST there was a small area of reflectivity between 5 and 25 decibels in the area of the turbulence encounter.

NTSB Probable Cause

The flight crew’s inadvertent encounter with a small area of cumulous clouds at night that resulted in moderate to severe convectively induced turbulence.