Boeing 777-200 N209UA accident near New Market, Virginia

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.

NTSB analysis

Prior to and after the turbulence encounter the captain reported smooth air while level at FL310. There were no reported sigmets, airmets, or pilot reports issued, nor were there “watch boxes” that warned of adverse weather. The onboard weather radar was functioning when suddenly the airplane went through a cloud at Mach 0.84 and encountered moderate turbulence for about 10 to 15 seconds. He said that there was an overspeed warning alarm and he immediately took over manual control of the airplane maintaining control, and activated the seat belt warning sign which had previously been off, since the flight had been smooth. The first officer statement was consistent with the captain’s and in addition he reported seeing “considerable St. Elmo’s Fire” throughout the duration of the turbulence.

The aircraft’s flight data recorder indicated that while the aircraft was level at FL310, the vertical acceleration changed from 1.25 g’s to -0.5 g’s in less than one second and then abruptly reached 2.0 g’s within a few seconds before reducing to minor oscillations that were between 1.25 g’s and 0.75 g’s which lasted for about 15 seconds. According to the FDR data, the turbulence lasted about 20 seconds.

The NTSB conducted a meteorological study to assess weather conditions in the area where the flight encountered turbulence, and 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.

Text from the NTSB. Open the NTSB docket for DCA10FA012 for the full record.