Boeing 747 N173UA accident near Anchorage, Alaska

Probable cause

an inadvertent encounter with clear air turbulence.

NTSB analysis

On February 19, 2010, about 1452 Alaska standard time (AKST) (2352 UTC), United Airlines flight 897, a Boeing 747-400, N173UA, encountered severe turbulence during cruise descent between FL310 and FL300 about 100 miles west of Anchorage, Alaska. Of the 243 passengers and 19 crew members on board, 17 passengers and eight flight attendants (FA) received minor injuries and one FA received a serious injury. The airplane received minor damaged. The flight was operating under 14 Code of Federal Regulations (CFR) part 121 as a regularly scheduled international passenger flight from Dulles International Airport (IAD), Washington, DC, to Narita International Airport (NRT), Narita, Japan.

Prior to the turbulence encounter, the flight crew received pilot reports (PIREPs) of moderate to severe turbulence above FL340 and that it was less at FL300. According to the flight crew, about one hour late, the flight began to encounter a few "light bumps" or intermittent light "chop." The sky conditions were clear with good visibility. The captain turned on the "fasten seat belt" sign and made a public address (PA) announcement to fasten seat belts. The flight crew requested a descent to FL300 and began to encounter moderate turbulence after starting the descent. The first officer then made a PA announcement directing the flight attendants to "take your seats." The autopilot remained engaged throughout the turbulence encounter, which lasted less than one minute.

After encountering the turbulence, the crew assessed airplane damage and checked the number and severity of injuries to passengers and crew. The crew was assisted in assessing passenger and crew injuries by a U.S. Army Special Forces medic who stated that all of the injuries were considered minor. All the FAs indicated that they were able to perform their required emergency duties for the remainder of the flight.

After discussing the injuries and the available diversion airports with the FAs, company dispatch and the medic, the flight crew decided to continue on to NRT since there were sufficient diversion airports along the route if the situation worsened.

Upon arrival in NRT, 16 passengers and one FA were transported to hospitals for medical evaluation. All were released with only minor injuries, however, the FA was re-evaluated by her personal physician when she returned home with the further diagnosis of a fractured rib.

Text from the NTSB. Open the NTSB docket for DCA10CA032 for the full record.