DCA98MA015

The flightcrew inadvertently flew into an area of clear air turbulence. Weather forecasts indicated that there was not to be turbulence in the area where it was encountered. The ability to forecast clear air turbulence over the ocean is limited. In addition, the Japanese-speaking flight attendant did not fully translate one of the captain's instructions to fasten seat belts into Japanese, although such an announcement was made and translated several times prior to the turbulence encounter. Most of the passengers spoke only Japanese.

History of flight

On December 28, 1997, at 1340 UTC, a United Airlines Boeing 747-122, N4723U, experienced an episode of what the captain described as wave action (see footnote 1) followed by severe turbulence (two closely spaced turbulence encounters) about 870 nautical miles east southeast of New Tokyo International Airport, Narita, Japan (NRT) on Pacific Ocean navigation track 12 (see footnote 2). The airplane was operating under 14 Code of Federal Regulations (CFR) Part 121 in VFR conditions at the time of the accident and was bound for Honolulu, Hawaii (HNL). Of the 374 passengers (including 5 infants) and 19 crewmembers on board, 15 passengers and 3 flight attendants received serious injuries and 1 passenger was killed. Also, 161 minor injuries were sustained by flight attendants and passengers. Following the turbulence encounter, the airplane returned to New Tokyo Airport for an uneventful landing.

According to the captain, the overall flight planning activity was routine but there was more concern about turbulence than usual. Because of this concern, the captain selected track 12 as the route of flight because there were no SIGMETs near that track. However, even with that selection the captain felt some turbulence might be encountered approximately two hours after takeoff. The captain briefed the purser of this possibility. He also stated that prior to takeoff during the "welcome aboard" announcement he told the passengers that there might be turbulence enroute. The Japanese-speaking flight attendant translated this announcement into Japanese. Also before takeoff, a safety video was played for the passengers; this included a warning to passengers to keep their seat belts fastened when seated. The instructions in this safety video were narrated in English and also clearly communicated to Japanese language passengers by means of Japanese subtitles.

The takeoff, departure, and climb were uneventful. The Captain said that he turned the seat belt sign off during climb because the ride was smooth. After he turned the seat belt sign off, the captain made a PA announcement that included information about common weather patterns during that time of year, the probability of turbulence and a request that each passenger keep his or her seat belt fastened when seated. The Japanese-speaking flight attendant responsible for translating the announcement went to the purser to discuss the best way to communicate this information to the passengers. The flight attendant did not translate all of this announcement into Japanese. She stated that she did not want to alarm the passengers. At that time, the aircraft reached cruise altitude and the seatbelt sign was turned on for approximately 15 minutes when minor wave activity was encountered. When the minor wave action subsided, the seatbelt sign was turned off. Customary announcements during this period were made in English and Japanese. The seatbelt sign then remained off for approximately one hour.

The captain stated that approximately one hour and forty minutes into the flight the airplane encountered what he described as "wave action" and he turned the seatbelt sign on, again, as a precaution. The customary public address announcements to have the passengers fasten their seat belts were made in English and Japanese. Prior to the encounter, the captain noted seeing stars above. He also illuminated exterior lighting to look for clouds and he saw none. The captain also radioed NW flight 90, ahead of him, requesting a ride report. NW 90 reported that the ride was smooth with an occasional ripple of light turbulence at their altitude. The first turbulence encounter happened no more than one to two minutes later. According to the flight data recorder (FDR), the aircraft encountered severe turbulence about 2 minutes after the "wave activity."

Seconds later, the flight experienced another turbulence episode. During the turbulence, the captain made a PA announcement for the flight attendants to sit down and then he made a PA announcement to the passengers telling them not to be alarmed. The captain ordered the first officer to reduce speed, and he complied by reducing the indicated airspeed to approximately 330-340 knots indicated airspeed. The captain said that he broadcast notice of the turbulence encounter to other flights. Northwest flight 22 responded by saying they were climbing to FL 350.

The captain said that at the time of the turbulence encounters, about 50 to 60 miles ahead and right of course, there were some light green echoes on radar. No red echoes were observed. At the time of the turbulence encounters, the radar was selected to the 80-mile range with the antenna tilted five degrees down. The aircraft's radar turbulence detection mode was also utilized using the turbulence detection switch. The turbulence detection mode allows the pilot to switch between Turbulence "Precip", Turbulence "Doppler" or "Both". The pilots stated prior to the encounter, both radar displays were on, the "Precip"/"Doppler" selector was in "Both" and was being switched back and forth between "Precip" and "Doppler" and that no "Doppler" returns were observed. Before and after the turbulence encounters, the captain saw a band of clouds with no lightning to the right and below the aircraft, but he did not see any lightning or clouds along the route of flight. Also at this time, Autopilot B was in Command mode and the flight director was in INS mode. Autopilot altitude hold was on and the autothrottles were not in use, according to company policy. The flight crewmembers did not remember the autopilot disengaging because of the turbulence. He said that the autopilot has a turbulence mode but it is rarely used on over-water operations unless the flight was in continuous light to moderate turbulence. At the time of the turbulence encounters, the flightcrew stated that total air temperature (TAT) read approximately minus 40-44 degrees C and did not change when the turbulence was encountered. He said that there was no rapid change in wind direction or speed before or after the encounter with turbulence.

At the time of the turbulence encounters, the flight crew stated that the overspeed warning sounded. The first officer reduced power with the captain assisting on the throttles. Warning lights illuminated on inertial navigation system (INS) numbers one and three. The number four hydraulic low pressure lights also illuminated. The captain said it concerned him that there might be structural damage to the airplane. The INS units worked normally after the error codes were cleared and no flight instrumentation was lost during the events. Upon investigation by the crew, it was determined the number four hydraulic lights illuminated because the hydraulic panel switches had apparently been bumped to the off position by something in the cockpit. The second officer believed that, during the turbulence, the switches may have been bumped into the off positions by a tray sitting on his desk. All other systems were indicating normal operation.

At the point at which the captain requested permission from ATC to climb to FL330, he asked the purser to provide him with information about the conditions in the cabin following the turbulence. The purser informed the captain that a flight attendant was in the aisle, the cabin was "a mess", and there were several injuries. The captain then asked the purser to see if a doctor was on board. Two doctors were found who asked for and were given the airplane's medical kit, oxygen bottles and first aid kits. One doctor stayed with the flight attendants in the back of the cabin who were performing CPR on an unconscious passenger. The other doctor assisted the flight attendants who were providing first aid to other passengers.

When the aircraft was out of the area of turbulence, the captain asked the second officer to examine the c