N174UA
BOEING 747-422
The flight crew reported that the flight experienced unexpected light-to-moderate turbulence during cruise flight for about 3 hours. The seatbelt sign was illuminated, and the passengers and flight attendants had been seated. A flight attendant, who was on her rest break and was resting in the bunk room, reported that the turbulence had stopped for a while and that she thought she heard a "faint ding" indicating that the seatbelt sign had been turned off, so she went to the lavatory. However, the flight then encountered moderate turbulence, during which the flight attendant sustained a serious injury while walking from her bunk to the lavatory. The flight attendant reported that announcements made over the public address system could not be heard in the bunk room and that it was not always possible to hear when the seatbelt sign was turned on or off. It is likely that, if the flight attendant had a visual indication that the seatbelt sign was on, she would not have attempted to get up until safe movement was assured.
HISTORY OF FLIGHT On February 19, 2013, about 1500 Universal Coordinated Time (UTC), N174UA, a Boeing 747-422, encountered turbulence during cruise flight over the Pacific Ocean while en route to San Francisco International Airport (SFO), San Francisco, California. United Airlines operated the airplane under the provisions of 14 Code of Federal Regulations Part 121, as an international scheduled passenger flight. One flight attendant sustained a serious injury during the airplane's encounter with moderate turbulence. The airplane did not sustain any damage. The flight departed the Sydney (YSSY/SYD), Australia, airport on an instrument flight rules (IFR) flight plan about 0540 UTC. According to United Airlines personnel, the flight experienced light to moderate turbulence; the turbulence started about 1330 UTC and continued until 1530 UTC. The seatbelt sign was illuminated, the passengers remained seated, and the flight attendants would make their way back to a jump seat for secured seating or sit on the floor until it was safe to walk back to their jump seats. About 1500 UTC, in the vicinity of BALKS intersection at flight level 390 (39,000 feet), one of the flight attendants fell as she was walking from her bunk to the aft lavatory when the injury occurred. One of the passengers was a nurse, and attended to the injured flight attendant advising of a possible broken wrist. An emergency was subsequently declared with air traffic control (ATC) and the airplane continued to its destination for an uneventful landing. After landing the flight attendant was transported to a hospital, where upon further examination a doctor confirmed the flight attendant's injury was a broken wrist. PERSONNEL INFORMATION The first officer flying, age 58, held an airline transport pilot (ATP) certificate for airplane multi and single-engine land and was type rated in the B747-400 and B777. His last Federal Aviation Administration (FAA) first-class medical certificate was issued on October 16, 2012. He reported a total of 15,500 flight hours. The second-in-command first officer (International Relief Pilot (IRO)), age 58, held an airline transport pilot (ATP) certificate for airplane multi-engine land and was type rated in the B747, B757, B767, and B777. His last FAA first-class medical certificate was issued on October 25, 2012. He reported a total of 17,094 flight hours. The captain was on a rest break during the time of the turbulence encounter. CREW STATEMENTS The IRO reported that the preflight planning included a discussion of expected turbulence during the first half of the flight, which occurred as briefed. He noted that an area of light turbulence was noted east of Hawaii at flight level FL 300 to 440 (33,000 to 44,000 feet) in association with a 120- to 140-knot (kt) jet stream. The IRO stated that the weather encountered was expected for the first half of the flight, but the weather for the second half of the flight, to include the encounter with moderate turbulence, was not expected based on preflight information. He reported that they received information of convective SIGMET (Significant Meteorological Information) from dispatch concerning areas of embedded CB (cumulonimbus clouds), which implied the potential for severe or greater turbulence, severe icing, and low-level wind shear close to the ground. After discussing a possible deviation of 600 to 1,000 nautical miles to clear the area, the decision was made that with functioning weather radar that the best course of action would be to continue on course. Prior to entering the area of the SIGMET the airplane encountered light turbulence at which time the seatbelt sign was turned on. Upon entering the area of the SIGMET, the airplane experienced light to moderate turbulence for approximately 3 hours. The SIC first officer stated that the injury to the flight attendant occurred near the BALKS intersection. The flight attendant who was injured reported that while on break in the bunk room the airplane had been encountering moderate turbulence for hours. She stated that "the turbulence stopped for a while and was very calm and she thought she "could hear a faint ding from the seatbelt sign." She further stated that from the crew bunk room they do not hear announcements made over the airplane's public address system, and that it is not always possible to hear when the seatbelt sign is turned on and off. She stated that she waited in her bunk for 10 to 15 minutes to make sure that it was safe before she climbed down. After climbing down from the bunk, the airplane "dropped and picked her up throwing her 6 to 10 feet" and she landed on her right arm. METEOROLOGICAL INFORMATION The National Weather Service NWS Pacific Surface Analysis Chart for 1200Z on February 19, 2013, depicted a high pressure system north of the Hawaiian Islands with a central pressure of 1035-hectopascals (hPa). A high pressure ridge extended southeastward from the high towards the Baja of California. The approximate accident location, which was located south of the ridge axis, had no significant surface boundaries indicated in the immediate vicinity of the turbulence location. The NWS High Level Significant Weather Prognostic Charts issued for the period were valid for 0600Z, 1200Z, and 1800Z, respectively, on February 19, 2013. The chart valid for 0600Z was issued to the flight crew as part of their weather briefing documentation and had the flight track overlaid. The charts depicted a subtropical jet stream moving northeast at FL410 with a wind maximum of 160 knots near the southwestern United States, with the jet stream near the accident location with winds of 140 knots. An area of clear air turbulence was expected along and on the poleward side of the jet stream from FL270 to FL430, which bordered the accident site on the charts between 0600Z and 1800Z. No significant frontal or tropical boundaries were noted in the vicinity of the turbulence location. No organized convective activity was depicted in the vicinity of the accident site, although several areas of isolated embedded cumulonimbus clouds were depicted west and south between 39,000 and 42,000 feet. All the charts depicted an area of moderate turbulence associated with the jet stream in the area of the event, with the route of flight crossed the jet stream in the vicinity of the turbulence event. These chart were also available to United Airlines Flight Dispatch during the period for flight following. There were several pilot reports of light to moderate turbulence between 31,000 and 38,000 feet over the region, and no reports of any encounters with severe turbulence. No reports were noted at 39,000 feet. Several of the pilot reports north of the Hawaiian Islands reported northerly winds, which indicated a trough of low pressure moving across the region and supporting upward motion and the Baroclinic leaf observed on satellite imagery. See attached NTSB Meteorology Report for this docket. A review of the NWS Aviation Weather Center's Oceanic SIGMETs (Significant Meteorological Information) indicated no advisories were issued for the Oakland Oceanic area, which extended over the airspace where the turbulence occurred. A NWS Oceanic SIGMET requires an area of severe or extreme turbulence to extend over an area at least 3,000 square miles at a time to be considered widespread and an advisory issued. The GOES-15 infrared image at 3X magnifications providing the general location of the turbulence event to San Francisco and the Hawaiian Islands to the west. A large area of cloud cover is depicted over the turbulence location. Several interesting cloud features were noted; a comma cloud pattern to the west associated with a Baroclinic leaf formation, a deformation zone, and defined transverse cloud band pattern running perpendicular to the upper level wind flow all of which have been reported with clear air turbulence encounters. The radiative cloud top temperature over the site was 233 degrees Kelvin (K) or -34.16 degrees C, which corresponded to cloud tops near 27,000 feet. An area of moisture channel darkening was noted to the west along the edge of the Baroclinic leaf or comma cloud pattern and on to the north associated with subtropical jet streams position where descending air and evaporation of moisture is occurring. United Airlines flight 870's position is noted along the northern edge of an approximately east-to west band of clouds that was perpendicular to the wind flow. Additional cloud features were identified on the image with the transverse cloud bands, which appear over the accident site. A Weather Research and Forecasting Model (WRF) simulation was run to simulate the weather conditions surrounding the accident site at the accident time. The model depicted Richardson Numbers values associated with moderate and greater turbulence over the region associated with breaking transverse waves a known turbulence producer. FLIGHT DATA RECORDER Flight Data Recorder (FDR) data indicated at approximately 14:16:00 UTC (about 8 hours 25 minutes after takeoff) until 17:13:30 UTC, there were vertical acceleration variations ranging between 1.3 g's and 0.65 g's. During this time, the pressure altitude started at about 37,000 feet (ft.) and about 8 minutes later, the pressure altitude increased to about 39,000 ft., and remained at 39,000 ft. for the next 2 hours 18 minutes until 16:41:40 UTC. Over the next 31 minutes 50 seconds from 16:41:40 UTC to 17:13:30 UTC, the pressure altitude decreased to a minimum of 37,575 ft., then increased to a maximum of 41,000 ft. before decreasing and settling at a pressure altitude of about 33,000 feet One hour 18 minutes later at 18:31:07 UTC, the airplane landed.
The flight’s encounter with moderate clear air turbulence while en route, which resulted in a serious injury to a flight attendant. Contributing to the accident was the lack of communication equipment in the rest compartment, which, if installed, could have alerted the cabin crewmembers that the seatbelt sign was on.
Source: NTSB Aviation Accident Database
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