CHI06LA099

The air carrier flight encountered turbulence associated with mountain wave activity during cruise at flight level 390 (FL390). One flight attendant located in the aft galley sustained serious injuries. The passengers and remaining crew were not injured. The aircraft was not damaged. The captain reported that the airplane was established in cruise at FL390 with smooth air conditions. She stated that the turbulence encounter began with a "slight buffet" and "rapidly increasing" airspeed. She attempted to maintain cruise airspeed by decreasing engine power and climbing to a higher altitude. She noted that as she initiated the climb, the aircraft experienced moderate turbulence. She reported that as the airplane reached FL400, the airspeed began a "rapid decrease" and she initiated a descent back to FL390. The flight subsequently descended to FL350 before the conditions smoothed out. She noted that the turbulence was encountered "suddenly and without warning." The flight data recorder indicated that the airspeed increased from 236 knots to 273 knots during the encounter. A vertical acceleration spike of 1.81 g's, followed by 0.61 g's, was recorded 64 seconds after the initial increase in airspeed. The weather forecast provided to the crew listed a risk of moderate turbulence between 20,000 and 38,000 feet. While ATC communications were describing light turbulence and moderate chop at lower altitudes, no reports from ATC, company dispatch, or other aircraft were received regarding mountain wave activity or turbulence at 39,000 feet.

On March 15, 2006, at 1614 central standard time, a Boeing 757-222 airplane, N518UA, operated by United Air Lines Inc. as flight number 8, encountered turbulence associated with mountain wave activity in cruise flight at 39,000 feet pressure altitude over Cheyenne, Wyoming. The domestic air carrier flight was being conducted under 14 CFR Part 121 on an instrument flight rules (IFR) flight plan. Visual meteorological conditions prevailed at the time. One flight attendant sustained serious injuries due to a broken wrist during the turbulence encounter. The 2 pilots, the remaining 5 flight attendants, and 90 passengers were not injured. The scheduled passenger flight departed San Francisco International Airport (SFO) at 1421 with an intended destination of John F. Kennedy International Airport (JFK). The flight diverted to Eppley Airfield (OMA), Omaha, Nebraska, following the turbulence encounter.

The captain reported that the airplane was established in cruise at flight level 390 (FL390), with smooth air conditions, when she felt a "slight buffet." She subsequently observed that the airspeed was "rapidly increasing," and she disengaged the auto throttles and pulled the throttles back to in an attempt to compensate. She stated that the airspeed continued to increase and she elected to disconnect the autopilot and climb to reduce airspeed. However, the airspeed continued to increase, momentarily exceeding the maximum operating speed for the aircraft. The airplane was experiencing moderate buffeting at that time, according to the captain.

The captain stated that as the airplane entered the climb, the flight encountered moderate turbulence. She noted that upon reaching FL400, the airspeed began a "rapid decrease" and she descended to FL390. The flight subsequently descended to FL350 before the conditions smoothed out.

The captain noted that during the descent, the flight crew was notified that a flight attendant had been injured. The decision was made to divert to OMA for medical treatment.

The captain noted that the turbulence encounter occurred "suddenly and without warning." She reported that flights at lower altitudes had reported light turbulence and moderate chop; however, most of the flight at FL390 was smooth until the turbulence encounter.

The flight dispatch contained an alert for moderate turbulence between FL200 and FL380 in an area bounded by Rock Springs, Wyoming (OCS), Billings, Montana (BIL), Des Moines, Iowa (DSM), Butler, Missouri (BUM), Rock Springs, Wyoming (OCS). No alerts were included for altitudes above FL380.

The flight data recorder was downloaded. Review of the data indicated that the flight was at 39,005 feet and 236.5 knots, approximately 1 minute prior to the upset. Fifty-one seconds later, with the airspeed increasing through 266 knots, the auto throttles were disengaged and engine speed decreased to 70 percent. The airspeed peaked at 273.5 knots, 57 seconds into the event. The autopilot was disengaged 1 second later and the pitch attitude increased from 2 degrees nose up to 12 degrees nose up. A vertical acceleration spike of 1.81 g's, followed by 0.61 g's, was recorded 64 seconds into the event. The airplane reached a maximum altitude of 39,967 feet pressure altitude 80 seconds into the event.