NTSB Factual Report

On July 25, 2004, about 0115 eastern daylight time (EDT), American Airlines flight 955, a Boeing 777, N797AN, encountered clear air turbulence during cruise at flight level (FL) 370 approximately 25 nautical miles north of intersection GELOG. Flight 955 was a regularly scheduled international passenger flight from Miami International Airport, Miami, Florida, to Sao Paulo, Brazil (GRU), with 204 passengers and 13 crewmembers on board. During the turbulence encounter, one of the flight attendants suffered a fractured shoulder. The flight continued to GRU, where the injured flight attendant was transported to a hospital. The flight was operated under 14 Code of Federal Regulations Part 121 under an instrument flight plan.

At the time of the turbulence encounter, which occurred about an hour after takeoff, flight attendants were providing meal service to the passengers. Statements from the flight attendants characterized the turbulence as "bad" or "severe" and indicated that there were at least two jolts that threw them to the floor. The injured flight attendant, who was located in the rear of the airplane, stated that she tried to hold onto an armrest in the last row after the initial jolt threw her down. She was tossed up and then down as the turbulence continued, and her shoulder struck one of the armrests. Two doctors responded to a request for help, and the injured flight attendant was treated with pain medication, ice packs, and a sling. The captain decided to continue to GRU after the doctors attended to the flight attendant and arranged for her to be transported to a hospital. The flight continued for approximately 6 hours before landing at GRU. The flight attendant was diagnosed with a fractured left shoulder.

Statements from the captain (the flying pilot) and the first officer indicated that the event involved clear air turbulence that lasted 3-4 seconds. The captain characterized the turbulence encounter as "moderate" and being similar to a wake turbulence encounter because of the extremely short duration. He indicated that there were no aircraft in front of them and the nearest thunderstorms were greater than 20 miles to the northwest. The first officer added that they had received no turbulence warnings from other aircraft and that they were clear of all weather.

The reserve pilot for the flight stated that he had been resting in the designated crew rest seat in first class at the time of the event. He stated that the turbulence lasted less than five seconds and involved approximately two groups of three rapid and abrupt jolts that appeared to diminish in severity over time. He indicated that the flight conditions were mostly smooth before and shortly after the event.

The seat belt sign was on at the time of the turbulence. There was no forecast turbulence for the entire route of the flight.

Flight Recorder Data

A review of the flight data recorder (FDR) data indicates that the turbulence event lasted about 10 seconds. The data further show the following about the turbulence encounter:

- there was a 60-degree change in wind direction and a +/-8 knot change in wind speed;

- the autopilot and yaw damper remained engaged and active throughout the event; (According to information provided by Boeing, the B-777's primary flight computer will disconnect the autopilot when it is overridden by manual inputs.)

- there were angle of attack deviations of +/- 3 degrees;

- there were airspeed deviations of -12 to +15 knots; and

- vertical accelerations ranged from approximately 1.6 g's to 0.2 g's; lateral accelerations ranged from approximately 0.06 g's to -0.2 g's; and longitudinal accelerations (which averaged about 0.04 g's prior to the event) ranged from approximately 0.06 g's to -0.01 g's.

NTSB Probable Cause

the in-flight encounter with unforecasted clear air turbulence.