CHI95IA204

While in level flight at 37,000 feet in cirrus clouds, the airplane encountered moderate to severe turbulence. The airplane was on autopilot and seat belt signs were off. The flight crew took abrupt action to control the airplane with control inputs. No damage was reported to the airplane; however 17 persons on board the airplane reported minor injuries.

History of flight

On June 26, 1995, about 2000 central daylight time, American Airlines Flight 58, a McDonnell Douglas DC-10-10, N131AA, encountered turbulence while in cruise flight at 37,000 feet, near Green Bay, Wisconsin. The flight departed Los Angeles, California, at 1400 pacific standard time with the intended destination of Jamaica, New York. The crew diverted the flight to Chicago, Illinois, where the airplane landed uneventfully at 2046 central daylight time. Subsequently 13 passengers and 3 flight attendants were transported to local hospitals for examination and treatment for minor injuries. There were a total of 103 passengers, 3 flight crew and 11 flight attendants aboard the airplane on departure from Los Angeles.

The flight crew reported that they were flying at flight level 370, twenty-five miles west of Gopher VOR, in thin cirrus clouds. The seat belt signs were off, and the radar reported no echoes, when the airplane encountered five to eight seconds of moderate to severe turbulence. They reported rapid vertical movement with a maximum altitude increase of 200 feet, with the autopilot on. Both pilots said they took control to maintain level flight. After the turbulence encounter it became smooth again. They reported that they then began to assess injuries and made the decision to divert to O'Hare Airport, Chicago, Illinois.

Flight recorders

The flight data recorder information was analyzed by the Office of Research and Engineering, Vehicle Performance Division of the NTSB. That report is attached as an addendum to this report. The report indicated that, "At (the request of the NTSB), American Airlines provided a (flight data recorder} readout of the event. The data were subsequently sent to McDonnell Douglas for further evaluation. (McDonnell Douglas) developed a vertical wind time history for use in their simulator. The elevator deflection and vertical wind time histories resulted in a simulated load factor time history that was similar to that recorded on the FDR (graph II, [attached]). When the elevator was held constant, the wind time history produced the load factor data seen in graph IV, (attached). According to the graphs, if the elevator had not been moved, the load factor would have decreased to near zero as a result of the wind flow field. Therefore, the elevator deflection resulted in the load factor decreasing to the -0.5 G range."

Additional information

The Federal Aviation Administration, Flight Standards District Office, Schiller Park, Illinois, was party to the investigation.