FTW97IA261

The airplane was in level cruise flight at FL370, flying in an area of widely scattered thunderstorm cells. The airborne weather radar was set to the 160 nm scale, and the tilt was approximately 1 to 2 degrees down. One cell was observed 20 nm to the left of the airplane towering above its altitude, a second cell was 40 nm to the right, and a third cell 'came up independently underneath' and in front of the airplane. The FASTEN SEATBELT sign was not illuminated, but earlier, the captain had delivered an address to the passengers advising them of the possibility of a bumpy ride ahead and to '...keep your seatbelts on while in your seats.' The flight encountered convection induced turbulence (CIT), which resulted in minor injuries to 20 passengers and 2 flight attendants. There were no injuries to the other 120 passengers and 5 crew members. Vertical acceleration was between +2.01 and -0.75 g's. American Airlines meteorologists prepared the flight plan and entered a turbulence index of '0' for that segment of flight (from Billings, Mt, to Dupree, SD), indicating smooth air on average for the segment. Convective activity had been forecast for the area.

History of flight

On July 10, 1997, at 1541 mountain daylight time, a Boeing 757-223, N660AM, owned and operated by American Airlines, Inc., encountered severe clear air turbulence while in cruise flight 35 miles southwest of Dickinson, North Dakota. There were no injuries to the airline transport-rated captain, airline transport-rated first officer, three flight attendants, and 128 passengers, but two flight attendants and 20 passengers sustained minor injuries. The airplane sustained minor damage to the passenger service units (PSU). Visual meteorological conditions prevailed, and an IFR flight plan had been filed for the scheduled domestic passenger flight being operated under Title 14 CFR Part 121. The flight originated in Seattle, Washington, at 1245 Pacific daylight time, and was en route to John F. Kennedy International Airport, New York, New York.

According to the captain's statement, the aircraft was 80 miles north-northeast of Billings, Montana, in level cruise flight at FL (flight level) 370. The captain was flying the airplane between widely scattered thunderstorm cells above a 2,000 foot undercast. The Collins WXR-700X weather radar was set to the 160 nm scale, and the tilt was approximately 1 to 2 degrees down. The captain observed one cell 20 nm to the left of the airplane, towering above its cruise altitude, another cell 40 nm to the right, and a third cell in front and below the airplane that "came up independently underneath them." He said that up to this point the flight had experienced a smooth ride, so the FASTEN SEATBELT sign was not illuminated. The airplane encountered severe clear air turbulence for approximately 10 seconds, and the autopilot disengaged followed by a slight altitude deviation. The captain was advised that there were injured passengers and flight attendants, and they were being treated by three doctors and a nurse. He diverted the airplane to Denver, Colorado, where an uneventful landing was made. The injured passengers and flight attendants were taken to several local hospitals for treatment. All were released that evening.

The following is based on the incident report and correspondence submitted by American Airlines. The flight plan indicated zero for turbulence in the area. The reference is related to the 'turbulence index' that appears on the flight plan on a leg-by-leg basis. The index is an indication of smooth air on average for the entire segment. The segment on which the event occurred extended from Billings (Montana) to Dupree (South Dakota), a distance of 295 nm.

Turbulence Indicators (TI's) are issued by American Airlines' meteorologists during the preflight planning phase to inform flight crews and dispatchers about the possibility of non-convective turbulence or clear air turbulence (CAT). They are developed using weather charts depicting jet stream location, satellite imagery and pilot reports. Turbulence Indicators are then input into the Flight Operating System and used by the flight planning system when calculating flight plans:

0 - Smooth 2 - Continuous light chop 4 - Light to moderate turbulence 6 - Moderate to occasional severe turbulence 8 - Severe to extreme turbulence

The 0, 2 and 4 TI's do not have any programming impact on the flight planning system. A TI of 6 will force the computer system to seek a more habitable altitude and slow the mach (speed of the aircraft). A TI of 8 will block the altitude or route from consideration of flight planning.

"It is very difficult to strategically forecast rapidly building/decaying convective induced turbulence," the report stated. "The knowledge of thunderstorms implies the possibility of moderate or severe turbulence, wind shear, hail, etc. Once the National Weather Service (NWS) issued their Severe Thunderstorm Watch (WW), American Airlines Weather Services issued a Thunderstorm SIGMEC (Significant Meteorological Condition) covering it. In fact, the presence of the incident cells was strategically forecast in two areas of the flight planning document supplied to the crew during the preflight planning process," (See METEOROLOGICAL INFORMATION).

At 1444, the crew contacted its dispatch office and asked them to transmit SIGMET 57C. It was received at 1448 (see METEOROLOGICAL INFORMATION). The crew acknowledged the message and indicated they were flying "direct to Carleton (Michigan) VORTAC and north of the affected area." They reported experiencing "occasional light chop at (FL) 370."

The crew statement indicates the flight was operating between widely scattered thunderstorm cells. "This cautious circumnavigation is a tactical maneuver consistent with the crew having knowledge of the thunderstorms and associated turbulence. Considering the normal life cycle of this convective activity, this type of phenomenon can only be detected tactically," the report stated. "A review of the regional composite radar map indicates the National Weather Service added a severe thunderstorm watch box to the incident area 20 minutes after the event. Initial analysis of ground-based radar data examined by the Center for Analysis and Prediction of Storms (CAPS) at the University of Oklahoma indicates that the particular convective event that is believed to have induced the upset had a life cycle of about 45 minutes."

One passenger gave an oral statement to investigators. He described himself as an "amateur meteorologist." He said he was seated at a window seat on the left side of the airplane. They were at 37,000 feet, and the flight attendants had just finished the meal service. The pilot had made an announcement 45 minutes before, advising passengers of the possibility of a bumpy ride ahead and suggested they return to their seats and fasten their seatbelts. He did not think the FASTEN SEATBELTS sign was illuminated. He looked out his window and observed a "cloud deck" approximately 90 degrees to the aircraft, or due north. He described this cloud deck as a cumulonimbus, towering higher than the airplane's altitude. The cloud was black inside, and had an anvil. He looked below and saw another cumulonimbus cloud with an anvil, climbing towards the airplane. The pilot then made a "slight right bank, certainly not an evasive maneuver." The passenger thought the pilot was circumnavigating clouds ahead. The airplane "skimmed" over the top of this cloud. The turbulence was encountered on the other side of this building cumulonimbus.

Meteorological information

Dickinson (DIK), North Dakota, METAR (Aviation Routine Weather Report) was as follows:

2054Z (1454L): Wind, 170 degrees at 13 knots; visibility, 20 sm; 25,000 feet scattered; temperature, 32 degrees C.; dew point, 21 degrees C.; altimeter setting, 29.72 inches of mercury; remarks: sea level pressure, 1004.0 millibars.

2150Z (1550L): Wind, 150 degrees at 13 knots; visibility, 20 sm; ceiling 25,000 feet broken; temperature, 32 degrees C.; dew point, 21 degrees C.; altimeter setting, 29.71 inches of mercury; remarks: towering cumulus distant southwest through west; sea level pressure, 1003.6 millibars.

American Airlines said the following NWS convective forecast was supplied to the flight crew:

1755 UTC (1155 MDT) convective SIGMET...none. Area 2...from 50 miles north of williston, north dakota, to rapid city, south dakota; to scottsbluff, Nebraska; to salt lake city, Utah; to winnemucca, Nevada; to 40 miles east-northeast of salmon, Idaho; to bozeman, Montana; to 40 miles northwest of havre, Montana; to 50 miles north of williston, north dakota. Increasing westerly flow and cooling at mid levels will support thunderstorm activity over central and eastern Montana this afternoon in the vicinity of cold front. Activity will spread into western dakotas.

The American Airlines Weather Services issued the following forecast that was also made available to the flight crew for preflight planning purposes:

Valid from