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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.
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 JULY 10, 0845 MDT, TO JULY 11, 0300 MDT. THUNDERSTORM OUTLOOK - ISOLATED THUNDERSTORMS THIS MORNING...WILL INTENSIFY DURING THE AFTERNOON. BY 1600 MDT, SCATTERED THUNDERSTORMS WITH SOME SMALL BROKEN LINES ARE EXPECTED OVER THE EASTERN HALF OF MONTANA, SOUTHERN IDAHO, AND NORTHERN UTAH. SOME THUNDERSTORMS SPREADING EASTWARD INTO THE EASTERN DAKOTAS BY LATE AFTERNOON-EARLY EVENING.
SIGMET 57C was transmitted to the crew at 1448:
CONVECTIVE SIGMET 57C VALID UNTIL 1555 MDT WYOMING COLORADO FROM 90 MILES EAST OF SHERIDAN, WYOMING, TO 20 MILES NORTHWEST OF DENVER, COLORADO; TO 30 MILES WEST-NORTHWEST OF LARAMIE, WYOMING TO 90 MILES EAST OF SHERIDAN, WYOMING. AREA THUNDERSTORM MOVING FROM 230 DEGREES AT 15 KNOTS. TOPS TO FL 240. HAIL TO 1 INCH (DIAMETER)...WIND GUSTS TO 50 KNOTS POSSIBLE.
A special weather study was conducted by NTSB's Operational Factors Division. According to its report, GOES 9 (Geostationary Operational Environmental Satellite) photographs showed developing convective activity in the area of the incident. In addition, doppler weather radar data from Bismarck, North Dakota, detected weak weather echoes about 9 nm west and 8 nm northwest, and a strong weather echo north-northwest, of the incident area.
The digital flight data recorder (DFDR) was removed from the airplane and read out, first, by American Airlines (with the permission of NTSB), and then by NTSB's FDR Laboratory. Both reports are attached.
According to the Flight Data Recorder Specialist's Factual Report, the airplane "was cruising at about 37,000 feet. At FDR Subframe Reference Number (SRN) 7831, while the aircraft was at 36,992 feet, the vertical acceleration began to oscillate between a maximum and minimum of 2.01 and -0.75 g's. The situation continued for approximately 12 seconds. During this time the lateral and longitudinal accelerations oscillated as well, the angle of attack varied from -4.4 degrees to -23.4 degrees, the aircraft began losing altitude, and the pitch angle dropped from 3.2 degrees to -3.2 degrees. The computed airspeed had increased from 255 to 271 knots just prior to the incident, and a left turn had been executed, changing the heading from 90.5 degrees to 86.0 degrees. Immediately following the incident, a right turn was made, and the heading returned to 90 degrees. The altitude dropped from 36,992 to 36,408 feet over a period of 20 seconds, and the aircraft then began climbing, returning to 36,992 feet at SRN 7886."
After reviewing NTSB's report, American Airlines made the following observations: "There was a dramatic G-force reversal beginning at time 88325.63 from a +1.773 to a -0.653 at time 88326.13, or 1/2-second. The negative g's persisted for only 3/8-second.
"There was a precipitous drop in computed airspeed from the peak 271 knots to 234 knots over a 4 second time increment. This becomes even more significant if this plunge is given an equivalent mach number at 37,000 feet."
American said the vertical oscillations lasted for 15 seconds, and varied between +1.975 g's and -0.653 g's, and altitude dropped to a low of 36,408 feet.
The facts surrounding this incident were reviewed by Mr. Archie Trammell of Airborne Weather Radar Seminars. Mr. Trammell has achieved an international reputation as an authority on airborne weather radar and has instructed various airline crews, including American Airlines, on its operation and usage. His report is attached as an exhibit to this report.