CHI02LA111

The air carrier passenger flight encountered severe turbulence while in cruise flight. Three passengers were seriously injured and nine passengers and three flight attendants received minor injuries during the turbulence encounter. The remaining passengers and crew were not injured. The airplane was traveling in a predominately northern direction at 37,000 feet altitude during the encounter. Weather products obtained showed that extreme intensity thunderstorms existed in the area where the turbulence was encountered. Weather data and aircraft position radar data show that the airplane was 9 miles south-southwest of a cloud buildup that extended to 39,000 feet. Additionally, the airplane was 5 miles west of an extreme intensity radar echo associated with the thunderstorms in the area. The airline's General Operations Manual states that thunderstorms that are identified as severe, or giving a intense radar echo should be avoided by at least 20 miles. No National Weather Service aviation weather advisories were in effect for the location and time of the turbulence encounter. Communications transcripts show that the flight crew requested and were granted a course deviation for weather about 10 minutes prior to the upset. However, the Digital Flight Data Recorder shows that the seat belt sign was illuminated only 10 seconds prior to the encounter.

History of flight

On April 18, 2002, about 1635 central daylight time, a Boeing 757-200, N516AT, operated by American Trans Air (ATA), as flight 208, encountered severe turbulence during cruise flight at flight level 370 near Springfield, Missouri. The airplane was travelling in a predominately northern direction during the encounter. The flight diverted to the St. Louis International Airport (STL), St. Louis, Missouri without further incident. Three passengers received serious injuries and nine passengers and three flight attendants received minor injuries. The remaining 106 passengers, 4 cabin crewmembers, and 2 flight crewmembers were not injured. The airplane was not damaged during the encounter. The 14 CFR Part 121 airline flight was operating in instrument meteorological conditions and was on an instrument flight rules flight plan. The flight had departed Guadalajara, Mexico, and was destined for the Chicago Midway Airport, Chicago, Illinois.

Meteorological information

A Meteorological Specialist with the National Transportation Safety Board, Operational Factors Division, obtained and examined weather products related to the turbulence encounter. The full text of the Meteorological Report is included in the public docket of the accident report.

Surface weather observations in the vicinity of the turbulence encounter indicated the presence of thunderstorms and lightning moving eastward.

The National Weather Service (NWS) Radar Summary Chart indicated multiple areas of echoes from Wisconsin south-southwestward across Iowa, Missouri, Kansas, Oklahoma, Arkansas, and into Texas. The maximum intensity of the echoes in southern Missouri reached video integrator and processor (VIP) levels 5 and 6 or intense to extreme intensity, with echo tops ranging from 36,000 to 41,000 feet.

The NWS Weather Surveillance Radar located in Springfield, Missouri, showed a band of echoes, at 1623, extending over the location of the turbulence encounter. The echoes had a reflectivity of 60 dBZ or VIP level 6 extreme intensity. This same radar station recorded, at 2138, a band of echoes in the area of the turbulence encounter. The strongest echo had a maximum intensity of 60 dBZ or VIP level 6 extreme intensity and was less than 5 miles east of the turbulence encounter. The echo intensities over the location of the turbulence encounter ranged from 36 to 45 dBZ or VIP level 3 to 4 strong to very strong intensity.

Satellite data was used to determine cloud tops in the area of the turbulence encounter. An infrared image from April 18, 2002 at 1632 shows that the cloud tops along the flight path of the airplane were in the range of 28,000 to 33,000 feet. The image also showed the highest cloud tops in the vicinity to be about 39,000 feet. The highest cloud tops were located about 9 miles east-northeast of the turbulence encounter.

At 1210, the Kansas City (ZKC) Center Weather Service Unit (CWSU) issued a Center Weather Advisory (CWA) for a line of VIP level 3 to 5 thunderstorms with tops to 33,000 feet. This CWA however, was only valid until 1410 and had expired by the time of the turbulence encounter. At 1640, after the turbulence encounter, the ZKC CWSU issued a CWA for an area of VIP level 3 to 5 thunderstorms with tops to 38,000 feet. This CWA covered the location of the turbulence encounter.

No Significant Weather Information (SIGMET) advisories were issued by the NWS Aviation Weather Center that related to the accident flight. No Convective SIGMET's were in effect for the accident location.

Communications

Transcripts of voice communications between Air Traffic Control (ATC) and the airplane were obtained. The transcripts show that the flight crew requested a course deviation for weather avoidance at 1627. The ATC controller approved the deviation and requested that the flight crew advise when they were able to return on course. At 1632, the flight crew advised ATC that they were able to return on course.

At 1637, the flight crew requested a deviation to the Kansas City Missouri International Airport for landing. The flight crew subsequently informed ATC that they, "got too close to that ah thundershower there and we've got uh we've got some passengers on board who've been injured and uh we're gonna have to have ambulances standing by for them." The ATC controller asked the flight crew if they "would rather go to Saint Louis or would you want to go to Kansas City." The flight crew decided to divert to Saint Louis.

When asked by the ATC controller as to what type of turbulence that the airplane had encountered, the flight crew replied, "we just caught the very corner of one of the build---ah or the only buildup that's out there just uh like got one wing into it on the right hand side and uh that was---got a good up draft and a good down draft." The airplane continued to STL where a landing was made without further incident. The full transcripts of the communications are included in the public docket of this report.

Flight recorders

The cockpit voice recorder was retained for examination. The 30-minute recording began as the airplane was on approach to STL and did not capture the turbulence event. No further examination of the recording was performed.

The digital flight data recorder was retained for examination of the data. A Flight Data Recorder Specialist in the National Transportation Safety Board Vehicle Recorders Division examined the data. Among the parameters recorded were the vertical acceleration, seat belt sign (ON/OFF), roll angle of the airplane, and pitch angle of the airplane.

The FDR Subframe Reference Number is a measure of relative time on the Flight Data Recorder. One frame is equivalent to one second. The data shows that the airplane experienced a series of vertical accelerations that began at a FDR Subframe Reference Number of 90903 and lasted for about 17 seconds. The maximum positive vertical acceleration of 2.099 g's occurred at a FDR Subframe Reference Number of 90906. The maximum negative vertical acceleration of -0.648 g's occurred at a FDR Subframe Reference Number of 90912. The data shows that the seat belt sign was turned on at a FDR Subframe Reference Number of 90894.

Tests and research

According to a written statement by the first officer, the crew requested to deviate left of course for weather and ATC approved. He stated that the captain initiated the deviation and that he and the captain were monitoring the weather radar. He said that prior to being abeam the weather, the captain made an additional deviation to the west and turned on the fasten seat belt signs. He stated that the airplane subsequently encountered the turbulence and that he did not notice any "appreciable excursions in airspeed or attitude." He noted that the airplane had descended about 50 feet below the cruise altitude.

The ATA General Operations Manual (GOM) states that thunderstorms that are identified as severe or giving an intense in-flight weather radar echo should be avoided by at least 20 miles.

Plots of the Aircraft Situation Display from ATA Dispatch were obtained. The plots of the airplane's flight path are overlaid onto Doppler weather radar data. The plots show the airplane's flight path in relation to the Doppler weather radar returns in the area of the turbulence encounter. The plots are included in the public docket of this report.