N745SK

On August 22, 2017, about 1721 central daylight time, a Bombardier CRJ700, N745SK, operated as SkyWest Airlines, d.b.a American Eagle flight 3167, encountered turbulence in cruise flight and diverted to Lambert-St. Louis International Airport (KSTL), St. Louis, Missouri. Of the 74 passengers and crew members aboard, one passenger received serious injuries; two flight attendants and eight passengers received minor injuries. The airplane was not damaged. The airplane was operated as a Title 14 Code of Federal Regulations Part 121 regularly scheduled domestic passenger flight.

The crew reported that they were in visual meteorological conditions with no turbulence, and that they could see the convective weather as they were approaching it from about 80-100 miles away. As they approached the area, the controller cleared the flight to deviate around the weather to the left, and shortly thereafter, the controller issued instructions to turn 10° right for traffic. The flight crew initially declined the turn due to their concern that the flight would enter adverse weather. The controller then repeated the instructions to turn right, and the crew subsequently complied.

After completing the turn, the airplane flew into the tops of clouds and encountered severe turbulence at 36,000 feet. According to the crew, the encounter lasted several seconds; the seat belt sign was not activated. At the time, the flight attendants were conducting beverage service, and the flight attendants and galley cart struck the overhead cabin. According to the flight crew, the encounter resulted in a 200-foot altitude loss and momentary activation of the stall warning system stick shaker.

The captain reported that company guidance stated that the seat belt sign be turned on anytime turbulence could be expected, and added that his decision to keep the seat belt sign off was based on the flight conditions leading up to the event, since there was no turbulence, and "there was nothing on the radar indicating what they were going to encounter" convective weather.

According to flight data recorder (FDR) information, the turbulence event occurred as the airplane was passing through about 35,750 ft. During the encounter, vertical acceleration ranged from -0.8g to 1.89g, the airplane lost 200 ft altitude in 2 seconds, and the airspeed varied by about 15 knots. The airplane's heading changed 5 degrees to the left in one second, and the airplane entered a pitch attitude of 11.6° nose-down, and rolled 26° to the left.

SkyWest Airlines Flight Operations Manual (FOM), Section F, "General Turbulence Procedures," specified that before entering an area of thunderstorms pilots were required to ensure that all personnel are seated and safety belts were fastened. Chapter 07 of the manual, "Adverse Weather," stated in part:

When turbulence is expected/forecast, the PIC [pilot-in-command] is responsible to ensure crew and passengers are adequately briefed and prepared. Briefings must include turbulence considerations, location, intensity, and possible duration.

When flight into forecast turbulence is unavoidable or an unforecast encounter occurs, timely notification to the FAs [flight attendants] and passengers is crucial. Notification is given as far in advance as possible using the interphone. Do not rely on the seat belt sign alone.

When asked about their company guidance for thunderstorm avoidance, the captain stated that their FOM specified 20 miles away and that the preferred distance was 10 miles. The FOM stated that prior to entering an area of thunderstorms, pilots were required to "monitor radar constantly and alter flight path to miss intense build-ups by 10 miles or more (20-mile clearance is best when possible, particularly at altitudes above the freezing level)."

When asked about the radar settings before the turbulence encounter, the captain stated that, "the gain would have been in auto, and with a pitch of 1 degree." He said that he adjusted the tilt as they approached the weather to give them the best picture to avoid ground clutter, and did not change the tilt once closer to the weather.

Radar techniques were included in the SkyWest Airlines FOM and stated in part:

- Use preset gain or, when this feature is not available, a constant low gain setting.

- Take particular care to avoid storms with hooked fingers, figure six echoes, sharp rainfall gradients, and rapidly changing shapes.

- Never fly under the overhang of a thunderstorm.

- A 40-mile range is good, but frequent check of longer ranges must be made to avoid flying into a blind alley.