DCA12LA091

The flight encountered an area of severe turbulence during the climb at about flight level 221 on departure that seriously injured two flight attendants (FA). The flight crew was aware of the forecasted convective conditions and potential for turbulence forecast along their route of flight. The captain and FAs differed in their recollection about the turbulence discussion during the pre-flight briefing. The FAs were not aware of the turbulence potential during the flight and began their service before the captain instructed them as he intended. The weather radar tilt setting of +1 degree used by the flight crew would be useful for detecting the tops of medium range and distant storms but would not be useful in detecting the rapidly developing nearby cells below and in front of the airplane. A negative tilt setting on the weather radar was recommended by the company to detect cells building in front of the airplane. As a result, the flight penetrated a rapidly developing area of "extreme" convective activity that produced severe convectively induced turbulence.

On June 12, 2012, about 2017 central daylight time (CDT), United Airlines flight 1632, a Boeing 737-724, N16709, encountered severe turbulence about flight level (FL) 221 near Winnie, Texas. Two flight attendants were seriously injured and the remaining flight attendant sustained minor injuries. The pilot subsequently declared an emergency and diverted to Lake Charles Regional Airport (LCH), Lake Charles, Louisiana. There was no damage to the airplane. The flight was operating under the provisions of 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight from George Bush Intercontinental airport (IAH), Houston, Texas, to La Guardia Airport (LGA), New York, New York. The flight was scheduled to depart IAH at 1744 CDT but was delayed on the ground due to strong thunderstorms at the airport. The captain conducted a preflight briefing with the three flight attendants (FA) about 1.5 hours prior to the actual departure time without the first officer present. The captain recalled in post-accident interviews that he informed the FAs that there would be turbulence on the departure and asked them to stay seated until he contacted them. The three FAs did not recall any mention of turbulence in the pre-flight briefing and none recalled being informed to stay seated in post-accident interviews. The flight departed IAH normally and proceeded east-southeast to avoid an area of severe weather located east of the airport. The FAs began their cabin service after the sounding of the chime and extinguishing of the blue light over the cockpit door indicating the aircraft was above the sterile cockpit altitude. Flight data recorder (FDR) data indicated the airplane was climbing through FL221 at an airspeed of 285 knots when it encountered an area of turbulence for about 10 seconds, with large fluctuations in vertical acceleration, lateral acceleration, and airspeed. The vertical acceleration fluctuated between about -0.77 g's and +1.89 g's, the lateral acceleration fluctuated between about -0.13 g's and +0.24 g's, and the airspeed fluctuated between about 261 knots and 290 knots. At the time of the turbulence encounter, the forward FA was out of her jumpseat making a public address announcement and was thrown against the ceiling during the turbulence encounter. One of the aft FAs was unbuckled and preparing to get up in the aft galley when she was thrown against the ceiling and galley and was knocked unconscious. The other aft FA remained buckled in her seat during the turbulence encounter and was struck in the head with the cabin phone during the event. The forward FA used the inter-phone to inform the flight crew of the serious injuries. The flight crew declared an emergency and requested a return to IAH, however, ATC informed them that IAH was closing due to weather over the field. As a result, the flight diverted to LCH and medical personnel met the aircraft at the gate. All three FAs were transported to the hospital where one wasdiagnosed with a fractured coccyx and a lumbar strain; one suffered a collapsed lung and fractures of her right clavicle, ribs, cervical vertebrae, and pelvis. National weather service forecasts showed a large area of convective activity over southeastern Texas and into southern Louisiana that was moving southeast. The thunderstorm at IAH that delayed the flight was part of the forecasted convective activity. The flight crew was provided weather information from dispatch that included Convective SIGMETs, Weather Watches, and company advisories warning of severe thunderstorms moving southeastward near the proposed route of flight. The flight plan routing received by the crew appeared to be clear of any strong convection, however, post-accident analysis showed the flight was proceeding east-southeast along the southern edge of the convective activity when it penetrated a rapidly developing cell of "heavy" to "extreme" intensity at their flight level. The cells developed within the 5 to 10 minutes prior to the event. The crew indicated they utilized the aircraft's onboard weather radar during the flight. Both pilots stated that the radar tilt was set to +1 degree during the flight and they identified areas of significant weather located 20 to 50 miles in front of the aircraft and to the north of their intended flight path. The weather radar tilt adjustment allows for pilots to compensate for aircraft altitude and attitude. Company guidance stated that the tilt should be set to 0 degrees or slightly down for operations between 10,000 and 35,000 feet.