Incident Overview
On June 12, 2012, at about 2017 central daylight time (CDT), United Airlines Flight 1632, a Boeing 737-724 (registration N16709), encountered severe turbulence while at flight level (FL) 221 near Winnie, Texas. The flight was operating under 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight from George Bush Intercontinental Airport (IAH) in Houston, Texas, to La Guardia Airport (LGA) in New York, New York. The aircraft was not damaged.
Preflight Briefing
The flight was scheduled to depart IAH at 1744 CDT but was delayed on the ground due to strong thunderstorms at the airport. Approximately 1.5 hours before the actual departure, the captain conducted a preflight briefing with the three flight attendants (FAs) without the first officer present. During post-accident interviews, the captain recalled informing the FAs that there would be turbulence on the departure and asked them to stay seated until he contacted them. However, all three FAs did not recall any mention of turbulence in the briefing, nor did they recall being told to remain seated.
Flight and Turbulence Encounter
The flight departed IAH normally and proceeded east-southeast to avoid an area of severe weather east of the airport. After the chime sounded and the blue light over the cockpit door extinguished—indicating the aircraft was above sterile cockpit altitude—the FAs began cabin service. According to flight data recorder (FDR) data, the airplane was climbing through FL221 at an airspeed of 285 knots when it entered an area of turbulence lasting about 10 seconds. During this period, vertical acceleration fluctuated between approximately -0.77 g and +1.89 g, lateral acceleration between -0.13 g and +0.24 g, and airspeed between 261 knots and 290 knots.
Injuries and Diversion
At the time of the turbulence, the forward flight attendant was out of her jumpseat making a public address announcement and was thrown against the ceiling. One of the aft flight attendants, unbuckled and preparing to get up in the aft galley, was thrown against the ceiling and galley, knocking her unconscious. The other aft flight attendant, who remained buckled in her seat, was struck in the head by the cabin phone. 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, but air traffic control (ATC) informed them that IAH was closing due to weather over the field. The flight diverted to Lake Charles Regional Airport (LCH) in Lake Charles, Louisiana, where medical personnel met the aircraft at the gate. All three flight attendants were transported to a hospital. One was diagnosed with a fractured coccyx and lumbar strain; another suffered a collapsed lung and fractures of the right clavicle, ribs, cervical vertebrae, and pelvis.
Weather and Radar
National Weather Service forecasts indicated a large area of convective activity over southeastern Texas and southern Louisiana moving southeast. The thunderstorm at IAH that delayed the flight was part of this activity. The flight crew received weather information from dispatch, including Convective SIGMETs, Weather Watches, and company advisories warning of severe thunderstorms moving southeast near the proposed route. The flight plan routing appeared clear of strong convection, but post-accident analysis revealed the flight was proceeding east-southeast along the southern edge of convective activity when it penetrated a rapidly developing cell of "heavy" to "extreme" intensity at their flight level. The cells developed within 5 to 10 minutes before the event. The crew reported using the aircraft's onboard weather radar during the flight, with the tilt set to +1 degree. Both pilots stated they identified significant weather 20 to 50 miles ahead and north of their flight path. Company guidance stated the tilt should be set to 0 degrees or slightly down for operations between 10,000 and 35,000 feet.