AirTran Airlines Flight 426

The properly certificated and qualified flight crew received flight release documents containing weather information from airline dispatch that was incomplete and did not contain pertinent Convective SIGMETs and a tornado watch bulletin. However, the flight crew stated that before departure, they looked at a computerized weather display and observed a line of adverse weather conditions along their intended route of flight. During initial climb after takeoff, the flight crew noticed that they were approaching a line of thunderstorms. The captain initially planned to fly around this line but noticed a 10-mile gap in the line that was depicted on the airplane's on-board weather radar display. The captain decided to fly through the gap, which he could also see when he looked outside of the airplane. The first officer did not challenge his decision. Both flight crewmembers stated that the weather radar was functioning normally. It is unknown exactly what was depicted on the aircraft radar. Radar and weather data indicate that the airplane penetrated an extreme weather echo (VIP level 6) that likely contained a severe thunderstorm, hail, and severe to extreme turbulence. The thunderstorm top was above 45,000 feet. Neither the flight attendants nor the passengers received an adequate or timely briefing to remain seated and to prepare for the possibility of turbulence. The aft flight attendant and a passenger were seriously injured during the turbulence encounter. Hail shattered the front windshield and damaged the pitot system, radome, wings, tail, and engines. The flight crew successfully performed an emergency landing at a nearby airport. The investigation revealed that the captain had been involved in two other air carrier incidents involving adverse weather conditions. It also revealed that the airline lacked adequate training and guidance regarding hazardous weather interpretation and avoidance, as well as adequate procedures to notify flight attendants about potential turbulence. According to FAA guidance, flight crews should avoid intense or extreme level radar echoes by at least 20 miles. This guidance also instructs pilots that such echoes should be separated by at least 40 miles before flight is attempted between the echoes.

Verbatim NTSB analysis and probable cause from the NTSB dataset