What happened
While cruising at flight level 310, USAir Flight 333 encountered severe turbulence. The pilot reported observing thunderstorm activity on the airborne weather radar. The flight was within 20 miles of the cell before action was taken to avoid the encounter. The recommended penetration airspeed is .70 Mach or 280 knots. The pilot reported maintaining .76 Mach during the encounter.
The investigation
The investigation focused on the operational decisions made by the flight crew during the turbulence event. Analysis of the flight data revealed that the aircraft was traveling at a speed exceeding the recommended penetration airspeed for severe weather conditions. The pilot maintained a speed of .76 Mach, which is higher than the standard .70 Mach or 280 knots recommendation.
Findings
The primary finding relates to the timing of the avoidance maneuver and the aircraft's speed during the encounter. The flight crew identified the hazardous weather on radar but delayed evasive action until the aircraft was already within 20 miles of the thunderstorm cell. Furthermore, the aircraft was not slowed to the recommended penetration airspeed before entering the area of severe turbulence.
Safety message
Pilots should initiate avoidance maneuvers as early as possible when thunderstorm activity is detected on radar. Maintaining the recommended penetration airspeed can help reduce structural stress and improve controllability during severe turbulence encounters.