What happened
While being vectored for a night instrument approach in visual meteorological conditions, the aircraft encountered severe turbulence. At the time of the encounter, no other aircraft were present in the vicinity. Although weather radar indicated thunderstorms located to the north of the flight path, the flight had been smooth prior to the onset of the turbulence.
The investigation
Analysis of the airplane's flight data recorder provided a detailed account of the atmospheric disturbances. The data showed a gradual increase in vertical accelerations over a 15-second period. This was followed by another 15 seconds characterized by a series of jolts, which included four vertical accelerations of approximately 2.0 positive g's and three measurements of 0.5 negative g's.
Vertical accelerations persisted for an additional 75 seconds. During this interval, the recorder noted eight instances exceeding 1.5 positive g's and two instances registering approximately 0.0 g. The investigation also identified lateral and longitudinal accelerations that occurred in synchronization with the vertical acceleration events.