What happened
On June 7, 2021, a Vistara Boeing 737-800, registration VT-TGE, was operating a scheduled flight from Mumbai to Kolkata. During the descent phase, the flight crew identified a 20-nautical-mile gap between two large cumulonimbus clouds and elected to fly through this opening to avoid surrounding line squalls and thunderstorms.
As the aircraft descended through FL200, it first encountered light to moderate turbulence for approximately 15 seconds. This was immediately followed by a sudden, intense burst of severe turbulence lasting nearly two seconds. The impact was significant enough to disengage the autopilot and cause passengers to strike the aircraft' and ceiling panels. While the aircraft remained structurally sound, the event resulted in 02 serious injuries and 02 minor injuries among the passengers. The crew subsequently requested a medical priority landing at Kolkata, where the aircraft arrived safely.
The investigation
AAIB India investigators examined the flight's history, weather data, and crew actions. The investigation confirmed that the crew had access to updated weather information via ACARS and was aware of the thunderstorms and gusty winds reported by the Kolkata ATIS. The investigators also reviewed the aircraft's flight recorders and the cockpit voice recorder to reconstruct the sequence of events.
Crucially, the investigation looked into the cabin securing process. While the crew had activated the fasten seat belt sign, the pilot in command instructed the cabin crew to take their seats due to the expected turbulence. This instruction prevented the cabin crew from completing a full seatbelt compliance check of all passengers before the severe turbulence hit.
Findings
- The flight crew chose to navigate through a narrow gap in weather cells despite being aware of the deteriorating conditions and the presence of heavy precipitation.
- The decision to fly through the gap instead of circumnavigating the weather led to the encounter with severe turbulence.
- The aircraft's weather radar showed magenta areas, indicating high-intensity precipitation and potential severe turbulence, yet no diversion to alternate airports was considered.
- The cabin crew could not finish securing all passengers because the pilot instructed them to sit down for their own safety.
- The operator's manual lacked specific procedures for weather avoidance during the enroute phase.
- Flight dispatch did not provide proactive guidance or weather trend updates to the crew during the period of inclement weather.