What happened
On the evening of April 12, 2020, a Boeing 737-800, registered JA64AN, was operating a scheduled flight from Fukuoka Airport to Osaka International Airport. The flight, operated by ANA Wings Co., Ltd. as a joint undertaking with All Nippon Airways, was cruising at approximately 27,000 feet when it encountered sudden and violent atmospheric disturbances.
While the flight crew had planned a cruising altitude above the primary cloud layers based on earlier meteorological reports, the aircraft began intermittently skimming thin clouds during its descent. At approximately 19:29 JST, the aircraft was struck by two bouts of intense shaking. Flight data recorder analysis showed vertical acceleration fluctuations ranging from +1.85 G to -0.30 G during the event.
While all 20 passengers remained safe because they were seated and belted, a cabin crew member standing in the aft section of the aircraft was caught by the turbulence. The sudden negative G-force caused the crew member to float into the air, and the subsequent impact caused them to strike the floor and ceiling, resulting in a partial fracture of the pelvis.
The investigation
The Japan Transport Safety Board (JTSB) examined flight data recorder (FDR) and cockpit voice recorder (CVR) data, alongside meteorological satellite imagery. The investigation focused on the visibility of cloud tops during night operations and the effectiveness of the aircraft's weather radar in detecting the specific convective activity involved. Investigators also reviewed the crew's use of the seat belt sign and the cabin crew's positioning during the period when the sign was turned off.
Findings
- The turbulence was generated by convective activity within cloud layers that reached up to 30,000 feet.
- Although the airborne weather radar did not show strong echoes, the aircraft was likely flying through clouds that were more developed than initially anticipated.
- The sudden vertical acceleration prevented the standing cabin crew member from maintaining balance or reaching a seated position before the second shock occurred.
- Nighttime flight conditions made it difficult for the flight crew to visually identify the exact boundaries of the cloud layers ahead.
- The safety of the passengers was maintained because they had adhered to company policy of remaining buckled even when the seat belt sign was extinguished.