Airbus Sas A350-941 N576DZ accident near Atlanta, Georgia

Probable cause

The flight crew’s unintentional encounter with rapidly developing cumulus clouds which resulted in an encounter with severe convectively induced turbulence.

NTSB analysis

Two passengers and two flight attendants received serious injuries aboard Delta Air Lines flight 175 when it encountered turbulence during descent into Hartsfield-Jackson Atlanta International Airport (ATL), Atlanta, Georgia. The flight was an international scheduled passenger flight operated as a Title 14 Code of Federal Regulation Part 121 from Milan Malpensa Airport (MXP), Ferno, Italy to ATL.

The flight crew’s performance leading up to the turbulence encounter was consistent with standard practices. The investigation evaluated the potential for crew awareness of the rapidly building cell where the flight encountered severe turbulence. The flight crew was aware and monitoring the general convective weather in the area and showed intention to deviate around other potential convective cells. However, the rapid development of cloud buildup that caused this turbulence event and the lack of precipitation associated with it, meant that the risk of turbulence was not apparent on aircraft weather radar nor on air traffic control scopes.

In anticipation of another convective cell a few minutes further down their route, the crew took steps to advise the flight attendants to secure the cabin early and the seat belt sign was illuminated. Unfortunately, the timing of the unexpected turbulence meant that cabin crew were not seated during the event, resulting in multiple injuries.

The investigation evaluated whether existing technology would have been sufficient to give the crew advance warning of the rapidly building cell. The Graphic Turbulence Guidance Nowcast (GTGN) or “Turbulence Nowcast” provides a computer-generated four-dimensional forecast related to the expected intensity of atmospheric turbulence in convection provided in eddy dissipation rate (EDR) as based on estimated vertical wind velocity or aircraft vertical acceleration to provide aircraft specific turbulence intensity values. While the accident aircraft did not have this capability, a Southwest Airlines flight immediately behind the accident flight on the route had in-situ capability and reported severe turbulence in the location where the turbulence event occurred. The GTGN can provide nowcasts corresponding to observed severity in near real time and could have alerted the flight crew earlier to secure the cabin.

Text from the NTSB. Open the NTSB docket for DCA23FA428 for the full record.