DCA23FA428

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

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.

The flight departed MXP about 1300 UTC. The aircraft climbed to cruise altitude and the cruise portion of the flight lasted approximately 9 hours. Autopilot was engaged shortly after departure and remained on until the final approach. The crew reported that several minutes prior to the top of descent, the seat belt sign was illuminated. The flight was cleared for the OZZZI One arrival approaching Atlanta from the northeast. At 21:59, the crew briefed the arrival and the ATL ILS Runway 8L approach. At 22:22, the crew discussed the cloud fronts in the vicinity and mentioned that they may need a weather deviation at some point in their descent. At 22:26, Atlanta Center instructed them to cross waypoint OZZZI at 14,000 feet and 310 knots or higher, which the crew confirmed. The crew then requested a deviation west of course at around waypoint KILRR. The controller directed them to continue heading to OZZZI and advised that the next sector controller could handle their request. The flight crew accepted this instruction noting only a small white cloud near OZZZI. The flight crew stated there were no PIREPs from preceding aircraft, no nearby weather radar returns, nor any significant turbulence indications on their Flight Weather Viewer application. At 22:28, the relief pilot contacted the flight attendants to inform them that they may encounter turbulence in about 5 minutes and suggested they complete their duties within that time. At 22:30, the controller requested a ride report passing through 17,000 ft prior to handing the flight off to Atlanta Approach. They reported “…its good right now but it’s about to get worse. There’s a cloud right up here.” The crew then completed the approach brief and descent checklist. During the descent, at about 2231:15 UTC, as the aircraft passed through a pressure altitude of about 15,100 ft and computed airspeed of 312 kts, the aircraft experienced a period of severe turbulence. The cockpit voice recorder (CVR) recorded a sudden sound of the flight deck being jostled for about 3 seconds with an audible reaction from the crew. In less than one second, the aircraft experienced vertical accelerations ranging from +1.69g to -0.97g and recorded vertical speeds from +3,000 fpm to -10,600 fpm. Immediately afterward one crew member checked in with the Atlanta Approach controller, while the other directed the jump seater to check in with the cabin crew to see how everyone was doing. The flight crew continued with entering their new assigned altitude of 12,000 feet. The initial report from the cabin crew was that one person was injured during the turbulence and that the crew was tending to them. The crew continued to coordinate with air traffic control (ATC) controller and then advised them that they “hit a pretty good bump over OZZZI”. The controller asked them what the degree of turbulence was, and they responded “severe.” The jump seater continued to coordinate with other members of the cabin crew, and at 22:32:55 informed the flight crew that they needed to declare an emergency due to an injured passenger needing assistance on arrival at Atlanta. At 22:33:29, the crew declared a medical emergency to Atlanta Approach, reporting one person injured needing paramedics at the gate upon arrival. They also sent a message to dispatch informing the company of the emergency. The crew continued the approach, coordinating with ATC and completing standard checklists. At 22:38:40, the captain thanked the jump seater for coordinating with the cabin crew and said that there was nothing showing on the radar at the time of the turbulence. The flight landed at 22:48. During the taxi the crew received an update from the cabin crew informing them of the number of injured passengers and their locations. Paramedics met the aircraft at the gate and transported injured passengers to the hospital. Atlanta (ZTL) Air Route Traffic Control Center (ARTCC) was a level 12 enroute air traffic control facility located in Hampton, GA, and part of the FAA’s eastern service area. The Logen Radar (R49) sector was the involved sector which is an inbound sector for Atlanta Terminal area arrivals from the northeast and is responsible for final spacing of aircraft to Atlanta Terminal Radar Approach Control (A80 TRACON). The following timeline was constructed from certified audio recordings and automatic dependent surveillance – broadcast (ADS-B) data were provided by the FAA. Times are in EDT and altitudes below 18,000 feet are in feet above mean sea level (msl), and altitudes of 18,000 feet or greater are in flight levels (FL). ADS-B data indicated that DAL175 departed MXP over nine hours earlier and flew without incident before checking in with ZTL ARTCC. The following documents only the portion of the flight when DAL175 was in communication with ZTL ARTCC and A80 TRACON. 1820:13 - The crew of DAL175 first contacted ZTL ARTCC and was instructed by the R49 controller to maintain FL 220. The crew acknowledged with a correct readback. 1826:19 - The crew of DAL175 queried the R49 controller regarding a CPDLC instruction. The R49 controller instructed the crew to disregard, maintain 310 knots or better, descend and cross OZZZI9 at 14,000 feet, and provided the altimeter of 29.82. The crew acknowledged with a correct readback. 1827:06 - The crew of DAL175 requested to deviate left around weather. The controller requested they maintain “pointed at” OZZZI, and that they could expect vectors from A80 TRACON. The crew acknowledged. 1829:54 - The crew of SWA4825 [a few miles in-trail of DAL175] asked how the rides were and the R49 controller advised he had received no complaints and that they could expect more information from approach when they were closer. The pilot mentioned they were looking at the “towering [indiscernible]” in front of them and they may have to slow down. 1830:15 - The R49 controller queried DAL175 about how their ride was through 17,000 feet. The crew said it was good but about to get worse and that there was a cloud up ahead. The R49 controller instructed the crew to contact approach. 1831:26 - The crew of DAL175 checked in with the A80 TRACON RA controller and reported out of 15,000 feet for 14,000 feet. The RA controller instructed DAL175 to descend and maintain 12,000 feet. The crew acknowledged with a correct readback. 1832:13 - The crew of SWA4825 checked in with the A80 TRACON RA controller and advised they were slowing to 280 knots for weather and that they were getting ready to enter clouds. 1832:33 - The crew of DAL175 advised the radar controller that they had just “hit a pretty good bump” over OZZZI and that they would not want to go through that again. The controller asked the severity of turbulence and the crew reported “it was pretty much severe.” 1833:01 - The RA controller immediately reached out to SWA4825 and advised them of the severe turbulence report from DAL175 and the crew of SWA4825 advised they had only experienced light to moderate through the same area. The remainder of the flight for DAL175 was uneventful, and the remainder of communications were routine landing information and the exchange of emergency information. There was minor damage to the interior of the aircraft as a result of the event. Of the passengers onboard, there were 5 minor injuries and 2 serious injuries. Of the 11 flight attendants on board, there were 8 minor injuries and 2 serious injuries. The 4 serious injuries involved combinations of rib fractures and spinal compression fractures. All injured parties were either not seated or did not have their seatbelt buckled at the time of the turbulence. Injuries were sustained from impacting the ceiling or overhead panel and falling back to the floor or seat during the turbulence event. The FDR recording was delivered electronically to the Vehicle Recorder Division in compressed .fdr format. The decompressed file contained approximate