N576DZ

HISTORY OF FLIGHTThe 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. METEOROLOGICAL INFORMATIONA stationary front on the western boundary of the area extending from Mississippi northeastward into Alabama, Tennessee, Kentucky, into West Virginia and Hurricane Idalia helped funnel a warm-moist airmass across the southeast and eastern U.S. that supported cumulus and cumulonimbus cloud development, which was the catalyst for the turbulence encounter. The 12-hour Surface Prognostic Chart valid for the period depicted an extensive area of precipitation and thunderstorms across the area, with a 60-70% probability of precipitation over the area. The NWS SPC’s Convective Outlook identified general thunderstorms over the region.

The NWS Regional Composite Radar Mosaic for 1830 EST depicted numerous echoes associated with heavy to extreme precipitation over South Carolina, central Georgia, and into Alabama, with no well-defined echo over the turbulence location. The image also depicts some of the echoes the flight had deviated around in South Carolina as the flight descended into Georgia. An animation of the NWS KFFC WSR-88D depicted the echoes rapidly developing over the area and were not identifiable until minutes prior to the turbulence encounter.

A review of all the pilot reports (PIREPs) surrounding the period noted several reports of light to moderate turbulence at various altitudes, predominately over southern Georgia, with two PIREPs noting thunderstorms. There were no urgent or severe reports prior to the accident flight. The NWS AWC plot of METARs and the regional radar display at 1835 EDT with the actual and reported turbulence event differed by about 30 miles.

The HRRR sounding for 1800 EDT depicted a moist and unstable atmosphere supporting convection with tops near 42,000 ft. The maximum vertical velocity (MVV) of the potential updrafts were estimated at 60 m/s or near 11,800 fpm and an indication of the rapid growth potential to the echoes.

The GOES-16 infrared and imagery depicted the accident site immediately north of an area of enhanced clouds associated with developing convection. The radiative cloud top temperature at 1831 EDT over the turbulence location corresponded to cloud tops neat 21,000 ft. The visible imagery identified the clouds over the location as cumulus congestus type clouds associated with developing cumulonimbus type clouds. The imagery also showed significant convection with tops near 44,000 ft about 50 miles south of the flight path.

The NWS KFFC WSR-88D composite and base reflectivity radar imagery depicted several well-defined extreme intensity echoes west and east of ATL, with several smaller areas of moderate to heavy echoes rapidly developing as the accident aircraft descended into the area. Echoes of 30 to 35 dBZ “moderate” intensity echoes are observed over the flight track at the time of the turbulence encounter. No echoes were detected until 1828 EDT or a few minutes prior to the turbulence encounter and intensified to 46 dBZ or “heavy” intensity by 1848 EDT.

The cross-sectional imagery also depicted the rapid development of echoes over the flight path during the period and showed radar echoes of 18 dBZ approaching 20,000 ft prior to the turbulence encounter and echoes over 40 dBZ under the flight path by 1841 EDT. Based on the radar imagery, airborne weather radar should have depicted some precipitation echoes immediately prior to the encounter.

The FAA CoSPA VIL display also failed to detect the developing echoes over the area until after the turbulence event had already occurred. The CoSPA growth and decay algorithm depicted growth over the area west of the accident site but did not highlight the turbulence location.

The NWS had predicted the potential for thunderstorms in the vicinity of ATL after 1400 EDT in the TAF, while the AFD indicated that scattered to widespread convection was expected over the region during the afternoon and overnight period. The NWS AWC issued Convective SIGMET 17E at 1755 EDT for a large area of thunderstorms with tops to 45,000 ft immediately south of the accident location, with the area expected to move northward at 10 knots.

The NCAR GTGN or “Turbulence Nowcast” rerun for the period depicted EDR values of 0.2 to 0.3 at 1830 EDT over the turbulence location with values up to 0.4 along the project flight track into ATL, with values of 0.4 to 0.5 at the event location by 1845 EDT. The Southwest Airlines flight immediately behind Delta 175, had in-situ capability and reported an EDR value of 0.42 at 1833:20 EDT at 14,070 ft at the location where the turbulence event occurred.

The ICAO turbulence reference puts the EDR values between 0.20 to 0.45 as moderate and values that exceed 0.45 as severe for a medium weight class aircraft. While EDR values of 0.42, or moderate intensity, were reported in the vicinity immediately after the encounter, the Flight Data Recorder (FDR) vertical acceleration data as well as the passenger and cabin attendants’ reaction inside the airplane suggests turbulence was actually in the severe to extreme intensity range.

Figure 1. National Weather Service KFFC WSR-88D Composite Reflectivity image for 2234Z with accident flight track overlaid in magenta. Aircraft position at the time the image was created is indicated by the aircraft symbol (bottom left). INJURIES TO PERSONSOf 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. DAMAGE TO AIRCRAFTThere was minor damage to the interior of the aircraft as a result of the event. COMMUNICATIONSAtlanta (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 foll...