DCA24FA143

The pilot’s inadvertent encounter with convectively induced turbulence from a broken line of embedded thunderstorms moving across the region. Contributing to the accident was the failure of the air traffic controller to issue pertinent weather information along the airplane’s route of flight due, in part, to the sector over-saturation and frequency congestion caused by damaged communication equipment and the inability to split the sector to alleviate some of the traffic volume.

The initial flight plan was created to minimize the exposure to any potential turbulence and thunderstorms. While en route, the flight crew used the available resources to deviate around visible weather; however, rapid development of cloud buildups prevented them from avoiding turbulence. The flight crew attempted to notify the flight attendants (FAs) of the potential for turbulence. The FAs were completing cabin service when the captain made the announcement, but only one FA was able to secure himself in a jumpseat before the airplane entered the turbulence. One FA and one passenger were in the aft galley area during the encounter and sustained serious injuries. Another FA was in the aisle and received minor injuries. A review of National Weather Service (NWS) products confirmed the passage of a squall line north of the location of the encounter with turbulence. Radar imagery depicted extreme intensity echoes below the airplane’s flight path, with light intensity echoes reaching the airplane’s cruising altitude at the time of the encounter. The NWS issued inflight weather advisories for the potential for severe embedded convective activity and turbulence associated with those storms, but there is no evidence that the flight crew was alerted. The convective echoes, high echo tops, and very unstable airmass defined this event as a convectively induced turbulence event. Jacksonville (ZJX) Air Route Traffic Control Center (ARTCC) had adequate radar coverage over the area at the time of the encounter to identify the severe weather hazard. The Center Weather Service Unit had issued a morning weather briefing warning of a broken line of thunderstorms that was expected to cross the area. A review of the air traffic control (ATC) audio revealed that the west radar sector (R30) controller did not advise the flight crew of the precipitation ahead of them as required, likely due to the increased workload the R30 controller experienced at the time. The R30 (NEPTA) sector at the time of the accident experienced increased traffic due to sector splits at adjacent ARTCCs. In addition, a fire in 2023 at the remote communications air ground facility (RCAG) site that serviced NEPTA caused damage to equipment that was critical to the NEPTA sector communications, so a temporary mobile RCAG was provided. The temporary frequencies were low-power 10-watt transmitters with limited coverage, which caused delays in transmissions, and multiple “dead spots.” These issues increased frequency congestion on an already overloaded sector because many transmissions had to be repeated multiple times. The need for the R30 controller on duty at the time of the accident to repeat missed transmissions increased the workload on the controller and increased frequency congestion.

The airplane was a Boeing 737-700, manufacturing Serial Number 32747. The airplane was manufactured in 2002, delivered to Southwest Airlines in 2014, and held a transport category airworthiness certificate. The airplane was configured with 2 flight crew seats, 3 flight attendant seats, and 143 passenger seats. At the time of the event, there were no deferrals from the minimum equipment list. Figure 1. Accident Airplane, N567WN (Source: planespotters.net) On April 3, 2024, about 0820 eastern daylight time (EDT), Southwest Airlines (SWA) flight 4273, a Boeing 737-700 airplane, experienced turbulence while enroute between Louis Armstrong New Orleans International Airport (MSY), New Orleans, Louisiana, and Orlando International Airport (MCO), Orlando, Florida. Of the 5 crew and 140 passengers onboard, one FA and one passenger sustained serious injuries, and one FA sustained minor injuries. The aircraft was not damaged. The flight was operated under the provisions of Title 14 Code of Federal Regulations Part 121 as a scheduled passenger flight. According to the captain, the incident flight was the first flight of the day and was scheduled to depart at 0740 central daylight time (CDT). Upon arrival at the airplane, the captain coordinated with the gate agent, then assisted the first officer (FO) with the preflight checklists while the FAs prepared the cabin. The FO then went to the ramp to complete the walkaround while the captain briefed the FAs, reviewing the flight route, weather, flight time, and information about special passenger needs. The crew collectively decided that the FAs would evaluate the flight conditions at 10,000 ft and start service when it was smooth. The captain informed the FAs that if they experienced turbulence, they should take their jumpseats. The captain and FO, who was the pilot flying, conducted a preflight briefing and reviewed the available information, including the planned destination alternate and enroute weather. The flight departed MSY about 0755 CDT and encountered light chop through the climb. The crew reported that the fasten seat belt sign remained on for the entire flight. The crew stated they leveled at flight level (FL)370, above the cloud tops, and continued to monitor the enroute weather on the airplane radar and the captain’s electronic flight bag weather apps. About 40 minutes later, while approaching an en route waypoint, the airplane encountered light turbulence and the flight crew coordinated with ATC to deviate around the weather. ATC initially suggested a right deviation, to the south, but the information available to the flight crew indicated that it was not the best route. The flight was handed off to another controller, who gave the flight crew the option to descend to FL350 and deviate to the right, or deviate left at their current altitude. The flight crew elected to remain at their altitude, above the cloud tops, and deviated to the left. After deviating left of course, the crew began to turn right, back toward the waypoint, and observed a rapidly developing cloud. The FO suggested they continue to turn to the right to avoid the buildup and the captain agreed. The captain used the public address system to request the FAs to take their seats. The captain informed ATC of the deviation and the FO reduced the airspeed in anticipation of turbulence. The flight crew stated that they penetrated the buildup and encountered about 10 seconds of severe turbulence. During the encounter, the airplane experienced about 30° of bank, airspeed fluctuations of 15-20 kts, and altitude deviations of +200 ft to -100 ft. The flight crew reported hearing the aural airspeed limitation alert. According to the A position FA, while completing cabin service, he heard the captain’s announcement and was able to make it to the forward jumpseat at the onset of the turbulence. He stated that the turbulence was initially moderate and he jostled around, but once he was able to secure the seat belt, the turbulence increased to severe. From his jumpseat he observed the C position FA sit in the aisle during the turbulence encounter. After the turbulence encounter, the A FA walked through the cabin to inspect for damage and check for injuries. The C FA, in the aisle, sustained minor injuries, but the B position FA and a passenger were lying on the floor in the vicinity of the aft galley. The B FA had bumped her head, potentially fractured her right arm, and had pain in her right hip. The passenger had pain in her neck and back. The A FA contacted the flight crew and reported the damage and injuries. The flight crew elected to declare a medical emergency and initiated a diversion to Tampa International Airport (TPA), Tampa, Florida. The FO continued pilot flying duties, and the captain coordinated the diversion with company dispatch. While preparing for landing, the C FA took the forward jumpseat and the A FA sat on the floor in the aft galley area with the injured B FA and passenger. The approach and landing at TPA were without incident. The airplane parked at gate 32 and paramedics met the flight. The captain made an announcement for passengers to remain seated, and the paramedics boarded the airplane. Once all passengers were deplaned, paramedics used a provisioning truck to lower the two injured people to the ramp level, from which they were transferred to ambulances and transported to the hospital. Air Traffic Control ZJX ARTCC was a level 11 enroute radar ATC facility located in the Jacksonville district of the eastern service area. The R30 sector was the involved sector. ZJX ARTCC personnel stated that there was an issue with the frequency in the sector, regularly resulting in missed transmissions. This was evident in the audio recording surrounding the time of the accident. The need to repeat missed transmissions increased the workload and frequency congestion of the R30 controller. Documentation provided by the FAA indicated that there had been a fire at the RCAG, which was located at Tyndall Air Force Base, on October 4, 2023. This fire caused the frequencies to fail. On October 15, 2023, a temporary mobile remote RCAG facility was placed into operation. As part of the temporary RCAG placement, FAA Telecommunications Infrastructure provided a satellite relay connection to assist with providing operational frequencies for the RCAG. In this instance, satellite support refers to a temporary supplement to ground-based services using satellite-based services (typically used in cases of natural disasters or other emergency situations such as hurricanes, etc.). This relay resulted in a delay of transmission by 600 milliseconds round trip, but it was noted that there would be no degradation of service. However, a systemic issue review report dated January 10, 2024, stated that the temporary frequencies were low-power 10-watt transmitters with limited coverage, delays in transmissions, and multiple “dead spots.” In April of 2023, a working group was formed to evaluate the need for and feasibility of splitting the R30 sector into two separate sectors. This was in part due to increased traffic caused by similar split positions at adjacent ARTCCs. The working group recommended a proposed split of the R30 sector, but at the time of the event the proposal had not been implemented. The history of the flight was constructed using certified audio recordings, automatic dependent surveillance-broadcast (ADS-B) data, and the aircraft accident package provided by the FAA. Altitudes below 18,000 ft are indicated above mean sea level, and altitudes of 18,000 ft or greater are in FL. ATC services provided by MSY airport traffic control tower and terminal radar approach control (TRACON), and Houston (ZHU) ARTCC prior to the airplane entering ZJX ARTCC airspace were routine and unremarkable. The following timeline summarizes pilot and controller communications, and other pertinent information while the airplane was in comm