N567WN – The Boeing Company 737-7CT – 2024-04-03 NTSB Accident Report

HISTORY OF FLIGHTOn 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. AIRCRAFT INFORMATIONThe 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) METEOROLOGICAL INFORMATIONA review of the NWS large-scale migratory weather systems at the time of the encounter with turbulence indicated that a strong cold front supported the development of a squall line that extended from the Florida Panhandle into the Gulf of America. The upper air charts depicted an upper-level low pressure system over Lake Michigan with a long wave trough extending southwestward into Texas. Winds over the area were from the west at nearly 100 kts over the turbulence encounter location, at the airplane’s cruising level of FL370. A strong 150- to 170-kt jet stream extended immediately north over eastern Texas and Louisiana and into Alabama and Georgia, with a likelihood of occasional moderate turbulence.

The Meteorological Aerodrome Reports for Apalachicola Regional Airport (KAAF), Apalachicola, Florida, about 110 miles north-northeast of the event location, reported a period of thunderstorms and heavy rain with outflow winds just below the severe storm limit of 50 kts prior to the accident, with rainfall of 1.33 inches. This coincided with the passage of the squall line over the area.

The High-Resolution Raid Refresh sounding indicated that the atmosphere was moist with a precipitable water content of 2.16 inches, a lifted index of -3.1 with a positive Convective Available Potential Energy of 1,079 Joules per kilogram, and maximum upward vertical motions up to 96 kts or 9,055 ft per minute. Estimated cumulonimbus cloud tops to FL410 were expected based on the sounding, while real-time satellite cloud tops were observed from FL380 to FL460.

The Eglin Air Force Base (KEVX) Weather Surveillance Radar-1988 Doppler (WSR-88D) and NWS composite radar imagery depicted echoes of 56 decibels of reflectivity (dBZ) under the airplane’s flight track, with the higher 1.8° base reflectivity imagery depicting echoes up of 22.5 dBZ at the accident airplane’s cruising altitude. Airborne weather radar typically displays echoes greater than 20 to 30 dBZ as light intensity echoes. Therefore, echoes were likely just beginning to appear on airborne radar near the time of the encounter. The KEVX WSR-88D long-range enhanced echo tops product depicted tops in the range of FL400 during the period.

Figure 2. Weather radar image with airplane track and location overlayed at the time of the turbulence encounter.

The FAA’s Corridor Integrated Weather System and Storm Prediction for Aviation and Offshore Precipitation Capability imagery were also examined and depicted extreme intensity echoes moving across the area with significant cloud-to-ground lightning. While there was no significant difference noted between the weather displays in the vicinity of the turbulence location within the ZJX airspace, several of the routes further south of the accident flight would have only been viewable using the Offshore Precipitation Capability data due to range limitations.

The ATC facilities weather services unit also provided a morning briefing to all oncoming controllers advising them of the broken line of thunderstorms that was expected to cross their airspace during the period.

A review of the pilot reports issued surrounding the period of the turbulence encounter indicated no specific reports along the route of flight of any significant turbulence prior to the accident flight.

The NWS convective forecast products provided some potential warning of thunderstorms across the Gulf of America in the vicinity of the turbulence event, which implied the potential for severe turbulence. While the NWS Storm Prediction Center convective outlook discussed the potential for severe thunderstorms over the United States, it provided no information or details of the convection offshore. The Traffic Flow Management Convective Forecast indicated a medium coverage of storms with tops above FL400 from southeast Georgia and the Florida panhandle to several hundred miles into the Gulf of America for the 2-, 4-, and 6-hour periods.

The NWS Aviation Weather Center’s graphic area forecast indicated scattered coverage of thunderstorms during the period. The NWS Graphic Turbulence Guidance (GTG) product predicted light to moderate turbulence associated with the front’s position; however, it did not include the convective potential. The current National Center for Atmospheric Research (NCAR) Graphic Turbulence Guidance-Nowcast (GTG-N) 1- and 2-hour testbed products also failed to depict the potential convective turbulence. The NCAR GTG-N v1 with lightning data and a finer domain, however, demonstrated better storm resolution and did highlight the potential for turbulence over the area.

The NWS Aviation Weather Center had Convective Significant Meteorological Information (SIGMET) 44E and SIGMET Bravo 1 current over an extensive area over the southeastern states into the Gulf of America for an area of severe embedded thunderstorms, which also implied the potential for severe and greater turbulence associated with the area.

The airplane measured a peak eddy dissipation rate value of 0.62, which indicated that severe turbulence was encountered while overflying the echo tops. The convective echoes, high echo tops, and very unstable airmass defined this event as a convectively induced turbulence event.

A review of the dispatcher’s statements indicated the flight had been planned for FL390 and on the company route south of the main area of expected convection. The flight crew statements indicated that they had briefed the cabin crew regarding the potential for turbulence along the route prior to departure. The flight crew also confirmed that the flight was filed with a requested altitude of FL390, but due to other traffic it was not available. AIRPORT INFORMATIONThe 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) INJURIES TO PERSONSOf the 140 passengers onboard, there was one serious injury. Of the 3 FAs on board, there was 1 minor injury and 1 serious injury. The passenger sustained broken bones in her neck, back, and ribs. The FA sustained broken bones in her arm. DAMAGE TO AIRCRAFTThere was no damage to the airplane as a result of the event. COMMUNICATIONSAir 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 communication with ZJX ARTCC airspace (times in EDT):

0809:32 A position relief briefing took place on the west area radar associate controller (D30) position, which included a statement by the controller being relieved that the “comms” [radio communications] were terrible.

0812:48 The R30 controller accepted a handoff from ZHU ARTCC.

0813:39 The R30 controller cleared the airplane direct to the next waypoint with no response from the crew.

0813:49 The R30 controller re-cleared the airplane direct to the waypoint, again with no response from the crew.

0813:54 The crew of the airplane advised that they were at FL370 and on a heading of 090º for [weather] deviations. The R30 controller asked the crew to verify that they had been approved for left deviations by the ZHU ARTCC controller. The crew advised that ZHU ARTCC had given them the option to deviate left or to descend and deviate right and that if they had been able to climb to FL390, they would not have needed to deviate. The R30 controller then re-cleared the airplane again direct to the waypoint when able.

0821:58 The crew advised the R30 controller that they were navigating direct to the waypoint and asked if there was “any chance for higher.” The controller responded with “not right now.”

0823:34 [According to the flight data recorder (FDR), this is the time the accident occurred.]

0824:05 The crew requested a heading of 130º, which the R30 controller approved.

0825:55 The crew advised that they were navigating direct to the waypoint. The R30 controller acknowledged.

0828:25 The ZMA ARTCC radar sector eight (R8) controller accepted an automated handoff from the ZJX ARTCC R30 controller.

0829:34 The R30 controller asked the crew to state their heading with no response from the crew.

0829:39 The R30 controller again asked the crew to state their heading with no response from the crew. [According to post-accident interviews, it was about this time that the R30 controller issued a frequency change to the crew via DataComm.]

There were no further recorded transmissions between ZJX ARTCC and the crew.

The following timeline summarizes pilot and controller communications and other pertinent information while the crew was in communication with Miami (ZMA) ARTCC:

0830:18 The crew checked in with ZMA ARTCC and the R8 controller cleared them direct to the waypoint.

0832:16 The R8 controller instructed the flight crew to change to a new transponder beacon code.

0833:36 The crew requested to divert to TPA and advised they had sustained a couple of injuries due to a turbulence encounter. The R8 controller asked the crew to verify that they were requesting to divert to TPA due to injuries, and the crew responded in the affirmative. The controller cleared them to TPA via a direct waypoint for that waypoint’s area navigation arrival. The crew asked the controller to repeat the clearance, which the R8 controller did. The crew then read back the instructions as issued. The controller asked the crew to provide the location and type of turbulence they had encountered, and the crew reported they had encountered severe turbulence approximately 50 nautical miles northwest of the prior waypoint.

0834:40 The ZMA ARTCC sector eight radar associate (D8) controller called the ZJX ARTCC D30 controller and asked them if the crew had reported any turbulence to them. The D30 controller responded in the negative. The D8 controller advised the D30 controller that the airplane had encountered severe turbulence 50 miles west of the waypoint and that they had reported injuries.

0834:41 The R8 controller instructed the flight crew to descend to 10,000 ft and issued them the TPA altimeter setting.

0837:08 The R8 controller asked the flight crew to provide details of the injuries when they were able. The crew advised that they had one crew member and one passenger who were injured. The R8 controller asked if they were declaring a medical emergency or if they just needed to divert. The crew verified that they were declaring a medical emergency.

0839:06 The R8 controller cleared the crew direct to the next waypoint, then direct to TPA. The crew read back the clearance as issued.

0841:19 The R8 controller asked the crew if they had been assigned a gate number at the airport yet and the crew advised that they had not. The R8 controller asked the crew for more information on the injuries and the crew reported there were two possible fractures and someone with a back issue.

0843:30 The R8 controller then instructed the crew to contact TPA approach control.

About 0856, the airplane landed at TPA.

FAA order 7110.65AA, Air Traffic Control, paragraph 2-6-4, provided procedures and guidance for controllers in issuing weather. The paragraph stated, in part:

a. Controllers must issue pertinent information on observed/reported weather and chaff areas to potentially affected aircraft. Define the area of coverage in terms of:

1. Azimuth (by referring to the 12-hour clock) and distance from the aircraft and/or

2. The general width of the area and the area of coverage in terms of fixes or distance and direction from fixes.

The paragraph also stated:

f. EN ROUTE. When issuing Air Route Surveillance Radar (ARSR) precipitation intensity use the following:

1. Describe the lowest displayable precipitation intensity as MODERATE.

2. Describe the highest displayable precipitation intensity as HEAVY to EXTREME.

PHRASEOLOGY-

AREA OF (Intensity) PRECIPITATION BETWEEN (number) O’CLOCK and (number) O’CLOCK, (number) MILES, MOVING (direction) AT (number) KNOTS, TOPS (altitude). If applicable, AREA IS (number) MILES IN DIAMETER.

If a pilot enters your area of jurisdiction already deviating for weather, advise the pilot of any additional weather which may affect the route.

The paragraph also includes a note, as follows:

NOTE-

3. When aircraft are deviating around weather and transitioning from sector to sector, unless previously coordinated, the receiving controller should not assume that the transferring controller has issued weather affecting the aircraft’s route of flight.

Since the NTSB’s on-site investigation, the facility has resolved the frequency issue by repairing and recommissioning the permanent RCAG site. They have also implemented an ultra-high NEPTA sector, alleviating some of the workload on the R30 controller.

Company Communication

The Southwest Aircraft Communications Addressing and Reporting System messages regarding the flight were as follows:

0757 Weather update – WSI SIGMET for turbulence over TX and WSI SIGMET for convection along route of flight. BKN LINE OF TSTMS AHEAD OF COLD FRONT. TOPS FL430- FL550. MOV NE 25KT.

0803Z ETA 0901 to MCO.

0804 Weather Request – MCO ATIS

0823 Updated ETA 0908 at MCO

0823 Weather Alert – WSI SIGMET-CONV. Area 85E MCN. BKN LINE OF TSTMS AHEAD OF COLD FRONT. TOPS FL430-FL550. MOV 067° 25KT.

0832 Aircraft to Dispatch – Need 2 paramedics, FA serious injury, and 1 PAX injured. SVR turbulence.

0833 Aircraft to Dispatch – copy at FL370 where

0836 Aircraft to Dispatch – going to TPA

0837 Dispatch to Aircraft – acknowledges diversion to TPA with landing data FLIGHT RECORDERSThe FDR data were delivered electronically to the NTSB’s Vehicle Recorder Division. The file contained the event flight, with a duration of about 1 hour. Timing of the FDR data was measured in subframe reference number (SRN), where each SRN equals one elapsed second.

The FDR data indicated that, about 30 minutes after takeoff and at a pressure altitude of approximately 37,000 ft, the airplane encountered an upset from about 0823:24 to about 0825:03. During this time, the minimum vertical acceleration deviation from 1 g (measurement of acceleration force relative to gravity) was -0.45 g at 0823:34, and the maximum vertical acceleration deviation was 1.80 g. About 32 minutes later, the airplane landed.