N567WN

On April 3, 2024, about 08:20 eastern daylight time, Southwest Airlines (SWA) flight 4273, a Boeing 737-700, 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 flight attendant and one passenger sustained serious injuries, and one flight attendant sustained minor injuries. The aircraft was not damaged. The flight was operating under the provisions of Title 14 Code of Federal Regulations Part 121 as a scheduled passenger flight.

The day of the accident, a major cold front was traversing the southeastern United States. The squall line of rapidly developing thunderstorms associated with the front extended south several hundred miles into the Gulf of Mexico. A convective Significant Meteorological Information (SIGMET) was issued at 1155 UTC for the area, warning pilots of cloud tops above 45,000 ft. A post-accident review of the weather showed a rapidly developing cell in the area of the accident consistent with the accident pilots’ recollection described in the next paragraph.

Figure 1. Weather radar image with aircraft track and location overlayed at the time of turbulence encounter.

The captain stated that when he met with the flight attendants before departure and briefed them on the weather conditions, they agreed that, at the 10,000 ft chime, the flight attendants would decide amongst themselves if the ride was smooth enough to start their inflight service. The captain informed them that if the ride became too uncomfortable that they should take their jump seats and let the cockpit know that they were seated.

As the aircraft climbed to altitude, the aircraft encountered occasional chop. They leveled the aircraft at flight level (FL) 370, above the weather system with the tops of the clouds at FL340 – FL350. The fasten seatbelt sign remained illuminated the entire flight. About 40 minutes into the flight and approaching the REMIS waypoint, the flight began to encounter light turbulence and the crew began deviations around cells before waypoint ROZZI. The visibility was intermittent instrument meteorological conditions (IMC) and marginal visual meteorological conditions (VMC) with occasional light chop. As they returned to their route to REMIS, the visibility improved and they visually acquired a rapidly developing cloud top that was on their flight path, but not painting on the onboard radar. The captain made a passenger announcement to ensure those in the cabin were seated and then contacted air traffic control (ATC) to request the deviation. The first officer (FO) began a right turn to avoid the cell, however, was not able to complete the turn in time and entered the cloud buildup. They encountered severe turbulence that lasted about 10 seconds that resulted in fluctuations up to 30°of bank, 20 knots airspeed, and 200 ft of altitude. Moderate turbulence followed for about 1 minute. According to flight data, vertical acceleration ranged from -0.45 gravitational force equivalent (g) to +1.8g during the turbulence encounter.

Following the event, the flight crew coordinated with dispatch and decided to divert to Tampa International Airport (TPA), Tampa, Florida. They informed ATC of their intentions and were routed to TPA where they performed an uneventful landing, and medical personnel met the aircraft at the gate.

The following NTSB specialists were assigned to investigate the accident: air traffic control, meteorology, and flight data recorder. The Federal Aviation Administration (FAA), Southwest Airlines, and the National Air Traffic Controllers Association (NATCA) are parties to the investigation.

The flight data recorder data for the flight was downloaded by the operator and sent to NTSB headquarters for analysis. This analysis is ongoing.

At the time of the encounter with turbulence, the aircraft was in Jacksonville (JAX) Air Route Traffic Control Center (ARTCC) airspace. While crew statements indicated they were aware of and monitoring rapidly developing cells, ATC audio indicated that the air traffic controller overseeing the flight did not provide specific information about the developing weather cell the aircraft penetrated to encounter the accident turbulence.

NTSB investigators travelled to JAX ARTCC and conducted interviews of ATC personnel controlling the flight during the turbulence encounter. The investigation found that while the target number of aircraft for the sector was set at 20 (down from 23 due to increased complexity), the controller had significantly more aircraft on the frequency in the timeframe surrounding the accident.

The controller’s workload was further increased due to technical limitations with the temporary very high frequency (VHF) communications antenna the ARTCC was relying on to communicate with traffic after the primary VHF antenna was rendered inoperative in a fire in October 2023. FAA telecommunications infrastructure (FTI) had provided a satellite relay connection to assist with provision of operational frequencies, typically used in case of emergency situations. This caused a delay of transmission by 600 milliseconds round trip, resulting in clipped communication with a number of aircraft on the frequency. The controller described that these limitations required him to issue multiple repeated calls to multiple aircraft on the frequency increasing his workload and frequency congestion.

The investigation is continuing.