DCA16LA032

the flight crew's early turn towards the assigned gate because taxiway lighting had been inadvertently turned off by the controller-in-charge which resulted in the airplane leaving the paved surface. Contributing to the accident was the operation of the screen-saver function on the lighting control panel that prevented the tower controllers from having an immediate visual reference to the status of the airfield lighting.

On December 15, 2015, at 5:23pm central standard time (CST), Southwest Airlines flight 31, a Boeing 737-300, N649SW, exited the taxiway while taxing to the gate and came to rest in a ditch at the Nashville International Airport (BNA), Nashville, Tennessee.  Nine of the 138 passengers and crew onboard received minor injuries during the evacuation and the airplane was substantially damaged.  The airplane was operating under the provisions of 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight from William P. Hobby Airport (HOU), Houston, Texas. Weather was not a factor, light conditions were dark just after sunset.  The airplane landed normally on runway 20R and exited at taxiway B2.  The flight crew received and understood the taxi instructions to their assigned gate.  As the crew proceeded along taxiway T3, the flight crew had difficulty locating taxiway T4 as the area was dark, and there was glare from the terminal lights ahead.  The crew maneuvered the airplane along T3 and onto T4, and then turned back to the right on a general heading consistent with heading across the ramp toward the assigned gate.  The flight crew could not see T4 or the grassy area because the taxiway lights were off and the glare from the terminal lights.  As a result, the airplane left the pavement and came to rest in a drainage ditch resulting in substantial damage to airplane.  The cabin crew initially attempted to keep the passengers seated, but after being unable to contact the flight crew due to the loud alarm on the flight deck, the cabin crew properly initiated and conducted an evacuation. As a result of past complaints regarding the brightness of the green taxiway centerline lights on taxiways H, J, L and T-6, BNA tower controllers routinely turned off the taxiway centerline lighting.  Although the facility had not received any requests on the day of the accident, about 30 minutes prior to the event the tower controller in charge (CIC) turned off the centerline lights as a matter of routine.  In doing so, the CIC inadvertently turned off the "TWY J & Apron 2" selector, which included the taxiway lights in the vicinity of the excursion.  The airfield lighting panel screensaver feature prevented the tower controllers from having an immediate visual reference to the status of the airfield lighting.

HISTORY OF FLIGHT On December 15, 2015, at 5:23pm central standard time (CST), Southwest Airlines flight 31, a Boeing 737-300, N649SW, exited the taxiway while taxing to the gate and came to rest in a ditch at the Nashville International Airport (BNA), Nashville, Tennessee. Nine of the 138 passengers and crew onboard received minor injuries during the evacuation and the airplane was substantially damaged. The airplane was operating under the provisions of 14 Code of Federal Regulations Part 121 as a regularly scheduled passenger flight from William P. Hobby Airport (HOU), Houston, Texas. The airplane landed normally on runway 20R and exited at taxiway B2. ATC instructed the flight to taxi to the ramp (gate C20) via taxiway B, T3, and T4. (Figure 1) Figure 1 - Taxi routeThe captain reported he used landing lights to assist in the taxi along B and T3 and then extinguished the landing lights as the airplane crossed runway 13/31, but left the taxi lights on. Video surveillance footage was consistent with the captain's report. The flight crew reported that they had difficulty locating taxiway T4 as it appeared dark and there was glare from the terminal lights ahead. About 15 seconds prior to the excursion the airplane began a left turn briefly to a heading of about 065 degrees. Taxiway T4 was oriented about 045 degrees. Crew reports, and the cockpit voice recorder (CVR) recording, indicate the crew was searching for the turn to the ramp when flight data recorder (FDR) data and video surveillance footage indicated the airplane then turned back to the right to a heading of 090 degrees, consistent with the heading toward gate C20. Three seconds later the airplane exited the taxiway into a grassy area with a large drainage ditch, east of the intersection of taxiways T4 and J. The nosegear collapsed, and the airplane came to rest angled to the right, on the left nacelle, right wing tip, and nose. (Figure 2) Figure 2 - Aircraft resting position Shortly after the airplane came to rest, the CVR recording indicated, an audible alarm began sounding in the cockpit which the flight crew indicated they could not silence (see Tests and Research section). The pilot advised ATC that "we've cut the corner here and are off in the grass" and asked for ARFF equipment. According to the flight attendant (FA) statements, there was a large jolt and loud bang when the airplane stopped and all the cabin lights went out for some time before the emergency lights came on. The FAs stated that they did not know what was going on so began yelling, "heads down, stay down" as they tried to call the pilots but there was no power on the interphones. The FAs initiated the evacuation a short time later. About one minute after the airplane came to rest, the pilots noticed that the slides were deploying and passengers were evacuating and the captain announced on the public address system "okay don't evacuate flight attendants, do not evacuate" to which the first officer responded "oh they are already going." Review of Air Traffic records and interviews with controllers revealed that about 30 minutes prior to the accident the taxiway lights for "TWY J & Apron 2" were selected "off." This resulted in shutting off the lights for taxiways L and J; taxiways T4 and T5 to the northeast of L; the lights along the edge of the ramp parallel to J; and the connector and circular area between Concourses B and C. INJURIES TO PERSONS Nine of the 133 passengers received minor injuries during the evacuation. None of the five crew members were injured. DAMAGE TO AIRCRAFT The nose landing gear collapsed in a rearward direction resulting in substantial damage to frames, stringers, and the bulkhead aft of the nose gear well. Additional minor damage to engine nacelles, fairings, and skin was also found. PERSONNEL INFORMATION The captain, age 58, had worked for Southwest Airlines since 1999. He held an Airline Transport Pilot certificate, multi-engine land, with a type rating in the B737. He held an FAA first class medical certificate with a limitation for glasses for near vision. Company records indicate that he had approximately 19,300 hours total time with approximately 14,100 hours in the B737. He had no previous accidents, incidents, or violations. He had flown to Nashville numerous times previously. The first officer, age 61, had worked for Southwest Airlines since 2006. He held an Airline Transport Pilot certificate, multi-engine land, with type ratings in the B-727, B-737, B-757, B-767, CL-30, and DC-9. He held an FAA first class medical certificate with a limitation for glasses for near vision. Company records indicated a total time of approximately 15,500 hours, with approximately 5400 hours in the B737. He had no previous accidents, incidents, or violations. He had flown to Nashville numerous times previously. The three flight attendants were all current and qualified on the B737. AIRCRAFT INFORMATION N649SW, manufacturer serial number 27719, was a Boeing 737-3H4 equipped with CFM-56-3B1 engines. The airplane had accumulated approximately 58,630 hours total time on the airframe. Recorded data and airline records indicated no relevant maintenance issues with the airplane. METEOROLOGICAL INFORMATION The Nashville Airport 5:05pm weather observation indicated clear conditions with 10 miles visibility, wind from 180 degrees at 3 knots, temperature 17º C. There was no precipitation. Night lighting conditions prevailed, local sunset was at 4:34pm. COMMUNICATIONS After the airplane came to rest, the pilot advised the tower that he that he had "cut the corner" and requested assistance. There were no further communications between the flight crew and the tower. The flight crew reported that a loud audible alarm in the cockpit, which they could not silence, impeded communication. AERODROME INFORMATION The Nashville International Airport (BNA) is located approximately 5 miles southeast of the city of Nashville, Tennessee. The airport averages almost 500 operations per day, mostly air carrier and air taxi activity. Runway 20R is 7,704 feet long and 150 feet wide, aligned to 201 degrees magnetic. The airfield lighting at BNA consisted of taxiway edge and taxiway centerline lights, runway edge and runway centerline lights, runway end identifier lights, approach lights, and a rotating beacon. The airfield lighting was controlled via two airfield lighting touchscreen control panels located near the local controller (LC) and ground controller (GC) positions in the control tower. The BNA air traffic control tower (ATCT) standard operating procedures (SOP), BNA Order 7111.1B, dated July 24, 2014, did not specify who had overall responsibility for the operation of the airfield lighting. The airfield lighting panels (Figure 3) were owned by the Metropolitan Nashville Airport Authority (MNAA) and were operated using software installed in 2010. The airfield lighting was operated in accordance with requirements contained in FAAO 7110.65, Air Traffic Control, and a letter of agreement between the control tower and MNAA dated December 16, 2013. Figure 3 - Lighting panel The lighting panel had preset intensity selections used according to current weather/day/night conditions and was operated by the tower controllers via a touch screen that selected lighting intensities as required by the current conditions. Specific airfield lighting configurations could be selected to operate independently of the pre-set conditions, such as turning off a specific taxiway lighting circuit. The touch screen control circuit that activated or deactivated the taxiway centerline lights was located next to the touch screen control circuit that activated or deactivated the lighting for the ramp/apron and a portion