History of Flight
On May 12, 2009, at 1945 central daylight time, a Boeing 737-3H4 airplane, registration N371SW, operated by Southwest Airlines as Flight 519, encountered a fire in the area of the right main landing gear during landing on runway 22 at William P. Hobby Airport (HOU), Houston, Texas. Three of the four main landing gear tires blew out. The airplane sustained minor damage. The flight crew consisted of 2 pilots and 3 flight attendants; 48 passengers, including one lap child, were on board. Two passengers sustained minor injuries during the emergency evacuation. The flight was a domestic scheduled passenger operation under 14 CFR Part 121, originating from Louis Armstrong New Orleans International Airport (MSY) at 1830. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed.
The airplane was dispatched with several inoperative items permitted by the minimum equipment list (MEL), including an inoperative automatic brake system. Landing on runway 22 was required due to MEL operational requirements. The captain reported a stabilized flaps 40 final approach and a normal landing in the target area. After touchdown, the thrust levers were placed at idle, the speed brake was raised to the full-up detent, reverse thrust was used, and manual wheel braking was applied. Both flight crewmembers believed the tires blew soon after touchdown.
Television news helicopter video showed the airplane touching down in the touchdown zone of runway 22 and stopping approximately 3,300 feet from the touchdown point. Smoke and fire in the right main landing gear area appeared on the video soon after touchdown while the airplane was still moving. After the airplane stopped, the crew received confirmation of fire and smoke from the control tower and was advised that aircraft rescue firefighting (ARFF) units were en route. Other aircraft also confirmed the presence of fire and smoke. The captain, after consulting with the first officer and flight attendants, ordered an emergency evacuation from the left side doors.
Flight attendants opened only the forward and aft entry doors on the left side, deploying the associated emergency slides. ARFF units arrived and began extinguishing the fire as passengers evacuated down the slides. All 48 passengers exited, followed by two flight attendants who then gathered passengers and screened for injuries. The remaining crewmembers verified the cabin was empty and then evacuated. All occupants exited the airplane 57 seconds after the evacuation began. The fire was knocked down by ARFF approximately 32 seconds after evacuation start and declared out four minutes later. After initial triage by emergency medical services, passengers and crew were bused to the terminal.
Flight Recorders
The cockpit voice recorder (CVR) and flight data recorder (FDR) were removed and sent to the National Transportation Safety Board's Vehicle Recorder Division for readout. FDR data recorded during the event showed that at 19:45:31, left brake pressure was 770 pounds per square inch (PSI). The maximum right brake pressure was 1430 PSI at 19:45:32. Maximum left brake pressure was 990 PSI at 19:45:57 and 19:46:03.
Aircraft Information
N371SW, a Boeing 737-3H4, serial number 26598, was manufactured in 1993. It was powered by two CFM56 series turbofan engines, each developing 20,000 pounds of thrust. At the time of the incident, the airplane had accumulated 52,892 hours. The most recent inspection was on January 8, 2009. The cockpit had four crewmember seats, there were four cabin crew seats, and 137 passenger seats, for a total capacity of 145 persons.
Meteorological Information
At 1950, a SPECI weather report at Houston reported wind from 150 degrees at 11 knots, gusting to 16 knots; 10 miles visibility; few clouds at 2,700 feet; scattered clouds at 25,000 feet; temperature 27°C; dew point 22°C; and an altimeter setting of 29.90 inches of mercury.
Airport Information
Runway 22/04 at HOU was 7,602 feet long and 150 feet wide, with a grooved concrete surface. Runway 12R/30L was also 7,602 feet long and 150 feet wide; runway 12R had a displaced threshold with a landing distance available of 6,568 feet.
Wreckage and Impact Information
Investigators from the Safety Board, FAA, and Southwest Airlines examined runway marks and airplane damage on May 13, 2009. Touchdown marks for the right main landing gear, approximately 20 feet long, were found 15 feet right of the runway centerline, 1,300 feet from the approach end of runway 22. Solid skid marks for the right main landing gear started at 1,550 feet from the approach end and continued for 300 feet, where marks consistent with blown tires were observed for both inboard and outboard main tires. Uninterrupted skid marks continued to the final stop at 4,650 feet from the approach end, 20 feet right of centerline.
Examination of the airplane revealed the left main landing gear outboard tire was ruptured with separated sections; the left inboard tire was inflated and undamaged. The right inboard tire was ruptured with minor damage to the wheel rim. The right outboard tire was ruptured, and about 3 inches of the wheel rim and brake assembly were ground away. Both right main landing gear tires, wheels, and brake assemblies had minor thermal damage. The wing flap section behind the right main landing gear sustained minor damage from tire debris. MEL labels for auto speed brakes (27-7), auto brake system (32-2), and on-board performance computer (46-2) were observed in the cockpit. No pre-event anomalies were found that would have prevented normal operations.
Tests and Research
All four main landing gear tires, wheels, and brake assemblies were examined at the Honeywell facility in South Bend, Indiana. Three of the four tires were blown. Both right inboard and outboard wheels sustained damage, and the right outboard brake assembly was damaged. All four brake assemblies were tested; pressure required for rotors tight and rotors loose was nominal. Brakes pressurized to 3,000 PSI showed no leakage. Brake wear pin measurements were nominal. No pre-event anomalies in the tires, wheels, or brakes were discovered.
Additional Information
The anti-skid system is designed to provide touchdown protection during the air-to-ground transition, preventing brake application prior to touchdown and wheel spin-up. With touchdown protection operative, brake pressure can reach the brakes only 3 seconds after the air-ground switch logic indicates ground, or once each wheel has spun up to 60 knots. Touchdown protection is not available when the antiskid system is off or inoperative. The manufacturer's and operator's manuals contain detailed information on anti-skid system operation and limitations. The operator's Quick Reference Handbook states, "Use minimum braking consistent with runway conditions to reduce the possibility of a tire blowout."