The accident occurred about 7:14 pm central standard time. The airplane departed the end of the runway, rolled through a blast fence, a perimeter fence, and onto a roadway. The airplane came to a stop after impacting two automobiles. One automobile occupant was fatally injured and another seriously injured. The flight was conducted under 14 CFR Part 121 and had departed from the Baltimore/Washington International Thurgood Marshall Airport, Maryland.

The on-scene portion of the investigation has been completed. Additional fact-finding, including tests and research, will be conducted at various component manufacturers. The Safety Board staff continues to examine the information provided by the flight data recorder and the cockpit voice recorder.

The two pilots in the cockpit were interviewed on Saturday. Each interview took approximately three hours.

The pilots stated that everything was normal through the point of touchdown. Approaching the airport, weather was of concern to them, and they listened to the ATIS (the recorded weather update) four times during the latter portion of the flight. They stated that they agreed with the dispatcher's assessment of the conditions for landing on runway 31C and backed up that assessment by inputting the numbers into the on-board laptop computer tool.

The computer confirmed that the landing would be within the operational parameters of the airplane and Southwest's procedures, they said. Autobrakes were set on MAX, and they activated after a "firm" touchdown. The flying pilot (Captain) stated that he could not get the reverse thrust levers out of the stowed position. The first officer, after several seconds, noticed that the thrust reversers were not deployed and activated the reversers without a problem. At some point, the Captain noticed that the airplane was not decelerating normally and applied maximum braking manually. The first officer also became aware of the poor braking effectiveness, moved his seat farther forward, and also applied maximum braking. They stated that they continued to apply maximum pressure to the brakes as the airplane went straight off the end of the runway and came to a stop.

Interviews were conducted with a number of other Southwest Airlines flight crews, including the crew of the last Southwest flight to land at Midway and a subsequent crew that diverted to St. Louis.

Airplane Performance

Preliminary calculations show that the airplane touched down with about 4,500 feet of remaining runway and was on the runway for about 29 seconds. Preliminary calculations also show that, for the runway conditions and use of brakes and thrust reverser that occurred, the stopping distance without hitting obstructions would have been about 5,300 feet (the actual stopping distance was about 5,000 feet). In addition, had the airplane landing into the wind, rather than with a tail wind, the stopping distance for a landing would have been about 1,000 feet less.

FDR data show that autobrakes were active and provided high brake pressure upon touchdown. Autobrakes and manual braking continued to provide high brake pressure throughout the landing roll.

FDR data show that thrust reversers were activated about 18 seconds after touchdown or about 14 seconds before contact with the blast fence. Testing and examination of the thrust reverser systems will continue.

Investigators have obtained the laptop computer tool used by the accident flight crew. It will be examined and calculations of landing performance will be compared to flight manual data.

Eleven security-type video cameras were identified on the airport that may show imagery of the airplane rollout or the surface of the runway and taxiway at the time of the accident. The videos will be reviewed.

Meteorology

National Weather Service forecasters and other personnel were interviewed. An enhanced snow band was in the area producing localized heavy snow due to lake enhancement. This apparently is a somewhat unusual weather phenomenon, as the band swath was only 20 to 30 miles wide with snow accumulations of 10 inches right over Midway Airport.

Midway Airport weather observation equipment and records were examined and all equipment was working normally during the evening of the accident.

Southwest Airlines dispatchers who were associated with the accident flight were interviewed. Prior to the takeoff from Baltimore, when weather conditions deteriorated and the runway switched to runway 31C, the dispatcher determined that runway 31C was approved for landing for flight 1248. Runway conditions, braking action, wind speed and direction, airplane weight and mechanical condition of the aircraft are typical factors considered in making such decisions. The flight was contacted twice on the way to Midway and the appropriateness of using the runway for landing was reaffirmed during both contacts.

Official weather observations:

Approximately 20 minutes prior to the accident, the winds were from 100 degrees at 11 knots, visibility was 1/2 mile in moderate snow and freezing fog, the ceiling was broken at 400 feet, and overcast at 1400 feet, temperature -3C, dew point -5C, altimeter setting 30.06 in. Hg. Remarks - runway 31C rvr (runway visual range) 4500 feet, snow increment - 1 inch of new show in the last hour, 10 inches on the ground.

Approximately 23 minutes after the accident, a special observation revealed winds out of 160 degrees at 5 knots, visibility 1/4 mile in heavy snow, freezing fog, sky obscured with a vertical visibility of 200 feet, temperature -4C, dewpoint -5C. Remarks - runway 31C, rvr 3000 feet.

Toxicology

Blood and urine samples were obtained from both pilots. The disposition of the blood samples is being reviewed.

Structures

The aircraft has been removed from the accident site and was transferred to a hangar at Midway Airport. The maintenance log revealed no writeups or deferred items for the accident flight or several previous flights.

Professional surveyors completed a survey of the accident scene and the geography leading up to the site to include the locations of parts shed by the aircraft after it left the paved runway surface and the blast fence destroyed during the accident sequence.

Powerplants

Both engines were visually examined at the accident site. Although the first stage compressor blades of both engines showed foreign object damage, they were all intact and present. Wood from the blast fence and other debris was present in both engines. A visual examination of the turbine sections revealed no missing blades.

The 60-day engine history revealed no deferrals or writeups. Each engine has two thrust reverser sleeves. FDR data indicated that all four sleeves were deployed until after the airplane left the paved runway overrun surface. Hydraulic system B (that runs the thrust reversers) revealed no leaks.

Systems

The Systems Group documented the switches, circuit breakers and controls in the cockpit. The leading edge slat, flap, and trailing edge flap extension measurements were taken and revealed symmetrical extension of all devices. The measurements will be compared to Boeing documentation to determine exact extension.

Chicago Fire Department personnel were interviewed to determine if any switch positions or other items were altered during the rescue effort. The Fire Department Chief stated that the only things his people did were disconnect the battery and turn off the crew oxygen source.

The brakes were found in good condition with adequate wear remaining. The main landing gear tires had acceptable tread depth and no indication of flat spots.

Air Traffic Control

The local controller, two tower controllers, and the tower supervisor were interviewed. All controllers stated that they saw the aircraft lights during the landing roll, but did not see the actual touchdown.

The investigation has revealed that runway 31C was used as the landing runway because it contained lower landing minimums for aircraft using the ILS approach. If runway 13C was used, the runway most aligned with the wind, pilots would have been unable to land because of insufficient landing minimums.

Survival Factors/Airports

All flight attendants were interviewed. They all said that they noted a smooth landing but that the deceleration feeling thereafter seem less than usual. They noted that the emergency lighting came on after the airplane came to rest, and one flight attendant opened the L1 door to begin the evacuation. The emergency slide deployed automatically, but its angle in relation to the ground was less than ideal. This caused passengers to begin to pile up around the bottom of the slide. Rescue personnel assisted people away from the slide. The first officer deplaned after about 5 passengers and also assisted in getting people away from the airplane.

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