Southwest Airlines Flight 1248

Investigation

The National Transportation Safety Board (NTSB) investigated the accident. Chicago Fire Department commissioner Cortez Trotter said that the aircraft would not be removed from the intersection until the NTSB gave clearance to do so following its on-site investigation. The nose of the aircraft was hoisted onto a flatbed tractor-trailer on Saturday, December 10, and the 737 was towed to a hangar for continued inspection.

The Boeing 737-7H4 was equipped with the latest anti-skid and braking technology. The report noted that Southwest had only very recently begun using the autobrake systems, and that pilot training on proper use of autobrakes had been inadequate.

The NTSB preliminary report determined that the aircraft touched down in the touchdown zone of the runway with 4,500 ft of its 6,522 ft length remaining; under the prevailing conditions of weather, wind, speed, and weight, the aircraft needed 5,300 ft of runway to stop safely. It was found that the tailwind was eight knots and exceeded the limit of five knots.

A preliminary NTSB advisory said:

"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. Flight data recorder information reveals that the thrust reversers were not deployed until 18 seconds after touchdown, at which point there was only about 1,000 ft of usable runway remaining."

Alternately, the crew could have held in the air, waiting for the weather to improve, or they could have diverted to another airport such as Chicago O'Hare International, which had substantially longer runways just 10 minutes away. Each of these options would have entailed considerable additional expense for Southwest, as well as missed connections and significant inconvenience for the flight's passengers. The NTSB identified the psychological pressure to complete their assigned task as one of the factors contributing to the crew's decision to land at Midway despite unfavorable conditions. Cockpit voice-recorder transcripts indicate that the pilots had been concerned about the weather and, before landing, jokingly alluded to the movie Airplane!, saying, "I picked a bad day to stop sniffin' glue."

The NTSB discovered that the air traffic controller had told the pilots that the braking action for the first half of the runway was good, and poor for the second half. However, investigators found that the controller had not furnished the pilots with all of the available required braking action reports because he did not take the type of aircraft from each report into account. One of these planes was a small plane that reported poor braking conditions. It was found that braking-action reports varied significantly based on aircraft type, changing weather conditions, personnel experience, the type of equipment used, and the time of report, and therefore should not be used as conclusive information on runway conditions.

The pilots stated that, based on the calculations that the crew had entered into the on-board performance computer (OPC), they believed that they could land and stop safely. However, investigators determined that the OPC had based its stopping margins on two assumptions: that the tailwind would be slower than it turned out to be and that the thrust reversers would be deployed when landing. It was found that the pilots had been unaware of these assumptions and that Southwest had not provided sufficient recurrent training on them. Therefore, the pilots did not use the critical braking-action term "poor" when assessing the landing performance.

Southwest Airlines had a policy that required pilots to consider more critical braking-action assessments when they receive mixed braking-action reports. However, the accident pilots were unaware of the policy and therefore did not consider it while assessing landing conditions. The NTSB also found that three other company pilots had landed before the accident with the same mixed braking-action reports. When interviewed, company pilots revealed that they too either did not adhere to the mixed braking policy or were not aware of it. Southwest also had not routinely trained its pilots on following or understanding the mixed braking-action report policy, and it was not referenced in the company manuals.

The NTSB concluded that even under the poor braking conditions and the tailwind, the plane could have stopped in time had the pilots deployed the thrust reversers on time. Examination of the reverser system found no evidence of malfunction. The NTSB determined that the probable cause was the pilots' failure to use available reverse thrust promptly to safely decelerate or stop after landing, which resulted in a runway overrun. This failure occurred because the pilots' first experience and lack of familiarity with the airplane's autobrake system distracted them from thrust-reverser usage during the challenging landing.

Contributing to the accident were Southwest Airlines' 1) failure to provide its pilots with clear and consistent guidance and training regarding company policies and procedures related to arrival landing distance calculations; 2) programming and design of its on-board performance computer, which did not present critical assumption information despite inconsistent tailwind and reverse thrust assessment methods; 3) plan to implement new autobrake procedures without a familiarization period; and 4) failure to include a margin of safety in the arrival assessment to account for operational uncertainties. Contributing to the severity of the accident was the absence of an engineered materials arrestor system, which was needed because of the limited runway-safety area beyond the departure end of Runway 31C."