The National Transportation Safety Board today determined that the probable cause of a fatal runway overrun at Chicago’s Midway Airport was the pilot’s failure to promptly use available reverse thrust to slow or stop the airplane after landing. This failure occurred because the pilots’ inexperience and unfamiliarity with the airplane’s autobrake system distracted them from using the thrust reverser during the challenging landing. The weather conditions were less than ideal. The landing required experience and explicit attention to multiple variables.

The urgent recommendation we issued today addressing landing performance is extremely pertinent to the safe operation of our aviation system," said NTSB Chairman Mark V. Rosenker. "As we approach the winter months we continue to push for acceptance of a minimum safety margin so that this type of accident does not occur again.

As outlined in the Board’s report, the investigation revealed that as the crew neared their destination, the pilots received mixed reports of braking action on the landing runway. The flight crew used an onboard performance computer (OPC) in the cockpit of SWA's airplanes to calculate the expected landing distance. They entered multiple scenarios, including wind speed and direction, airplane gross weight at touchdown, and reported runway braking action. Observing OPC indications that they would stop before the end of the runway with either fair or poor braking action, they decided that they could safely land at MDW.

However, as stated in the report, the accident pilots were not aware that the OPC’s stopping margins for poor runway conditions were, in some cases, based on a lower tailwind component than that presented. Also, the accident pilots were not aware that the stopping margins computed by the SWA OPC incorporated the use of thrust reversers for their model aircraft, the 737-700, which resulted in more favorable stopping margins. Therefore, the Safety Board concluded in the report that had the pilots known this information, they might have elected to divert to another airport.

Contributing to the accident were Southwest Airlines’ failure to provide its pilots with clear and consistent guidance and training regarding company policies and procedures related to arrival landing distance calculations; programming and design of its on board performance computer, which did not present critical assumption information despite inconsistent tailwind and reverse thrust assessment methods; plan to implement new autobrake procedures without a familiarization period; and failure to include a margin of safety in the arrival assessment to account for operational uncertainties.

Contributing to the accident was the pilot’s failure to divert to another airport, given reports of poor braking action and a tailwind component greater than 5 knots.

Also contributing to the severity of the accident was the absence of an engineering materials arresting system (EMAS), which was needed due to the limited runway safety area beyond the departure end of runway 31C.

On January 27, 2006, the Safety Board issued an urgent Safety Recommendation to the Federal Aviation Administration (FAA) to prohibit airlines from using credit for thrust reverser use when calculating stopping distances. In today’s report, this recommendation was classified “Closed-Unacceptable Action/Superceded” by a new urgent safety recommendation that calls on the FAA to immediately require operators to conduct arrival landing distance assessments before every landing, based on existing performance data and actual conditions, and to incorporate a minimum safety margin of 15 percent.

Additional new recommendations to the FAA contained in the final report include:

• Require all Part 121 and 135 operators to ensure that all onboard electronic devices they use automatically and clearly display the critical performance-calculation assumptions.
• Require all Part 121 and 135 operators to provide clear guidance and training to pilots and dispatchers regarding company policy on surface condition and braking action reports, and the assumptions affecting landing distance/stopping margin calculations, including the use of airplane ground deceleration devices, wind conditions and limits, air distance, and safety margins.
• Establish a minimum standard for operators to use in correlating an airplane’s braking ability with braking action reports and runway contaminant type and depth reports for runway surface conditions worse than bare and dry.
• Develop and issue formal guidance regarding standards and guidelines for the development, delivery, and interpretation of runway surface condition reports.