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NTSB Recommendations Likely to Impact FAA Runway Safety Margin Discussions
October 4, 2007
On December 8, 2005, Southwest Airlines (SWA) flight 1248, a Boeing 737-74H, (N471WN), ran off the departure end of runway 31 center (31C) after landing in a snow storm at Chicago Midway International Airport (MDW), Chicago, Illinois. The airplane rolled through a blast fence, an airport perimeter fence, and onto an adjacent roadway, where it struck an automobile before coming to a stop. One occupant in the automobile was killed, one received serious injuries, and three others received minor injuries. Eighteen of the 103 persons on board the airplane received minor injuries and the airplane was substantially damaged.
The National Transportation Safety Board (NTSB) determined that the probable cause of accident was the pilot's failure to use available reverse thrust in a timely manner to safely slow or stop the airplane after landing. 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.
As outlined in the Board's report, the investigation revealed that as the crew neared their destination the pilots received mixed braking action reports for the landing runway. The flight crew used an on-board laptop performance computer (OPC) provided in the cockpit of Southwest Airlines’ airplanes to calculate 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 stopping margins displayed by the OPC for poor runway conditions were in some cases based on a lower tailwind component than that which was 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, the pilots 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 the reports that included 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 because of 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 the use of thrust reversers 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, actual conditions, and incorporating 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 on board electronic devices they use automatically and clearly display 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, to include 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 to 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 interpretations of runway surface condition reports.
A synopsis of the Board's report, including the probable cause and recommendations, is available on the NTSB's website, www.ntsb.gov, under "Board Meetings." The Board's full report on the accident will be available on the website in several weeks.