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December 2005 Southwest Chicago blizzard accident leads US NTSB to urge for revised landing margin policy on contaminated runways

US airlines are being urged by the National Transportation Safety Board (NTSB) to voluntarily adopt changes in the way they calculate stopping distances on contaminated runways to avoid a repeat of the December 2005 Southwest Airlines Boeing 737 accident at Chicago Midway during a snowstorm.

NTSB chairman Mark Rosenker’s recommendation, which mirrors a previous call from the US Federal Aviation Administration, is a response to that agency’s decision to suspend a mandate that would have restricted domestic operators from landing at some shorter runways.

The FAA operational specification was issued following the overrun of a Southwest Boeing 737 at Chicago Midway’s runway 31C in December 2005 during a snowstorm. Procedures used to calculate the safety parameters of such a landing were questioned in the resulting investigation.

While approaching Chicago Midway, the Southwest pilots used an on-board laptop performance computer (OPC) to calculate expected landing performance. Information entered into the computer included expected landing runway, wind speed and direction, airplane gross weight at touchdown, and reported runway braking action. The OPC then calculated the stopping margin. Depending on whether “wet-fair” or “wet-poor” conditions were input, the computer calculated that the remaining runway after stopping was either 560ft (170.8m) or 30ft.

These calculations were based on the assumption that the engine thrust reversers would be deployed immediately upon touchdown. However, flight data recorder information revealed that the thrust reversers were not deployed until 18s after touchdown, at which point there was only about 1,000ft of usable runway remaining.

In the aftermath of the accident, the NTSB urged the FAA to prohibit airlines from using credit for the use of thrust reversers when calculating stopping distances on contaminated runways. It said that if the thrust reverser credit had not been allowed in calculating the stopping distance for the Southwest flight, the OPC would have indicated that a safe landing on runway 31C was not possible.

Although the FAA’s subsequent operational specification did not address the NTSB’s recommendation, the mandate would have stopped US airliners from landing at any airport unable to offer at least 15% more runway than required to land safely under the prevailing conditions.

But the FAA’s mandate was suspended a month before its 1 October start date after operators and industry bodies argued against the agency’s attempt to use operational guidance to change policy.

Instead, the FAA issued a safety alert urgently recommending that operators of turbojet aircraft voluntarily comply with the margins immediately, and said it would proceed with issuing a notice of proposed rulemaking (NPRM) on the matter.

Noting that the process of handling the issue through the NPRM process will delay implementation of the change, Rosenker is calling for airlines to “voluntarily adopt the procedures contained in the FAA's [operational specification] now, as we are entering another winter flying season”.

Rosenker notes that Midway has improved the safety margin at its runways by installing the Engineered Materials Arresting System, which uses materials placed at the end of a runway to stop or slow an aircraft that overruns the runway.

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