National Transportation Safety Board
Office of Public Affairs
Washington, DC - National Transportation Safety Board Chairman Mark V. Rosenker is urging airlines to voluntarily adopt changes in the way they calculate stopping distances on contaminated runways, in accordance with Federal Aviation Administration (FAA) guidance issued in response to an urgent NTSB recommendation.
On December 8, 2005, Southwest Airlines flight 1248, a Boeing 737-7H4, landed on runway 31C at Chicago Midway Airport during a snow storm. The aircraft failed to stop on the runway, rolling through a blast fence and perimeter fence and coming to rest on a roadway after striking two vehicles. A 6-year-old boy in one of the automobiles was killed.
While approaching Chicago on a flight from Baltimore, the 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 remaining runway after stopping at either 560 feet or 30 feet.
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 18 seconds after touchdown, at which point there was only about 1,000 feet of usable runway remaining.
As a result of the accident, on January 27, 2006, the Safety Board urged the Federal Aviation Administration to prohibit airlines from using credit for the use of thrust reversers when calculating stopping distances on contaminated runways. (Safety Recommendation A-06-16)
The FAA does not allow operators to take credit for the assumed deployment of thrust reversers upon landing when performing the dispatch landing distance calculations. However, once airborne, the FAA permits operators of certain transport category airplanes like the 737-700 to assume that deployment of thrust reversers will occur immediately upon touchdown and take credit for the reduction in landing distance when pilots re-calculate landing distance just prior to landing.
If the thrust reverser credit had not been allowed in calculating the stopping distance for flight 1248, the OPC would have indicated that a safe landing on runway 31C was not possible.
The FAA responded to the recommendation by publishing a notice in June 2006 that it would issue a mandatory Operations Specification (OpSpec) concerning landing distance calculations that would be effective September 1, 2006. Although the OpSpec did not address reverse thrust credit, it addressed many issues in calculating landing distance including incorporation of a 15% safety factor.
The FAA subsequently announced that it believes it needs to propose this change through the rulemaking process, which will delay implementation of this change. On August 31, 2006, the FAA issued a Safety Alert for Operators (SAFO) urgently recommending that operators of turbojet airplanes voluntarily comply with the OpSpec immediately.
"We think airlines should voluntarily adopt the procedures contained in the FAA's OpSpec now, as we are entering another winter flying season," Chairman Rosenker said.
Chairman Rosenker noted that Midway Airport has improved the safety margin at its runways by installing the Engineered Materials Arresting System (EMAS). EMAS uses materials placed at the end of a runway to stop or slow an aircraft that overruns the runway.
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