NTSB urges safety margin for landing calculations

 - November 1, 2007, 4:34 AM

The NTSB last month made an “urgent” recommendation to the FAA in response to the December 2005 fatal runway overrun at Chicago Midway Airport, calling for the agency to require operators to conduct arrival landing-distance assessments before every landing based on existing performance data and actual conditions, and to incorporate a safety margin of at least 15 percent. The recommendation supersedes the January 2006 recommendation that the agency prohibit the airlines from factoring in credit for the use of thrust reversers when calculating stopping distances.

“The urgent recommendation we issued today addressing landing performance is extremely pertinent to the safe operation of our aviation system,” NTSB chairman Mark Rosenker said October 2.

The NTSB made five more recommendations to the FAA:

• require Part 121 and 135 operators to ensure that all onboard electronic devices automatically and clearly display critical performance calculation assumptions;

• require all Part 121 and 135 operators of thrust-reverser-equipped aircraft to require the non-flying pilot to confirm the thrust reverser status immediately after touchdown on all landings;

• 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; and

• develop and issue formal guidance regarding standards and guidelines for the development, delivery and interpretations of runway surface condition reports.

Reverse Thrust Not Used
On Dec. 8, 2005, a Southwest 737-7H4 overran the departure end of Runway 31C during a snowstorm, rolled through a blast fence, an airport perimeter fence and onto an adjacent roadway. The airplane hit an automobile on the roadway before coming to a stop. The crash killed a child in the car, and resulted in injuries to 20 others.

The Safety Board determined that the probable cause of the accident was the “pilots’ failure to use available reverse thrust in a timely manner to safely slow or stop the airplane after landing.” It added that the pilots’ lack of familiarity with the autobrake system distracted them from using the thrust reversers during the landing.

Contributing factors included Southwest’s failure to provide training and guidance regarding its policies for calculating landing distances and assumptions (such as tailwind factors) affecting distance/ stopping margins; the programming and design of the onboard performance computer, which did not take into account the eight-knot tailwind evident at the time of the crash and therefore displayed an inaccurate stopping margin; the airline’s plan to implement a new autobrake system without a familiarization period; and the pilots’ and operator’s failure to include a margin of safety in the arrival assessment to account for operational uncertainties. The pilots’ failure to divert to another airport and the absence of an engineered materials arresting system (EMAS) at the end of the runway were also listed as contributing factors.