The FAA listed reasons why it believes that the 15-percent landing distance safety margin policy is needed:
• The agency surveyed operators’ manuals (not the aircraft flight manual) and found that approximately 50 percent of operators don’t have policies for assessing whether there is enough landing distance at the destination at time of arrival, even when conditions (weather, airplane configuration, runway condition) change en route.
• Not all operators that calculate landing distance have procedures that account for conditions that are worse
• Not all operators that assess landing distance apply a safety margin; those that do aren’t consistent about increasing that margin as conditions worsen. In other words, not all operators calculate the margin using a percentage, which automatically increases the required distance as the needed runway length increases.
• Some operators use contaminated-runway performance data that is not as conservative as that provided by the airplane manufacturer or is not current.
• Operators don’t always understand the correct application of reverse-thrust credit and don’t apply it uniformly.
• Landing-distance data in aircraft flight manuals is developed by test pilots flying rigorously controlled flight scenarios such as touching down at high sink rates and approach angles of -3.5 degrees “to minimize the airborne portion of the landing distance,” the FAA explained. “Therefore, the landing distances determined under section 25.125 [certification rules] are shorter than the landing distances achieved in normal operations.”
• The numbers provided for wet and contaminated landing distance are calculated based on dry runway performance. “Therefore, the wet and contaminated runway data may not represent performance that is achieved in normal operations.” Because the regulations don’t specify a safety margin, this is left to the judgment of the operator and flight crew.
• Landing distance information is not provided in a standard format from manufacturer to manufacturer.