Required Landing Distance. The required landing distance is established during certification and is the distance needed to completely stop from 50 feet above the runway threshold (i.e., the point at which the obstacle clearance plane intersects the runway. In establishing landing performance data, the airplane must approach in a steady glide (or rate of descent) down to 50 feet at speed not less than VREF for the chosen landing flap setting.
Operational Landing Distance. These distances are advisory performance data (i.e., not required by regulation) which are intended to provide a more accurate assessment of actual landing distance at time of arrival, considering factors which cannot be accurately predicted at time of dispatch, such as runway contaminants, winds, speed additives, and touchdown points. These distances may be based upon the use of reverse thrust, ground spoilers, autobrakes, etc.