Originally Posted by
Capn Bloggs
I am attempting to understand what you people understand about the 15% and why it seems OK on a routine basis, that is all.
The dispatch case is based on the certified landing data, which is predicated on maximum manual braking. It's a
planning requirement that is very conservative and is not normally representative of
actual landing performance. You previously said: "
unless you shorten your flight time, you'll always have close to a 67% buffer when you land anyway...". You won't, unless you're in the habit of using maximum manual braking on every landing. Pilots normally use less braking force (either auto brake or manual), which invalidates the dispatch calculation. The pilot does not know how much margin is available on the runway length.
For the in-flight case, the regulations allow the use of a 15% buffer if actual landing distance data is available. Both Airbus and Boeing provide advisory data that includes information for a variety of brake settings. The pilot can input the actual brake setting, environmental conditions and aircraft configuration, and calculate an accurate landing distance, to which the 15% buffer is applied. The pilot then knows exactly how much margin is available, assuming the landing occurs within the normal touchdown zone.
The following might be of interest:
https://ifalpa.org/media/2030/12adob...g-aircraft.pdf