Doors to Automatic, thanks (#20). This reinforces the need for accurate qualification of data presented in Pprune (#11), i.e. ‘ground roll’ vs the ‘landing distance (factored/unfactored)’.
The assumption that the landing will occur 1000ft beyond the threshold is an important criterion, but it is affected by threshold crossing height, airspeed compared with ref, and wind; thus these and other aspects have to be considered before the ground roll distance.
Most importantly you do not qualify the runway condition, particularly where an increasingly wet runway results in deteriorating braking performance.
Although safety factors in landing distances indicate a 15% increase (1.67-1.92) between dry and wet, this does not mean that the achievable distances vary in the same proportion. Dry data is the result of flight test measurement, wet data uses a factor and might be checked with calculations, but this does not account for all combinations of ‘wet’ friction.
Thus with an airborne distance greater than 1000ft, ground roll (wet) considerably longer than 3000ft, and perhaps with a safety factor approaching 2.0 (braking level, flying accuracy, etc, etc), all indicate that a landing within 8000ft is not without risk, i.e. not a ‘good margin’.