KN, clarity is not necessarily safety.
In EU-OPS, the commander is required to ensure a safe approach and landing (EU-OPS 1.400). Thus for wet/dry ops, the open question is if the chosen 15% of something (QRH) will be as safe as the pre-takeoff factor (1.67/1.92). There is no particular factor or distance margin specified for an inflight check. The pre-landing decision requires (re)assessment of all relevant conditions and the choice of safe action.
This assessment requires knowledge of what the QRH figures mean, what assumptions have been made and any limits in use. The Boeing unfactored ‘actual’ distances might only equate to a practical minimum distance flown in average conditions by an average pilot.
The pre-takeoff factored distances may enable greater variability in operation, and define a standard which the authority accepts as acceptably safe (acceptable risks – severity of event / frequency of encounter). Guidance on this is in UK AIC 14/06
http://www.ead.eurocontrol.int/pamsl..._2006_P_014_en
Thus, does the QRH +15% (wet/dry conditions) provide an equivalent level of safety as would the pre-takeoff factored distance?
Contaminated operations are different. In the EU the authority accepts a lower level of safety assuming infrequent landings. However, the commander is still required to ensure safety.
The UK CAA have provided guidance which indicates that the (reduced) dispatch factor could be an acceptable safety margin, but cautions that because the distances may include the use of reverse, a reverse failure should be considered. Also, that some unpublished data suggests that contam+15% may not be achievable as a realistic minimum. The general advice in the EU for contaminated operations is to avoid the conditions (see CS-25.1591).
Comment on previous posts:
Wet AFM (unfactored) performance is more likely not to include the use of reverse (#8) - CS 25.125.
The use of ‘Good’ braking action in wet conditions is not the same as ‘good’ in dry conditions; ‘good’ is a relative term according to the actual conditions and not comparable across conditions (#10) -
Managing Threats and Errors during Approach and Landing slide 26.