Several posts imply that because there are safety margins (factors) then these can be ‘used’ before the event. The margins in the pre landing calculation provide an acceptable level of safety; a small additional tailwind beyond that planed will reduce the level of safety, but by how much may not be known.
A crew can elect to accept the increased risk but must be prepared to justify it if there were to be an incident, i.e. justify the risk to yourself before deciding.
Such a decision requires a good understanding of the components and origin of the risk. In the calculation of landing distance the relevancies are difficult to establish because there are many unknowns, inaccuracies, and assumptions. Check the small print in the book values of landing distance; what is assumed in the calculation, what is the reference manual.
RAT makes a good point re AFM limits vs advisory landing data. If you have an incident after deviating from the AFM without good justification, then you are answerable in civil law – including manslaughter.
Deviation from advice could similarly escalate, but hopefully nothing more than a chat with the chief pilot because your justification should have greater validity - economics and passenger comfort are never justifiable after an incident.
FMS wind is historical, it only represents conditions where you have been (often averaging higher altitudes and greater distances); airfield wind is where you are going, or somewhere close, but this is not without error.
The approved safety margin maintains a suitable level of safety for normal daily operational variability (try listing the factors in this), but this margin is insufficient to accommodate simultaneous limiting deviations and must not be planned to be used.
An increasing tailwind is a high risk category as this can interact with several factors; a tendency to stretch the flare distance, a slower reduction in airspeed, a different visual perception, and probably unfamiliarity with landings in the higher wind values; and also remember that the energy to be reduced is proportional to ground speed squared.
https://www.scribd.com/doc/285864189...ind-Operations
http://www.nlr-atsi.nl/downloads/som...f-tailwind.pdf
http://www.nlr-atsi.nl/downloads/run...-of-runway.pdf