tournesol, to avoid any misunderstanding about landing from Vref-5 at the threshold; I stress that the certification tests only demonstrate that a landing will be safe, and thus provides a margin of safety below the normal minimum speed of Vref at the threshold. Vref-5 is not intended for normal (or even abnormal) operations, only as a margin to counter speed inaccuracy, wind, etc.
It would be interesting to understand why manufacturers and operators add an additional margin above Vref for normal landings. It could indicate a difference in views of the safety margin between the operational and certification assumptions. If Vref is ‘safe’, and landing performance is based on this, then what is the justification – the balance of safety from the speed margin in landing at Vref+5 against a reduction in the landing distance safety margin?