Regarding the Smiths FMC mainly used on 737s. On the decent forecast page there is an option to put the QNH of destination and ISA deviation. In relation to the ISA deviation, is this on the ground at your intended destination or as I have also been told, an average ISA deviation figure for were you currently are ie in the cruise at FL360etc (deviation from ISA atmosphere of -56.5)
Also, in relation to the Vref speed that airliners fly on approach, in the Boeing manuals for the 737 NG it says if the approach is planned to be flown manually at any stage add a 5 knot additive onto your vref, even if it is calm, not considering headwind/gust added factor here. As per company protocol we do this but a point that has come to light is when is this extra 5 knots bleed off? as recent training documentation has stated that any extra speed above vref crossing the threshold will result in using more runway than was planned or possibly available.
I am speaking about the 800 series here but would like to other views on all other types, is this a type specific thing and either way what is the reason being this extra 5 knots even when it is calm as we do this even when planning a CAT 3 autoland, is it anything to do with auto throttle not correcting quickly enough?. Is it a common thing for all airliner types/companies? since I have also heard some planes touch down at a speed below vref.