Hello,
The FMS uses this temperature to refine its computation of the climb profile (ISA model). E.g: Higher temperature means, higher density altitude and ROC will be less than in standard atmosphere. The temperature at your cruising level if you don,t insert anything it will assume ISA condition. Inserting the actual cruise temperature will give you a more accurate Optimun and recommended Max Altitude. You can try on the ground and check how it affects the progress page altitude.
Same as for the temperature you insert in the performance page for the approach. It's there to compute more accurate descent profile. Again higher temperature means lower rate of descent. Not sure if the temperature is used also for pressurization segment tho.
The QNH is purely for pressurization segment. Nor the Temperature or the QNH has an effect on your VAPP. Only the wind you insert has with the groundspeed mini function. Again you can try and see by yourself. =)
That's how I understand it. I'm no expert tho.
Last edited by pineteam; 13th Nov 2017 at 04:04.