Help - Boeing OPT Vref question
Hi,
Been scratching my head for a while on this one. It's about the VREF ADD box on the Landing Enroute page of Boeings OPT. I have flown for 2 747-8 operators, one has 0 as default and the other has +5. Here's the case for both:
Case for default 0 additive - OPT data give you LDR based on touching down at Vref, The OPT performance data will have taken into account the deceleration from approach speed to touchdown speed during the flare. This is not a guess but clearly indicated by the fact that when you select AUTOLAND with a VREF additive of 0, the system adds 5kts (because it knows speed won't be reduced to VREF during the flare). The VREF ADD option is for wind gust corrections and control surface/configuration malfunctions.
Case for default +5 additive - The speed that is displayed in the results window is the speed at which the threshold is crossed.The threshold speed should be Vref plus half the steady headwind component plus the gust intensity, not to exceed Vref + 20. The total addition to Vref is entered in the VREF ADD input window. In absence of wind, the threshold is crossed with Vref + 5. For this reason, Vref ADD is set to 5 by default.
Any boffins out there who can tell me who is correct?