Originally Posted by
main_dog
Jammedstab, I think you're on the B744? I'm not 100% sure I've understood your question but I'll give it a try. Keep in mind that Vr must always be in excess of 1.05 VMCA, among other things.
Vr however is more limited by its minimum of 1.05 VMCA, and as the default setting in the FMS is full takeoff thrust, the FMS precalculated Vr may initially be several knots higher than the company provided one (since VMCA at full TO will of course be significantly higher than VMCA at TO2). When you then input into the FMS the actual rating you will use, especially if it's TO2, its precalculated Vr should drop significantly and may then closely match the company provided one.
To follow up....
I did some FMC calculations in Chicago the other day. For a takeoff weight of approximately 327 tons this is what the FMC showed as default speeds for runway 10L with no ATM:
FMC Flap 20 TO thrust: V1:136 Vr:150 V2:165
FMC Flap 20 TO2 Thrust: V1:143 Vr:154 V2:165
According to the performance computer for TO thrust Min Vr: 125
According to the performance computer for TO2 thrust Min VR: 117
So the min Vr decreases by 8 knots by reducing the thrust according to the performance computer which makes sense according the VMCA reasoning stated by some. However the FMC speeds are way above VMCA so I would assume that this has nothing to do with it. And instead of decreasing VR for derated thrust, the FMC increased it. So the first question is why does the FMC VR speed change with a thrust rating change when Vr is well above VMCA and why is Vr increasing as thrust rating is decreasing.
I think Lateonenite knows the answer based on his earlier response. Would appreciate you letting us know.
Thanks