Originally Posted by
Superpilot
The FTD is consistently reducing aircraft speed to that flap's min speed upon flap selection with VNAV PTH. We see this behavior if we make selections 5 miles before reaching the Fx symbols on the ND. I think I'll wait until I get to the FFS!
The B787 FCOM states:
During approach maneuvering with IAS/MACH window closed, VNAV sets command speed to flap maneuvering speed as the airplane passes the flap profile point indicated on the ND. The command speed change occurs even if flap selection is not made. If flaps are not extended before passing the flap profile point, the IAS/MACH window should be opened and the desired speed set to avoid an underspeed condition. When flaps are extended prior to the flap profile point, VNAV commands flap maneuvering speed for the flap position selected.
In other words, if you extend the flaps
before crossing the flap profile point (F1, F5, etc), VNAV will command the respective flap manoeuvring speed when you extend the flaps. If you cross the flap profile point and
don't extend the flaps, VNAV will command the respective flap manoeuvring speed regardless of the actual flap position.