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777 VNAV SPD/PATH question

Chaps, Boeing VNAV question for you:

In a descent with VNAV PTH on the FMA, opening the speed window will put you into VNAV SPD. However if you shoot an RNAV approach you can of course use VNAV as the vertical mode and have VNAV PTH showing and open the speed window at the same time.

At what point does it allow you to do this? There's clearly a transition from one VNAV 'mode' to another at a certain point, but I'm struggling to find the reference for it. Anyone?

Cheers
RU

From the FCOM

Quote:

The FMC transitions to "on approach" mode for any of the following conditions:

the descent phase is active and flaps are out of up, or

the airplane has sequenced the first waypoint (or FAXXX) of the active approach, or

the airplane is on a direct-to or intercept course-to the active waypoint and the distance to go is less than 12 NM, or

the missed approach point is the active waypoint and the distance to go is less than 25 NM

.....

Quote:

When the FMC is “on approach”, the following features are available..the IAS/MACH window can be opened and the command speed can be set while VNAV remains in VNAV PTH descent; VNAV commands the set speed.......

I need to get better specs. Thank you. That’s perfect.