Use of flight director BOEING 777
 

when the flight director is not required, the pilots in left and right seat
should reset it according to the procedures:
Flight Director Switches (Both) .......................................OFF PM
Flight Director Switch ......................................................... ON PM

what's the purpose of turning PM's FD ON?

EDIT: Ignore previous... Just remembered it's to ensure the FDs stay displayed when TOGA mode finishes (other mode selected / becomes active). They will auto pop-up with FD switch off, but disappear again later.

thanks for answering, could do you explain when pm side turning on, the FMA is changed showing "ATT" what' s this used for?

When both F/Ds are turned off, the FMAs will be blank and no (vertical/lateral) modes active. When a F/D is then turned on, it has to go to some default mode. With wings close to level (<5º bank), that will be TRK HOLD | V/S; beyond 5º it will be ATT | V/S, to approximate the existing flight path until other modes are selected.

Thanks a lot, that's exactly what i want to learn, could you give me more details or where i can find more? by the way, when the v/s mode activity, will it be changed by attitude changed? or just stuck at the moment when turning one side FD on?

That's basically all the FCOM says. If the bank angle is greater than 30º it will command a roll back to 30º, otherwise maintain the bank angle and V/S that existed at the time the F/D was switched on. So, when you do that while the PF is flying a visual approach, the flight director indications very quickly become irrelevant as it's trying to maintain what was happening at that instant. As Good Cat said, the purpose of turning the PM's F/D back on is simply so that the (useful) indications are not lost during a missed approach when the TO/GA mode ends. In most other situations, immediately after turning on the F/D you would make another selection and not remain in ATT.

very useful, thanks for helping.

it’s for SPD. It turns on V/S vertical mode, thus the autothrottle will maintain the speed and you can use until touchdown

Shouldn't the auto thrust change to SPD with flight directors off anyway?

Correct. The first action (flight directors off) ensures the autothrottle either remains in, or switches to SPD (very easy to demonstrate this from e.g. HOLD / (roll mode) / FLCH SPD).
The second action (PM flight director on) ensures continued go-around guidance as has been stated.

It’s in the circling approach (and maybe other) sections of the FCTM.