B777 Unreliable Airspeed Checklist

[wilco77]

Was wondering in the B777 Unreliable Airspeed Checklist, why does the checklist ask us to use Alternate Flaps Extension?
Thanks

WIlco
https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1780x2000/unreliable_airspeed_f5cf31ef008d78fbb83ddf518834e88e99713116 .png

[BuzzBox]

If the airspeed is unreliable, the FSEU cruise inhibit and load relief functions might not work correctly, which could affect slat/flap operation. Those functions are bypassed when the flaps are in ALTN mode.

[wilco77]

Thanks that makes sense.

Why do we also disconnect PFC if the airplane cannot be trimmed?

[FullWings]

Because you will be continually fighting the aeroplane in pitch if you leave the PFCs in. Far better to go into “direct” manipulation of the flying control surfaces at that point and use the natural longitudinal stability of the airframe to help.

[Dufo]

Interesting. On 747-400, extension of flaps is in normal mode until 20. Then alternate for 25, which is used for landing.

[BuzzBox]

Thanks that makes sense.

Why do we also disconnect PFC if the airplane cannot be trimmed?

In normal mode, pitch trim is achieved by changing the trim reference speed. The PFCs move the elevators to trim the aircraft and the stabiliser then moves to streamline the elevator. The PFCs need airspeed data to perform that function. If you can trim the aircraft, well and good. If not, then you must disconnect the PFCs and revert to direct mode to restore pitch trim. In direct mode, the pitch trim switches move the stabiliser directly.

[wilco77]

Thanks for the excellent answers

[FlyingGourmet]

How is it possible that the GS is still reliable if it comes from the ADIRU?